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Posted by: Van B Oct 30 2021, 04:45 PM

Evening everyone. So, I installed the proper intake boot from auto Atlanta this afternoon but the car still isn’t behaving properly.
Basically, when the car is warm and I go from driving at speed to a stop, the rpm hangs around 1600-1800. Once stopped, I can sometimes blip the throttle and get it to fall to around 1100rpm. If I load the engine by partially releasing the clutch, I can get the car to idle normally at 800-ish and it stays there.
Also, there is no hunting, surging, or problems with idling over a long term.
I just can’t understand why it hangs when I’m out driving.

I’ll run through the things I’ve checked so far and hopefully someone with more experience can help.

First I took off the Aux Air Regulator and confirmed it is functional.

Replaced the boot and discovered the one installed was not correct for a 74 1.8L
- that meant I had to replace and sort out some vacuum lines that were not correct length and get a substitute for the small fitting on the boot.

Put clamps on connections that didn’t seem tight enough.

Had the car smoke tested when I bought it and the mechanic said the boot was the only thing leaking.

Could the barn door on the AFM be hanging up or too loose?

Thanks!

Van

Posted by: wonkipop Oct 30 2021, 05:33 PM

did your mechanic tell you where the boot was leaking?

there are four connection points.

Posted by: Van B Oct 30 2021, 05:35 PM

All over. It was old and cracked to hell. But like I said, that’s all new and sorted as of this afternoon.

Posted by: ndfrigi Oct 30 2021, 05:46 PM

maybe flap inside throttle is dirty, or return spring or your gas pedal sticking.

Posted by: SteveL Oct 30 2021, 06:10 PM

I have experienced this same issue, 74 1.8 L-Jet.
Sorry, don't have the answer, but interested as heck at what you get for replies.
Thinking about it, since I replaced all vacuum hoses, I haven't noticed it happening.....

Posted by: wonkipop Oct 30 2021, 06:18 PM

sorry. thought you meant you had replaced it and it was still leaking.


could be the afm.

could also be decel valve sticking? not sure.

as suggested above mechanically the throttle is not returning to fully closed and is hanging up either at the throttle or back at the pedals. lubricate.
moving the clutch and having an effect might be pointing to something?

check one thing at a time and test run it.

you can check the decel valve by disconnecting the hoses to it and plugging the hoses.
drive it and see it it returns to idle. decel slows down the return to idle for emissions.

you can also check your aav, even though you say you have cleaned it out.
once you have it warmed up and are driving it and it sticks at high revs, see if while its stuck at high revs if you pinch the hose feeding into the aav from the intake boot whether anything happens.

someone here would know a bit more about worn afms.

Posted by: emerygt350 Oct 30 2021, 06:42 PM

I had the same idle hang on my 2.0 djet (and posted about it). I found it to be a pile of problems. Dirty pcv, which I don't think you have, combined with sticking advance and retard plate, which I think you may have.

If you have vacuum retard, check the line, check the diaphragm, and then clean and lubricate the retard plate in the distributor.

Posted by: timothy_nd28 Oct 30 2021, 07:13 PM

Really sounds like something is going on with your throttle body, more specifically the idle circuit control switch. This switch may be intermittently not closing when at idle.

Pretty easy to test, unplug the ECU connector and put your meter leads (set for resistance) on pins 2 and 18. I would set the meter so it would beep when it senses continuity. Jump in the car and play with the accelerator pedal, make sure you hear beeps everytime you let go of the throttle.

I'm not sure how much you are a stickler for originality, but I recently ditched my throttle body and replaced it with a Gowesty throttle body. Some modifications needed, but man what a difference it made.

Posted by: Van B Oct 30 2021, 07:56 PM

Ok, lots of things for me to look at then lol. I’m new to the car and flying blind with no manual or other 914 guys around me.

Wonkipop,
Where is the decel valve? I’m doing this with no manual and I’ve only had the car two week so, hopefully you guys can give me some talk-ons?

Emery,
I don’t think I’m ready to mess with the distributor yet. I haven’t had a car with a cap in about 25yrs and I don’t have anything to retime the car. I do have a diaphragm on the distributor though so I’ll unplug and blow out all the lines. At least that will remove one variable.

Tim,
Uh you got anything to show me what the PIN numbers are? Otherwise, I think this might be outside my ability right now.

Feeling pretty frustrated and useless at this point. With everything I’m tooled up to do on my 996, it’s pretty demoralizing to be so ineffective on a car that is mechanically simple by comparison.

