Ive delt with a bitchen miss thats driven me crazy for way too long. (about a year after a dropped valve repair) Ive read many posts and followed many suggestions and found many small fixes (some of I woudl have bet fixed it). Ive rebuilt the FI harness, soldered the relay board, ohmed every part and Tstat sensor. Cleaned and tested TPS and Triger Pts. Intake vac at 20" at idle. All new Ign OEM wires plugs etc.. No points (eletronic) all in distributer. Idles good reves good but crusing speed bucks. (the TPS tracks I bent arms slightly to start a new track then ohmed all). And the intake temp sensor is unplugged (runs way bad with it) MPS checks great both Vac and ohms. Sealed settings never touched.

Symptoms now idles good reves good but hold the throdle steady almost anywhere off idle and it sounds like massive ign failure.

I just pulled the plugs to check them and they look snow white. I turned the fuel pressuer up did not seem to help. I experimented on the MPS and made up a test I unplugged the hose from the MPS and stuck a vac gauge on it reading the intake, then used my new hand vacuum on the MPS.

Here is what I learned. (following all related to MPS controled by the hand Vacuum) If holding vacuum 15" w/n start right and starts fine at 0 vacuum as in normal cranking conditions. At 0 on the MPS barely idles, At 15" on MPS idles great. But at 20" on MPS idles bad (20" is my intake vacuum at Idle) Well then I went for crusing mid range rpm held the throdle missed like mad slowly pumped up the vacuum on the MPS from 0 and could tune out the miss totaly by adjusting vacuum to the MPS.

Helped eliminate my ign problem I thought I had. Running so lean and 90%RH in the air sounded like an ign breaking down. Problem alot less noticeable during dryer weather.

Seeing how I could tune out my miss using vacuume on the MPS I decided to open er up. My 72 has been stored longer than it was ever used. First thing I noticed was grease. Oil migrated and solidified in the control. I unsoldered the coil and cleaned it out. I removed the check valve like disk form the vacuum port and cleaned the grease from it. Mine had a smaller check plate and spring built into the disk and it was stuck! (first problem I found) I did not touch the sealed adjustments or the mystical sealed disk where all the adjustments rest as it vacuum test perfect. The next minor problem I found was the core stop arms had pad like things at the tips one missing and one falling off. I glued hard drive jumpers on the ends that touch the ever moving core. Reassembled and vac test perfect and tried it out exact same results no better and luckly no worse.

I am going to remove the glued on pads and try it on the car again, as they wrap all the way around the arms and add about 5 to 7 thousandths of an inch to the arms width. Im gona go with no pads and light touch on the shaft core with a touch of lube.

And as if to add to my confusion I tried an MPS in the car that has a terrible vacuum leak (wont hold for 10sec ) and is the wrong number all together for my system and my car runs great compaired to the OEM MPS. WHY??

Im leaving the bad MPS on for now. I would love to get this thing working right. Without spending alot on parts that dont cure my woes.

Should the MPS be sent off to a pro to be tested and set to specs on the bench or adjusted in the car with an exaust analyzer?

Pls HELP, advise me of any experinces (Good or Bad) any have you have had with the real brain to the Djet EFI system the Manifold Pressure Sensor the MPS.

Hey

Sound like your on the right track...
I would still suspect the MPS even if it checks out...

Idealy the solution is for a fellow teener to stop by and let you try to swap parts and compair before spending $$$ on a rebiult unit.

Your vacum reading is stable? no "thumping" ??

Later

Real stable reading and steady idle!

Send it to me as I have probably 25 good control sides of MPS's laying around. A good diaphram is the key to a working MPS besides having a properly calibrated unit.

The mid range is in my experience a bad mps.

I found oil in mine as well...it seems that it could be from either to much positive pressure in the intake, or the high vacuum.

I get a 2.0 liter one, stole the diaphram and put in in place of the 1.7 liter diaphram, all settings the same...I did not turn any screws etc...

So I now have a 1.7 liter body, plugs and scews etc...but a 2.0 liter diaphram.

It runs perfect now...

