So, after a month in the shop at TRE motorsports who did some awesome work, I pick up the car and on my way to the DMV the next morning to get this thing reg I have an idle that goes from about 200 to 1800 then 200 to 1800 and so on.

How did this come out of the blue after 24 hours prior it was running perfectly!?

All input is greatly appreciated.

By the way, I called woodland Hills Porsche since I live 2 min from there and they want $280.00 an hour.. That is a bit extreme.

thx
Blair

Check all your vacuum hose connections first thing.

Check your vacuum hoses. Sounds like one may have come disconnected.

What engine and are your running fuel injection?

…or a cracked MPS diaphragm. Probably not the issue, but anything is possible and a big idle swing out of the blue is a symptom of a bad or misadjusted MPS.

More info about your car will help.
And yes, farming out the work on these cars is expensive. Best to do all the repairs yourself if possible. Everything you’d need to know is likely on this site somewhere.

Good luck!

It's all about heading to the DMV


I had the 82 turbo 740 Volvo, the only time it would misbehave was when I was heading to get the emissions tested.

Well, ok, it was because all the vacuum hoses were rotten, but the only time it mattered was when I was heading to the DMV.

What year? What motor?

a reasonably extreme case of hunting!!!!!

what is it. D jet or L jet.

initial advice as above.
snoop around at hoses.
check that throttle body is sitting even/tight and has not loosened screws.
if L jet,
check the air cleaner to t/b boot is on tight too.
check oil cap on tight.

sort of sounds like since it just come back from the shop that something was only on finger tight? - and has worked loose or popped off.

Silly question I bet.... Are the vac hoes all visible from the top side? I looked in there with a flashlight and did not find a loose connection.

thx alot
!!!

sorry 1971 1.7

thx

Everything you see from the top side needs to be checked. It’s either carrying air or fuel. Every hose should be coming from somewhere and going somewhere. Nothing should dead end or lie disconnected.

True something could be broken, but since you said you just picked it up from a shop, I think an unnoticed loose connection is more likely.

i am not a djet owner.

but had a look in factory manual about d jet troubleshooting.



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as Van B says and i hinted earlier, its just come back from the shop.
a bit of a classic moment would be to not have tightened up the big air boot to the throttle body or the throttle body itself? give that a very close look.

as Van says, even if you have no idea what the hoses are for or where they are going just track each one and makes sure it is still going from somewhere to somewhere.

hopefully its that simple.

If you have a stock setup, and don’t find any air leaks, you need to adjust the air bypass screw.
You will need a long thin slotted screwdriver, next to the air cleaner elbow on the throttle body towards the rear of the car, the screw needs to be turned clockwise until the idle evens out.
Make sure engine is warmed up so the AAR would be closed.

If Djet, extra air getting in from a disconnected hose, open AAR, or throttle body bypass screw too far out I think would result in a steady high idle, not a surge from 200 To 1800 rpm.
Surging as described will happen if the MPS is not adjusted correctly or has a bad diaphragm, or if the mixture knob on the ECU is too far counter clockwise (lean).

Check the stacked elbow as well. They crack. Then more than likely the hunting begins.

Update:

So I find a local German shop by my house open on Sat. There is a young man who had not worked on a 914 previously who says, I'll take a look. He is looking around at all the hoses and all od a sudden thr idle drops back to about 500 and does not move. Wow, I ask , what did u do... he shows me a hose he pulled off and had sis finger over the opening. Brilliant, so now I ask what the hell does that mean? He plugged it and I drove home. Please tell me what this hose does and jow do I proceed?
Thx for all the input!!!!

Update:

So I find a local German shop by my house open on Sat. There is a young man who had not worked on a 914 previously who says, I'll take a look. He is looking around at all the hoses and all od a sudden thr idle drops back to about 500 and does not move. Wow, I ask , what did u do... he shows me a hose he pulled off and had sis finger over the opening. Brilliant, so now I ask what the hell does that mean? He plugged it and I drove home. Please tell me what this hose does and jow do I proceed?
Thx for all the input!!!!

Update:

So I find a local German shop by my house open on Sat. There is a young man who had not worked on a 914 previously who says, I'll take a look. He is looking around at all the hoses and all od a sudden thr idle drops back to about 500 and does not move. Wow, I ask , what did u do... he shows me a hose he pulled off and had sis finger over the opening. Brilliant, so now I ask what the hell does that mean? He plugged it and I drove home. Please tell me what this hose does and jow do I proceed?
Thx for all the input!!!!

I1 more picClick to view attachment

1.7 CARB

thx alot

That is fuel injection and that is your AAR. Aux air regulator. Plugging it will only make your cold starts hard but not affect anything else. Fixing it, if that is all, is kind of easy and kind of difficult.