Thanks everyone!

Van

Posted by: timothy_nd28 Oct 30 2021, 09:27 PM

I wouldn't worry about the manual, and don't be timid as there is a wealth of info on this site. The ECU is near and under the battery, the main harness connector unplugs pretty easy from the ECU. If you can locate the two pins in question, you'll be golden!

Posted by: Van B Oct 30 2021, 10:12 PM

Money, thanks Tim!

Posted by: wonkipop Oct 31 2021, 02:29 AM

don't worry mate.
when i bought my 914 30 odd years ago i only had a haynes manual.
i didn't have a clue about L jetronic.
there were a few rare VW fastbacks that ran D Jetronic in Aus.
It was all a mystery. (still is).
i went from a twin carb vw squareback to the 914.

i sort of half get it but rely on my mechanic to really sort it when i get stumped.
particularly electronic test stuff.

timothy nd28 knows his stuff.

i'll dig up a pic of the decel valve and location.
i have mine disconnected.
i belong to the mob who thinks they are not so good.
it could be your problem but not saying it is.
you just got to go through things one thing at a time.

you can find the factory manuals on the net.
i can't remember where i found mine.
i downloaded them about 10 years ago.
the wonders of the internet. before that something like a factory manual was like finding gold at the end of the rainbow.
they are helpful. but won't necessarily solve your problems.
but its good for finding out what everyone is talking about.
decel valves, aar, etc etc.

Posted by: emerygt350 Oct 31 2021, 06:05 AM

Start simple. The retard brings the idle down to where it should be at idle and if it isn't working you get exactly that behavior. Get the idle to sit right by loading the engine. Then put it in neutral with emergency brake. Pull the vacuum line from the diaphragm and quickly plug the line with your thumb.

Idle goes to 1600?

Pulling the distributor is super easy, and you can mark the position of the cap and rotor before you pull it so no timing necessary.

Posted by: Dave_Darling Oct 31 2021, 01:45 PM

I would start by checking the throttle valve. Blip it by hand. If the idle hangs up, grab the bellcrank (the arm that the throttle cable hooks to) and twist it in the direction the spring is trying to pull it. If the idle drops then, you have a sticking throttle. Cleaning may help, or rebushing might be needed.

Your ignition timing might also be sticking. Hook up a timing light and look at what happens to the blades on the fan when you rev up the motor and close the throttle. If the timing doesn't go back to where it started at idle, then there's a good chance something in the distributor needs help--either the vacuum unit, or the mechanical-advance weights inside the distributor.

Most air leaks will make the 1.8's L-Jetronic run lean, making for a really crappy idle, so those are lower down on the list of suspects. The exceptions are places where air can leak through a component that connects the intake boot upstream of the throttle body with the manifold downstream of the throttle body.

--DD

Posted by: Van B Oct 31 2021, 01:55 PM

Hey everyone!
Working through the list of variables:
AFM looks healthy and happy. It has a clean reman sticker on it and the barn door is smooth with tension all they way through it’s range of travel.

Checking vacuum lines now. About to head to NAPA and get some bulk line for the ones that seem too hard or cracked at the connection point.

Throttle works fine, no hang. Snaps closed pretty aggressively.

Here’s a strange find; anyone know what this guy is or where he goes? Been unplugged a long time. It comes out of the right branch of the main loom.


Attached image(s)

Posted by: ClayPerrine Oct 31 2021, 02:16 PM

That is the wiring for the oil temp gauge. Gauge and sender were optional on 914s.

I would suggest you check the decel valve, the distributor advance, and make sure the plumbing is correct.

Posted by: emerygt350 Oct 31 2021, 02:17 PM

Yeah, we all have that. Apparently for a temp sensor none of us had or something like that.

Posted by: emerygt350 Oct 31 2021, 02:19 PM

Check the retard. It was made to bring the idle down. I don't believe your car has an advance. 1.8 might, 2.0s lost it in 74.

Posted by: wonkipop Oct 31 2021, 03:43 PM

@http://www.914world.com/bbs2/index.php?showuser=8571 B here is the decel valve and location.