I chased this for a few years...

Rich

Yea Im about a year into it myself. What I dont get is why MPS that d/n match my setup via #'s and has a massive vac leak works soo much better than the OEM one that tests fine both vac and electrical?

And another thing that I dont get is how running lean (via plug burn) can make such an erratic miss in all cyl sounding like an major ign failure.

Ive went to votek auto mech in hi school in the days of occilascopes, worked in garages, rebuilt many motors from scratch, rebuilt alot of manual trans and even an automatic in a 1ton Dodge van with all used parts from 2 free bad spares then packed it to the hilt even on top of the roof to move back to Fla. On the way a wristpin went out with a hurricane following me and I fixed it behind a truck stop with a used rod and piston. Pulled the head used the same rings and did the whold repair for under $50. It got me to fla and ran a couple years after untill I sold it with a bad freeze plug between the trans and motor. I would have fixed that too but I did not need it anymore.

Any way I consider myself well inclined to fix about any motor or at least find the trouble. And I would have bet $$ I found and fixed the trouble at least 5 times. I still dont have alot of $$ to be a parts changer or I would put in a 911 motor and 4 speed trany. I love my little 1.7 liter 80hp if that FI motor and gas millage and great handleing its good enough for me. I just wish I could unlock the mysteyes secrets of tweeking the MPS on a budget. I have read this link at least 6 http://members.rennlist.com/pbanders/manif...sure_sensor.htm It has been the best help so far. I would like to hear of others experiance with the MPS.

To answer you question as to why the "doesn't hold a vacuum"mps works better is that its running way richer than the sealed oem MPS. Once the diaphram is cracked or torn and it doesn't hold a vacuum, the MPS is figuring that you have less engine vacuum.
Idle is about 15hg of vacuum on a engine in good condition
Partload is anywhere from 10 to 4hg of vacuum
Wide open throttle is 0hg

So the "bad" MPS is not able to register the idle vacuum (15hg) properly so it is thinking its only maybe 10hg which needs a richer mix.

The first thing I would do on your car is use a good vacuum gauge to see what you engines vacuum readings are. You should be pulling a steady needle at 15hg.
Next, check for vacuum leaks using a little squirt of starting fluid aimed at the likely suspects, throttle body, seals, hoses etc. Leaks will raise the idle rpms

my car did the same thing (D-jet 2.0)

it really sounded like a ign miss....but it never was. I think my harness was heading south, oh and my 1.6L Type III mps didn't help my situation either. plugs were WHITE white and everything ran real hot and it pinged like crazy.

i think my fuel pump went bad too.

good luck.

b

Install a set of new plugs and see what color they are after a steady drive for 5 minutes

white-lean
tan-good
black- rich
oily- leaky rings/valve guides

Thanks Bleyseng for the reply. The oil in my OEM MPS was solidifying in the MPS internal check valve as well. In my first statment I where I finally realized it was my MPS and not the ign system or relays or relay board or ign switch or FI harness or TPS or Trigger points or fule pressure and all the leads I have followed for a year was by simply putting a new vacuum gauge on the intake MPS port and an hand vac on the MPS. At idle intake port at a steady 20" (high?) and pump the OEM MPS to 15" and it idled smooth but pump the MPS to 20" as the intake port gauge reads and the idle gets rough. Then when I hold the throdle to a higher rev it for midrange miss Im having I can use the hand vac on the MPS to totaly eliminate the miss. Like setting an adjustable jet on a carb I could use the hand pump to go above and below the sweet spot where my miss was gone.

Ive done a valve adj twice since repairing a dropped valve and doing both heads and all new springs. I know a poor adjustment can lower vacuum pressure but I have never heard of having anything wrong with too much vacuum. I know before the repair it never had this good of intake vacuum and always ran smooth compaired to now. Running smooth the last couple of days with the wrong and blown MPS on it.

thats the oddest thing ive ever heard of....

its almost like something else is making the system so lean that a blown mps makes things right again....

hows your fuel pump....?

did you mess something up on the harness....the primary and secondary mps windings??

b

Its old and used to leak but I resealed it and tested it in the car pressuer and regulator fine. When I first saw how lean looking the plugs were I raised the fuel pressure up to near 40psi motor running. It did not help the miss so I set it to 35psi. I then tested the pump by blocking the return line with the cap off pump relay I held the relay on without cranking it and it looked like it was heading for 100psi.