Chupee, you would be well served to at least learn the basic configuration of your car.

don't feel self-concious about it but.
you may as well try and get your head around the basics.
you need a shop manual.

attached is diagram of the hose layout.
i'll leave it to the D jet guys to explain.
the item in question is arrowed.
(i can't understand properly the effect of a leaky aux. air valve on D jet - i can on L jet but its a different world).

forced to stab a guess at it - and given sudden emergence of problem.
electrical connection on AAV has come loose? it has one? over to D jet guys.


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100000% correct. Trying...... thx much. So now that we k ow the issue, did it just go bad or what needs to be repaired ?

Thx

The problem here is that just a leaking AAR isn't going to cause that issue, what should happen is that if your AAR goes you end up with a high idle period.

What I would do is go ahead and leave that AAR plugged for the moment. Trace every vacuum hose on that engine and make absolutely sure they are going where they should, using that diagram above. If you have any questions, snap pics and post them here. Pay attention to the condition of the lines and how well they fit (snug, loose, etc). You can buy a whole new set for cheap when the time comes.

After you do that, go ahead and warm the car up with the plugged AAR, it might require you to massage the throttle a bit. Drive her till she is good and fully warm. Then let us know what your idle is at. With your AAR out of the circuit (and plugged good) you should have a nice 900ish rpm idle.

dunno about that emery!

dr. d jet describes how it can happen? sort of?
something to do with the ECU sensing rev rise and then going into fuel cut off.
repeat cycle. but d jet is just something i do not understand.

https://jetronic.org/en/d-jetronic/aav

@Chupee .
if you want to try and start getting your head into how the old d jet works.
doctor d jet (a german) has this fab website.
i got into his website when i was rebuilding an original fuel pump.
not much help for L jet, but its great history if nothing else.
but he does go step by step through each component in the d jet system and what it does.
the doctor is coming at it from the mercedes benz viewpoint - but the system is the same in the VW by and large.

https://jetronic.org/en/D-jetronic

here is a direct link to youtube video that dr. d jet has on his website.
this is attributed to a stuck AAV on a d jet mercedes benz engine.

@Chupee can watch it to confirm, but sure sounds like what he came on with from his original post.

https://www.youtube.com/watch?v=EVWxqn8c1WQ&t=12s

I tried to read this. I failed because of overload of info. It has to be as simple as a vacuum leak. It is air getting where it wasn't supposed to be FROM.

yes.

air is getting in from where its not supposed to be coming from.
(thats what the section on AAV in dr djet site is saying.
go straight to section 9.3.1 of link to AAV on dr d jet.)

via the auxiliary air valve. which has stayed open instead of gradually closing.
there is two ways those things can fail.
fail shut = hard cold start.
fail open = really screw with the amount of air the engine is supposed to be getting via the measurement of air into engine by ECU.
i know how L jet works. the AFM measures the amount of air coming in.
how D jet measures air coming in is a bit of mystery to me.

the mechanic he took it to proved that the extra air was the problem by pulling the hose from the aav and plugging it.

as emery says if @Chupee takes the aav permanently out of the equation by plugging it he solves it. sort of.
but its going to be harder starting it cold.
which is ok.

i'd have to read that d jet article a few more times to half grasp it myself.
you are not alone.
but since i have L jet i can't actually be bothered fully understanding it.
.......someone with experience of d jet problem solving would know what is being said.
and could confirm it or deny it.
i kind of half get what is being said.
i am thinking it hinges on whether 914s have fuel cut off on deacceleration like mercedes benz. that is the scenario being described.
so really i was just replying to emerygt350.
he thought a screwed aav would just be elevated idle. he may be correct.
but it seems a screwed aav on d jet can also cause the hunting idle thing.
i dunno.

it does fit the scenario being described.
a wildly sawing (german for hunting?) engine at idle.

anyway problem solved already by plugging the hose.
but if cold start wanted back - thats another matter.
sounds like he would have to look into the aav valve?

ps
@Chupee says he has a 71 1.7
i seem to remember reading something about the early d jet 914s having fuel cut off.
maybe chupee has that - just like mercs being talked about by dr. dread jet.
learned d jet guys would know this better.

There isn't any fuel cut on our early djet, she will just keep revving. The car just behaves like you cracked the throttle open. You would think they would have put some kind of check on that with the tps but apparently not. Now, anders says the computer ignores the mps if the tps is in the idle position, so it could be a lean run up but you don't need .much to get to 3k with no load.

I would also add that AAR stuck open should really only get you to1800kish rpm max (although I haven't tried it on a hot engine before), that's why I want him to do some more digging....