Posted by: wonkipop Oct 31 2021, 03:55 PM

here is a vacuum hose diagram for the 74 1.8

Posted by: Van B Oct 31 2021, 04:16 PM

Wonkipop, thanks! I’ve been going back and forth referencing the other thread:
http://www.914world.com/bbs2/index.php?showtopic=355743&hl=Decel+valve

So, I replaced both vacuum lines from the distributor diaphragm to the throttle body. I used some close but not quite right tube they had at NAPA… for my NARP lol.. sorry couldn’t resist.

Anyway car felt a little better on acceleration so I know those lines were too old and hard. But, I still got the 1600rpm idle once the car was warm and only when I was slowing down from driving. In the garage it idled like it should even after bringing revs to 3k, holding, then letting go.
While I was driving I even tried clutching at speed and the revs still didn’t fall to idle.
Unplugged line 7 per your updated diagram and the car stalled so, does that mean decel valve is working?

Posted by: wonkipop Oct 31 2021, 04:22 PM

you ought to unplug line 7 and 8 from the decel valve and then block the lines with a plug, use something like a bolt that fits snugly to plug the lines.

that should mean the action of the decel valve does not come into play.

for good measure unplug the small line 9 and plug.
then its really out of the circuit.

----

as a separate test you can simply pinch the line from the aav.
with the decel all hooked up.
line 6.

there is more to testing the decel valve.
i'd have to re read the factory manual.
get back to you on that.

Posted by: emerygt350 Oct 31 2021, 04:24 PM

You say you replaced the hoses, did you verify they were going to the right places? It really matters.

Posted by: Van B Oct 31 2021, 04:24 PM

What am I looking for when I do that?

Also, I just had another thought; when cold starting these cars, should it high idle for warm up? Mine definitely does not. When cold starting it lopes along trying to hold a normal idle?

Posted by: Van B Oct 31 2021, 04:26 PM

Yeah, they’re in the right orientation. The last thread were you guys posted the diagram was helpful. My car is a 12/73 build date, but it has the two lines without EGR.

Posted by: wonkipop Oct 31 2021, 04:28 PM

yours should be the same as mine. mine is a jan 74.


as to factory manual testing decel valve.
its really a basic test to see if its working or not working and nothing inbetween.
you can pull just the hose where it goes into air cleaner boot.
rev the engine to mid range (reach down and twist the throttle).
then snap the throttle off.
you should feel some suction through the hose.

i'm not exactly sure how a sticking decel valve hangs the revs coming down to idle, others can explain better and it may not be your problem.
but if you can take it right out of the circuit as i suggested by disconnecting all hoses and plugging them for one test and see if your idle doesn't keep hanging it might be the problem. in any case you will have tested and eliminated.

Posted by: emerygt350 Oct 31 2021, 04:30 PM

Kinda. The aux air system handles that but it is a real fudge of a way to do it.

I put a valve on mine so I can block it quick while troubleshooting.

The retard needs to go to vacuum below the butterfly in the throttle body. That port should pull strong vacuum at idle. The advance port should have no vacuum at idle. If you have plugged your advance into any old port it will advance your idle and do nothing to advance your ignition when it needs it.

Posted by: Van B Oct 31 2021, 04:33 PM

Ok, this might be something. See the difference in configuration? These lines are all real old so, it’s been that way a long time.
http://www.914world.com/bbs2/uploads/post-24231-1635717303.jpg


Attached image(s)

Posted by: emerygt350 Oct 31 2021, 04:34 PM

Again, notice the diagram, that dotted line leasing to the diaphragm on the distributor is because they left the advance disconnected post 73.

Posted by: Van B Oct 31 2021, 04:36 PM

Line 7 is the same, but 8, 9, 10, 11 are exactly the opposite.

Posted by: emerygt350 Oct 31 2021, 04:37 PM

Yeah, the t is real. That should be there. That is plenum vacuum. One should go to the decel. The other can go to the retard if needed but God I swear I have recently read about it going somewhere else.

According to the diagram it goes to the fuel pressure regulator... Hmm.

Posted by: Van B Oct 31 2021, 04:38 PM

Say what now?

Posted by: emerygt350 Oct 31 2021, 04:43 PM

I have seen several revisions of that diagram in the past three weeks too, so there maybe some variation. Best to just go by what kind of vacuum it needs and checking to see if it is getting it.

Posted by: emerygt350 Oct 31 2021, 04:43 PM

Yeah seriously. They just left it dangling.

Posted by: rhodyguy Oct 31 2021, 04:44 PM

Your's is the one with the finger point? Is the white sheath in the throttle cable supposed to stick out that far?