The oil in the MPS is from it being run in a worn out motor where the ring blow by is high. Lots of internal pressure will force more oily air into the plenum where the MPS will recieve some as its vacuum hose in located there. Oily air inside a MPS isn't good.

I did have all plugs off the harness to inspect the terminals (some had green on the wire to the terminal) but marked and tripple checked all the pin positions of every block. I also rewrapped the whold harness and made new boots for all. But I did all this because of the very same miss. And while I did find things that needed fixing it was not the cure.

So far the closest thing I have found to a cure is running a blown MPS and the testing I developed that proved to me it is an MPS trouble. (isolate the MPS on a running car while using two tools at once a gauge on the intake port and an hand vac and gauge to control the MPS)

Thanks for all the replies at first I thought no one wanted to touch this MPS thing.

I feel this MPS is the real brain running this thing. The ECU depends on two main imputs the mystical MPS and the head sensor. You could toss the intake sensor for all the good it does my lean machine. Both sensors read correct ohms cold and hot.

What symptom does a bad head sensor give ie shorted or open? Can they stick at a certain value inbetween?

usually a bad CHT means too rich waay too rich.

There has to be something else going on to make your engine run soo lean

The settings are on panders site for each specific CHT sensor.

It can get stuck in a wide variety of positions...not uncommon.

I would suggest that the wrong MPS that you have may be an early type4 VW mps...this version had no diaphram.

It was the first version of the MPS that was manually adjusted, but I believe it would never hold a vacuum...not sure, but without a diaphram it would have a hard time.

It was adjusted by a longer allen head screw...it had a wide range of adjustments...

So, have you taken the original mps apart and fixed it?
Does it hold a vacuum?
If so, and if you have the correct meter...you can set it yourself.

Get it setup for your car. the 1.7 runs rich..its ok.

I would suggest also that you have a vacuum leak somewhere else.
Dissconect the Aux Air valve and plug the end that goes to the throttle body...seal off the plenum, make sure you are not getting a different leak.

I have replaced all the hoses...each of the old hoses leaks a little bit, but it adds up.

I have a few friends that have clamped every hose to ensure no leaks.

Look at all the rubber boots at the plenum and the intake runner boots, they could all be leaking a bit.

To test, wrap electrical tape around them...do a good job and you will stop a leak...

You should not have 20 hg of vacuum at idle..
I would search there...
If you can make the car run fine while applying 15hg of vacuum to the mps, that is what you need to have at that hose, not 20....search there first...find the leaks...

you may feel like you already did, but you may have missed something...

Here is what I did.
I disconnected every line to the plenum except the MPS and the intake runners.
None are needed to idle, once you get the car warm.
Even take off the cold start injector...it could leak through the body...
The intake runners could leak at the rubber connection to the plenum and at the intake gasket at the head.

The FI injectors have two seals...replace them..they cause a vacuum leak.

Go back and re-adjust your valves, pay close attention to detail and for the purposes of this test and only this test, make them looser than normal....this way you know they are closing...
Dont over compensate, just set them at .008 or .006 (I cant remember the setting)...but go one or two larger to make sure you get them to close...

The valves will also create a vacuum leak. odd but true.
If you have a burned exhaust valve, you will be pulling air through that valve while the car is running, thus creating a larger vacuum signal.

Get it down to just the intake and engine...then test the vacuum....
You may have a bad valve, a leaky head/cylinder connection....certainly rings...

Do a leak down test if you have the gear...

I would be searching for the 20hg vacuum issue first...

Or, if you are ok mentally running the engine till it dies in the current condition (nothing wrong with that) then adjust the MPS to your specific engine vacuum signal...

I.E. replace the 15hg setting with 20hg..
It is the transition that needs to be massaged...the WOT is no longer relying on the MPS...the idle and transition are all MPS driven...