It doesn't. It measures manifold pressure as an indicator of engine load (hence the manifold pressure sensor).

It could be the case that two things are wrong (e.g., vacuum line, mixture).

Owners inexperienced with D-Jet run into big problems when multiple things go wrong, or when something was changed to compensate for something else going wrong. It happens with 50 year old systems that haven't been maintained properly.

Real solution is to fix the obvious problem (the disconnected hose) but also start testing/inspecting other components systematically to check for function.

emery - yours is a 73 2.0 L isn't it. thats what i would call later d jet.

i'll see if i can find it. but i think the early d jets have fuel cut off on decel.
by early i mean the first year or perhaps first two years.
i've read about it somewhere.

if you read the link, and the short particular section on the aav,
its discussing how that fuel cut off sparks the surging.
its particular to that form of d jet with the fuel cut off.
the short version of it is the ECU gets fooled into thinking you have just jumped off the gas at higher revs and goes into decel mode and shuts off the gas, because the throttle switch is in the idle position - but the revs have been high, thats how it resolves the contradiction - it thinks its being "driven". (thats explaining the second half of the surge - ie how it dies down). once its back at idle it then repeats the whole cycle because the fuel comes back on. i found it interesting to read about. and i am glad i don't have a d jet as a result.

not arguing against the systemic full check views of others.

but given the hose is to the aav.
and disconnecting and plugging the hose taking the aav out of circulation stabilised the idle.
i'd start there.

pull the aav and test it.
confirm it is working properly.

i should have said "measured".
or a better word would be it guesses? - in the case of you guys

the hose was not disconnected. that seems to be his problem?
when it was connected he got the idle nightmare.
what the local mechanic did was disconnect the aav by plugging the hose.
that provided some level of "fix". idle surge went away.

emery is musing over why the errant aav just didn't produce a high idle.
which is what you would also expect with L jet. the idle will just rise.

but......maybe with the fuel cut off version you get the cut off thing.
and it repeats. and repeats....

or it could be the more serious thing you guys suggest.

i will see if i can find the reference to the fuel cut off version of d jet in 914s.
or someone who knows for sure might come on here with the info and set that straight.

Mine has a decel valve as well but there is no fuel cut early or late, just more air added on decel. By early I was referring to 'on a 914' not some later car.

He just needs to look through all his hoses systematically,not worth doing anything else till that job is done.

WOw! Now that I know my issue I see there are quite a few threads on this exact issue. thx a lot for the diagnosis!!!!

@emerygt350

found it re fuel cut off.
think date is all engines before EA engine gets introduced in USA for 1.7?

the hose diagram i had posted previously on this thread is for pre aug 71 1.7.
shows the decel valve. has a decel as well as fuel cut off.
there is a different hose diagram for post aug 71 in factory manual.

Chupee says he has a 71 model.

so his is different to your d jet?


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the surging idle thing can be triggered by the revs rising to level that triggers fuel cut off if throttle switch is remaining in idle position. which it is in this case.
resulting in cycle of up and down revving. faulty aav can cause the rev rise.

no idea what revs are that trigger fuel cut off. could not find that.

Interesting, I had never heard of that before. I can only assume it was gotten rid of for simplicity and or emissions. So that could simplify things. If that is cutting fuel that would do it. I would still check all of those hoses. Fixing the aar can be easy and if too far gone, very difficult.

@emerygt350 .

only reference to rev range for the surging idle is in troubleshooting section of manual.
says considerable revs surging from 1000 to 2000.

this is listed as a separate and particular symptom to uneven idling.

attributes 3 causes.
leaking aav. (leaking hose from aav to air distributor - intake plenum).
leaking hose seals on intake manifold pipes to intake plenum.
leak at throttle valve housing.

chupee's intake hose seals look pretty good condition in his photo.

the dr. dread jet site expands on the aav problem to say that as the aav ages, it produces same result as leaking hose from it to plenum.

i guess if the electrical connection to the aav is not making good contact its also going to do the same thing and leave the aav open as another possibility?

Yeah, seems like the wrong way, you would prefer a failed component to not cause a massive air leak. Having it require that toaster coil to keep it closed doesn't seem the best design to me.

yep.

i had a nice drive this arvo.
at least my car was not doing impersonations of a race start at every traffic light.


was anzac day down here - i solemnly observed all road laws in rememberance of vets. all the old blokes were out and about with their medals. these days the old blokes are the nam vets. they were all down at the pub at the end of my lane hooking into it.
they gave me a cheer as i drove the 914 past them returning to the lane.
took em back to the 60s/70s?

i am raising my beer to them all tonight.

No vet in any nation wants you to drive your 914 slowly.

Just felt like that should be plainly stated for future reference lol....