Posted by: emerygt350 Oct 31 2021, 04:45 PM

I have heard of the port being plugged on the tbi, the hose just laying on the motor.

Posted by: wonkipop Oct 31 2021, 04:51 PM

looking at your photos the decel valve is correctly hooked up.

as to the business with the distributor retard lines.
mine is the same as the diagram.
i have two ports on the throttle body.

some cars have only one retard line to one throttle body port.
the other port is blocked and the other line from the distributor is an open hose that is just tucked under distributor.

don't worry about it. its some kind of emissions tweak again.
i suspect the 74s built in calendar year have the open hose.
after that it goes to a two port throttle body.
i did have this explained to me by my mechanic about a year ago and have forgotten it.
its probably something like vw getting ready for 74 calendar year emissions regs with the set up for L jet but not doing the final tweak until they had to.
my mechanic explained to me what the second line hooked up to the second throttle port would do. but again don't worry.

your decel lines are correct from the photo.

Posted by: Van B Oct 31 2021, 04:51 PM

The T on the green line does go to the fuel pressure regulator. Took me a minute to trace it because it disappears in the bowels of the engine bay, but I was able to confirm by pulling on it.

I have both lines from the distributor diaphragm going to the throttle body just like wonkipop shows. All the dashed lines are actual lines on my car.

I think we’re at the end of the road on vacuum lines. Everything is as it should be.

Posted by: wonkipop Oct 31 2021, 04:55 PM

i see what the problem is with the diagram.
its badly drawn schematic/representation of the decel valve itself.

your photo is correct.

Posted by: Van B Oct 31 2021, 04:56 PM

That’s me, and probably not. But the throttle works so, I’m not gonna mess with it.

Posted by: wonkipop Oct 31 2021, 05:01 PM

@http://www.914world.com/bbs2/index.php?showuser=8571 B

rhodyguy is on to something.

Posted by: emerygt350 Oct 31 2021, 05:03 PM

Verify that you are not getting vacuum to the advance at idle and you are getting vacuum to the retard at idle. Also, figure out which side is advance and retard. If I remember right, the outer part is the advance, the inner is the retard..

Posted by: Van B Oct 31 2021, 05:04 PM

Ok so, Recap for a problem still not solved:

-Vacuum lines are GTG
-AFM healthy
-TB ops normal

Symptoms:
-No high idle on cold start
-high idle when driving but not in the garage
-normal engine ops in all other aspects

Not yet checked:
-decel valve
-continuity between pin 2 and 18
-confirm vacuum at TB

Posted by: Van B Oct 31 2021, 05:06 PM

Copy. I’m outta time for this week. I’ll be away from home all week so, better luck next weekend I guess.

Posted by: Van B Oct 31 2021, 05:08 PM

How so? Throttle works fine. Full range, smooth, and it’s even shiny.

Posted by: emerygt350 Oct 31 2021, 05:39 PM

You will get it figured out.

Posted by: Van B Oct 31 2021, 05:44 PM

Oh I’ll still be leaning on you guys pretty hard.

Posted by: emerygt350 Oct 31 2021, 06:00 PM

I have only been working on mine for 6 months but this forum is fantastic. I have 35 years of other car experience, so that helps (just be happy you are not working on a Ford CFI HO 302), but the people on this forum are really great and super helpful.

Posted by: timothy_nd28 Oct 31 2021, 07:29 PM

I like the progress and sounds like you are headed in the right direction. Consider ditching the decel valve all together, I threw mine out many years ago. The decel valve feathers the RPM back down slowly, as if you were driving some Toyota Camry. Without the decel valve, gives a more spirited type of feel, like driving a Porsche.

I'm not sure if I'm the only one that had this issue, but these reproduction intake boots were not the exact copy of the OEM. The fittings that plugged into the boot were very loose and very lossy with vacuum leaks. I ended up machining some new fittings that fit the reproduction boot much much much tighter. Since I'm not using a decel valve and I don't have a EGR, I made a plug for one of the openings. It was the first time I ever used a metal lathe, and happy that I didn't injured using the machine


It may be worth spraying some starting fluid on the intake boot fittings while the engine is running to see if anything changes, or do a smoke test again. I don't think this is your current problem, but is worth double checking.

Posted by: Van B Oct 31 2021, 07:42 PM

Hey Tim,
Yeah I saw the thread where you posted about your fancy fittings. I like em! I had the OE large fitting and just put some gasket maker on it before pressing it in the new boot. You are right though, the reproduction pieces are a little too short in length and the ports are a little too loose.