You can tune your MPS to be correct with your car...
Geoff could as well..



Rich

QUOTE (r_towle @ Dec 20 2005, 08:22 AM)

I would suggest that the wrong MPS that you have may be an early type4 VW mps...this version had no diaphram.

You are thinking of the type 3 one as I have a early type 4 one number #000 000 002 off my old early 70. The insides are very similar to all the other MPS's just some small differences.

I agree on all the other stuff...find the leaks or the reason for the 20hg vacuum at idle.

Im thinking of the one that has no diaphram...I got one on my hunt...

It works, but I think I read somewhere (possibly you wrote it) that this was the first production version of djet...in the vw...and that this particluar diaphram was discontinued pretty quickly due to the lack of tunability...

I have a question...

If you raise the compression ratio, stock cam retained...does the manifold pressure change?

Rich

These are the early MPS:

http://www.914world.com/bbs2/index.php?act=...f=2&t=42290&hl=

Hey Rich

Fuck this is a bitch...
Giving this some thought I realy don't beleave you have too much vacum. Bottom line is the cam duration desides maximum vacum, its probably the highest I've seen but thats a good thing. If you shaved the heads to taco size, increasing compression, it'll ping like a whore ...
I don't beleave you have a vacum leak because that would show up on the vacum guage, and we are sure your guage is accurate.
Me thinks "Fuck the MPS" You sound realy on the ball machanicly (sp). Why not open all the plugs and try to adjust you MPS on the fly ? One reason or not (I have my theories) its not working for YOUR car. Your a smart guy, I am betting you can tune it a hell of a lot better then its running now If not you still need to send it out or buy a rebiult unit....at this point your car doesn't sound very enjoyable to drive.
You know I never looked into a ECM for this car ...analog worries me for older cars like ours. IMOP analog signals can degrade for sooo many reasons related with age...even inside the ECM...but then again I am rambling

Wadduya say ?

Later



PS ... oh ya...have you thought about a temp repair by installing a small bleed off in the MPS hose. I used to play with wastegates like that.

Yes, you can tune the MPS by removing the epoxy and then carefully remove the WOT stop and inside are the two screws.
Start the motor and have a friend hold it at 2500rpms running.....
then slowly adjust the inner screw till it runs smoothest possible.
Now you need to install the WOT stop to a calibrated setting using a Wavetek or a wide band.......or the diaphram will crack as it must be supported.

this whole operation is best done where you can use a wideband to read the A/F mix.
13.7 to 1 at partload
12.5 to 1 at WOT

Bleysing is the wavetek or wideband an exhaust gas anilizer?

Ive been leary of peeling the seal and changing the factory setting of the MPS, I have read alot of neg posts about that. I was leary about opening it up and I did find a fix, solidified oil/grease inside and into my internal check valve had a retrun check disk stuck by the oil.

I think with an intake vacuum of 20" there are no leaks. All new hoses and new rubber plenum boots and injector seals. Just got done overhauling the heads, 1 new intake valve 1 new guide all new springs (one cracked and dropped the valve another cracked in two in my hands inspecting it) all valves and seats ground and lapped, all new seals every where inc front and rear mains all execpt oil cooler. The crankcase PCV system can cause vacuum leaks I paid attention even to that, right down to the seal in the cap of the dipstick its self. Oh and a new PVC valve and new Vacuum advance for the distributer (it was shot). Ive checked the valve clearance twice. When having driviability troubles sometimes starting at square A and work your way through a process of elemination gets the job done.

Im going to read the plugs again today after running great with a bad leaky MPS. I dont want to just give up and run the bad MPS as a crack or leak can only get worse changing the values my car seems to like now.

A Wavetek in an inductance meter per Brad Anders
A wideband exhaust gas meter like the ones used at a dyno shop or other auto mechanic shops


maybe you should install a leaky AAR valve....

My AAR valve is out of circut already. It idles low when cold and the equivalant of the choke injector is making it rich till it warms up. I live in Fla it warms up quick.