For my car, 1974 1.8L, the upper port is smaller. So, I was able to find a plastic fitting that seals up. The one that was on the car was for a 1975 1.8. It was so old and beat up… sealed with brass threaded fittings and plumbers putty! I mean probably two previous owners drove this car in a poor state of tune for decades!!!

I went the other way and found a new old stock decel valve on eBay. I’m pretty pumped about that actually. It should be here when I get back home later this week. My aim for this car is to make it feel like it did when it was brand new in 1974. I have another Porsche that is heavily modified for when I want to go fast.

Random thought for everyone still helping me solve this riddle:
Even though the throttle body works smooth, could the TPS be causing a problem? Or, would a bad TPS manifest with other symptoms like hesitation and stumbling?

Posted by: timothy_nd28 Oct 31 2021, 08:42 PM

The TPS (not sure if it is truly a TPS) has two microswitches. It tells the ECU that you are at idle via pins 2 and 18. The TPS can also tell the ECU when you are at wide open throttle at pins 3 and 18 thru the second micro switch. The ECU receives no other info when the throttle is in between idle and wot.
When at idle, one of the TPS switches will electrically close and command the ECU to drop in RPM. This part almost never fails, but these cars are getting up in age, anything is possible, especially intermittent problems.

A bad TPS will cause a high idle, but I don't think it will effect much of anything else. It's worth testing to rule this variable out.

Posted by: emerygt350 Oct 31 2021, 08:44 PM

TPS isn't going to do the throttle hang. It's worth thinking about, but not for this issue. Try unplugging it...

Posted by: emerygt350 Oct 31 2021, 08:49 PM

Again though... You have all kinds of non intrusive troubleshooting before you start mucking with the TPS...

Posted by: wonkipop Nov 1 2021, 03:34 AM

@http://www.914world.com/bbs2/index.php?showuser=8571 B - L jets forever.

i wish i had another 74 1.8 sitting around to hot rod like timothy nd28.
sounds like the gear.

you will crack your problem.
stick to the methodical attack of one thing at a time.

Posted by: Geezer914 Nov 1 2021, 05:54 AM

I have a 75 1.8L from CA. I live in NJ. The first thing I did is get rid of all the pollution crap. Not running the decell valve. Only vacuum line is to the distributor. Do another smoke test. The crack in the boot took the pressure out of the system. You can make a smoke tester for under $20, look on you tube. L jets hate vacuum leaks. Could be injector seals?

Posted by: StarBear Nov 1 2021, 06:57 AM

When it happens on my 1.8, a drop of light household oil on the felt pad below the dizzy rotor usually works. Otherwise, a vacuum leak somewhere.

Posted by: emerygt350 Nov 1 2021, 10:37 AM

I know the decel helps the longevity of the MPS on djet. If it isn't protecting something on an ljet, I would get rid of it too.

Posted by: Van B Nov 6 2021, 02:49 PM

Riddle me this Batmen:
Why would clamping just that line stop the high idle??
Keep in mind #7 and #9 should be reversed on the diagram.

I replaced the decel valve and nothing changed. But when I clamp that line, idle will drop to 1200rpm immediately after releasing the throttle and then slowly down to idle.




Attached image(s)

Posted by: emerygt350 Nov 6 2021, 04:40 PM

But what is clamped? Where is it coming from and where is it going on your motor?

Posted by: wonkipop Nov 6 2021, 04:48 PM

ok mate.

i think clamping that line is telling you its something else that is responsible for hanging up the idle or its a combination of things and you still have to find it.

i've looked at your photo, the decel is plumbed in correct.

clamping test you have done in photo reliably takes decel out of the circuit.
if the decel valve was the problem it would drop to proper low idle straight away.
but it wants to drop fast with the decel valve out as you would expect.
something else stops it a 1200 - not the decel valve.

my conclusion would be the decel valve is good.

your original decel valve was likely fine too.
having it in the circuit exaggerates or masks what is really causing the problem.

have you done the other things suggested by other members.

for instance lubricating the distributor.

keep doing one thing at a time.

Posted by: rjames Nov 6 2021, 05:00 PM

My guess is the throttle body shaft where the butterfly pivot rod goes through is worn and it's letting in extra air. Until I fitted mine with a bushing I had a similar issue. Blipping the throttle would some times bring it down, but not always.

Posted by: wonkipop Nov 6 2021, 05:02 PM

he has got it right mate for the L Jet.

the decel valve opens up an additional air feed line from the induction boot through the valve to the inlet plenum.

Van has clamped off the air feed line to plenum.
has taken the decel air feed line out of the circuit.
its just another one of the air feed lines that by-passes the throttle valve when you snap the throttle shut - the idle tube in the throttle body being the other one.
what he has done is the same as disconnecting a decel valve and plugging all the lines.

it means the engine will/should drop straight to idle without delay (at least it will if it was the decel valve causing the problem).

Posted by: Van B Nov 6 2021, 05:13 PM

As is most often the case, the answers stare you in the face, but never speak up!

Bottom line: the decel valve is adjustable.

I thought the threaded end for the small diaphragm side was just a design element for assembly but in reality it is your adjustment for valve pressure.

I took my old one and used my mighty vac to put a vacuum on it and saw that it began opening at 18inhg and was full open by 20inhg.

Then I tested the new one, which I could see was threaded out more and it began opening by 15 and was full open by 18.

That was the problem all along. It was partially open even at idle.

I threaded it in until I got it to start opening at 20inhg and that did the trick. Idle is steady a tick under 1k rpm and the valve still catches when I chop throttle from speed.

Now I can start figuring out why don’t get a high idle on cold start like I’m supposed to.
I’ll beg for help on that matter on a new thread soon lol!

Thanks guys! All the diagrams were especially helpful!

-Van

Posted by: wonkipop Nov 6 2021, 05:18 PM

@http://www.914world.com/bbs2/index.php?showuser=8571 B

you can do the same simple clamp test with your AAV now you got the clamp thing going.

when engine is warmed clamp line that branches at the T where one line goes off to the decel valve (up near the intake boot from the air cleaner). make sure your AAV is not leaking air if its not closing properly.

easy to do and tick off.

then move on to looking closely at the throttle body itself as others suggest.

Posted by: wonkipop Nov 6 2021, 05:28 PM

ripper mate.



i have a decel valve lying around here myself that i pulled off an old K jet from a 911.
its the right looking unit but has a different mounting clamp than the 914 unit.
i have not got around to installing it as i kind of like the way mine runs and its been that way for over 30 years. the original owner did an emissions cheat on it long long ago.
basically subbed in a fuel pressure valve as a decel valve. what that does is take the decel circuit out but looks very similar. suspect it was an emissions visual inspection cheat.

that decel valve i have has got the same adjusting nut on it, so your tip might be handy for me.
basically just keep turning that nut until it drops fully to idle by the sounds of it?

i think the later decel valves that went on either in 75 or part way through 74 that look like a bigger flat pancake are non adjustable and throw away. i'd forgotten about that nut on the earlier decel valves.

Posted by: Van B Nov 6 2021, 05:45 PM

The big nut is a lock nut. There’s parallel flats on the small tube that fit a 6mm wrench. The lock nut is a 13mm.
I took the intake tube hose and blew in it while I pulled vacuum and listened for the air to start blowing through the tube I clamped in the photo. So, you’re basically playing a bagpipe lol!

Just be slow on your vacuum bleeder so you don’t jump past the number when the valve starts to open. (Small trigger pulls)

Getting it adjusted from 15 to 20 took several turns.
I have no idea what Porsche specifies for that valve, but I do know 20inhg works nice.

Posted by: wonkipop Nov 6 2021, 05:58 PM

yes i was trying to find those values before i had a go at installing it since it came off a 911 and i figured the values it was set for might be different.

i did come across a discussion on an old samba thread a while back when i was looking into it. there seemed to be a view held by the vw guys that the decel valve served purposes related to more than just emissions clean up on deacceleration. some of it was to do with matching engine revs for cars fitted with auto gearboxes, for instace VW 412 sedans. even stuff about decel valves for operating at different altitudes.

what i could figure out was even though there are lots of different part numbers for various decel valves they all look to be basically the same unit, just variations on mounting brackets - but maybe also pre adjusted from factory for different vacuum settings maybe too?

here is the one i have that i stripped off a 911. nice thing was i picked it up for free.
its got the bracket in the wrong orientation, but i am sure i can rig something to make up for that. i don't know if it works properly - i was going to plug and play this christmas when i have time. will be getting the car up on the hoist to install the original fuel pump i managed to rebuild and was planning to play with the decel valve then.

Posted by: Van B Nov 6 2021, 06:19 PM

That should fit fine; it’s only clocked 90 off from mine.
I was lucky to find a NOS Bosch part. Which confirmed that they are tuned differently. Per application. I’m betting the original from my car used to open around 20 in-hg but weakened over the years.

I might clean up the original, set it and put it back on. The NOS Bosch part is very very rare so, could be smart to keep it safe until the original completely gives up.

Posted by: emerygt350 Nov 6 2021, 07:57 PM

Awesome. Is it necessary on an ljet?

Posted by: wonkipop Nov 6 2021, 08:22 PM

i believe not.
though there are different opinions on that.


i understand they are necessary/desirable on D Jet. but i could not tell you why.

EDIT
was thinking a bit more about the test @http://www.914world.com/bbs2/index.php?showuser=8571 B did.
even though the decel valve hose was clamped off it was probably still able to draw a bit of air given its hard to close off a hose completely when you clamp it (air can still pass through where the hose curves at either end of the clamp).
was probably enough to do the quick drop to 1200 rpm and then go down slowly from there as the valve wasn't closing quick enough. when i was testing my various valves etc a couple of years ago i was taking the hoses off and plugging them with a bolt. when Van had the hose clamped it was taking the improperly adjusted decel valve approx 75% out of the equation? why i was thinking there was something funny about it stopping at 1200 rpm in revs drop.

good thread, i got some info before i screw around and play with re-installing a decel valve.

Posted by: StarBear Nov 7 2021, 07:23 AM

@http://www.914world.com/bbs2/index.php?showuser=24231 and @Big Len : How do your early/mid 74 L-Jets compare to the diagram?
1. Mine is mounted reverse as shown with the little end facing the rear, and
2. Is plumbed as in the diagram (which I kinda think I made)? On mine the diameters of the hoses wouldn’t fit if the lines 7 & 9 were to be reversed as the commenter above noted.
Thanks!

Posted by: ClayPerrine Nov 7 2021, 07:24 AM

Engineers don't put things on cars that are not needed. Adding stuff adds to the price of the car. So yes, the decel valve is needed. It is designed to slow down the return to idle if the throttle is snapped closed. It will keep the RPM drop from going too low and causing the engine to die.

Modern fuel injection doesn't use one because they have the idle air control motor held open on sudden throttle closures to do the same thing.

The info on the vacuum settings for the decel valve is good info. Thanks for figuring it out. I will be using it in the future.

Clay

Posted by: StarBear Nov 7 2021, 07:35 AM

Clay- I see further above the setting is Start at 20 in but don’t see a Full setting number. Did I miss it? Thanks.

Posted by: Van B Nov 7 2021, 08:59 AM

Star, there is no adjustment other than the small vacuum tube. My observations were that initial opening and full open occurs across a ~2inhg spread.
Keep in mind, my flow meter was the physical feeling in my lungs while I watched the vacuum gauge. Ultimately I decided that there must be a spring in there and all I’m doing when I thread in the adjuster is adding preload. The range from partial to full open got shorter as I added “preload” which is exactly the behavior of a spring. But it didn’t change much.
So, I elected to focus on initial opening values. And 20 is the number. 21 is right out, nor shall the count be 19 unless proceeding directly to 20.

-Van

Posted by: Van B Nov 7 2021, 09:29 AM

Wonkipop, that observation was what unstuck my mind. After I made the post (sometimes just trying to explain it can help you understand it) I realized that the only explanation was that the valve is open at Idle. And something else it made me realize is that the new valve didn’t respond to a throttle blip like the old one did. On the old valve, a light blip would often bring the rpm down.

So, taking the old one and putting the mighty vac on it I started pulling vacuum while blowing into the side tube. And like I said, flow started at 18. Thus, I formed the hypothesis the new valve would be open at less vacuum. Tested, confirmed, 15inhg.

Putting the data together in my mind, I decided 20 was my target value. 15 was way too low. 18 still wasn’t working, but was closer to the edge. 19 seemed like too fine a margin.


Emery, Clay,
What I’m starting to understand about this old school fuel injection design is that it makes all its decisions on air pressure. So, the T in the small vacuum line informs both the fuel pressure regulator and the decel valve; but does the opposite action.
If I do a full throttle pull to 5000rpm and chop throttle or power shift, the rapid change in manifold pressure will simultaneously cut fuel and add air. That makes sense to me since the old school AFM has a barn door that must “swing” closed. During that swing fueling doesn’t immediately stop because the computer only understands to fuel based on how open the door is.

These two little devices add air to burn that uninvited fuel and shut the regulator to prevent a runaway fuel situation. Simple but still clever.

-Van

Posted by: StarBear Nov 7 2021, 09:33 AM

Thanks! Will check mine out with a Mity-Vac (from a FLAPS borrow, if possible) as mine idle's been a bit wonky (pun intended) for a while.

Posted by: StarBear Nov 7 2021, 03:31 PM

@http://www.914world.com/bbs2/index.php?showuser=8571 B; I like where you’ve gone with this and will try to borrow a hand vac unit. Two questions:
1. Is it tested while in place ( I presume so)?
2. To Which port does the vac unit get connected - the small end, the large end or the side?
Thanks ever so much.

Posted by: wonkipop Nov 7 2021, 03:48 PM

yes mine does occasionally do that mr p. it will drop down under idle and feel like its going to die then almost instantly comes back up to idle. but only occasionally!

i think it is meant to be mostly an emissions thing. i do know that all the 911 guys back in the day disconnected them on the K jet cars here. they were used to carbs and the response off throttle.

this thread has been informative with the vac values, i'm guessing frustrations with decel valves back in the day might have been down to adjustment? instead folks just disconnected them.

Posted by: Van B Nov 7 2021, 03:49 PM

I tested the old one when it was off and the new one on the car. You draw vacuum on the small threaded end. That’s the diaphragm side.
Blow into the side port. Air comes out the larger port on the rear. In actuality, when the valve opens, it can flow either way through the large ports. But the way I described is how it functions on the car. Procedurally, you’re blowing the whole time you start pumping the mighty vac. But you’ll get nothing until you hit the point where the valve starts to open. Then you can easily hear and feel air flowing. Creep up on it with the mighty vac and as you steadily blow, you’ll be able to see exactly when it starts to open and then get full opening.
When it’s on the car I disconnected the hose that runs to the intake boot at the boot Y and used that as the blow tube lol…
The only other trick is rigging up the mighty vac to fit. I used a clear tube from the mighty vac and a straight barb to bridge to the smaller vacuum line for the decel valve.

Posted by: StarBear Nov 7 2021, 04:01 PM

Awesome. Thanks ever so much. Now to find a vac unit to borrow….

Posted by: wonkipop Nov 7 2021, 04:31 PM

yep - how it works. manifold pressure affects fuel pressure with L jet.
part of the fuel pressure regulator test for L jet is to test with the small vac hose off at idle. should be approx 35ilbs fp.
then put the vac line back on, fuel pressure should drop to 28ilbs at idle.
rises again to full pressure with the throttle open and engine powering.

Posted by: 914werke May 28 2022, 05:55 PM

Resurrection.
It seems as though much of this troubleshooting (& solution) was based on the use of a earlier adjustable Decel valve.
The later motors (depicted in the vac line diagram) use an larger non adjustable valve.
Aside from Clay's warning what are the detrimental effect of removing it from system?
One of the things I preferred about D-Jet was the snappier "feel" that system provided compared to the smoother more progressive response that you get from L-jet?

Posted by: 914werke May 28 2022, 06:00 PM

@http://www.914world.com/bbs2/index.php?showuser=8154
Im curious of the detail on how you plumbed the Gowesty unit. It comes with only one vac. port? I already created a TPS mounting plate

Posted by: Van B May 28 2022, 07:29 PM

75 uses the big can. The 74’s use the standard small one.
I think my car has a pretty snappy feel now.

The setting I’m using on the decel valve works pretty well for my taste. It makes the decel valve more subtle in its function. It will still catch the revs, but they drop more normal (quicker).

Posted by: porschetub May 28 2022, 10:52 PM

Don't remove parts !!! ,they are part of an engineered system which was developed by Bosch for the type of injection used,Porsche never add stuff that is surplus and the supplied fuel management is what you need.
IMO the L-jet is a great system,pretty simple and stands upgrades like bigger B+P's ,bigger throttle body,good tuning exhaust system,the cam used has to remain fairly mild as the afm will give issues.
My next build is a 96mm bore FI block will be running this system as a customer wants an uprated motor for his bus.
My 2 go to guys for info on L-jet are mark henry and clay cause they exceed more experience on T4 application than I have learnt many years back on this system.
Cheers.