Porsche 944 (1985) - dies in idle under startup

[PorscheCollecto]

Hi fellow 944’ers,

I need a “little” help with a friends Porsche 944 from 1985 (late model), which I’m trying help fixing a weird start-up / idle running problem.
The car starts, but will only keep alive if throttle/accelerator is being pushed. And even with that, sometime it dies. And sometimes it can be brough to live before dying by pressing accelerator, but no matter what, if you let go of accelerator it dies for sure.
When pressing the accelerator and getting high rpms, engine sounds healthy. Problems appears below 2.000 rpms.
Important to mention, car has been starting and running fine, but suddenly this problem appeared.

These are the things we have tried until now (not listed in order of trying it):
- Verified all fuel pressures are ok.
- Disassembled and verified the AFM.
- Verified generator at the battery with volt meter.
- Did compression test – all good.
- Dissembled DME to verify if water had gotten into the box or other electrical problem. Did find any obvious culprits.
- Tried the simple test of the FPR of disconnecting one of the fuel injectors. Didn’t seem to be the culprit.
- Tested the CPS – not the problem either.
- Managed to simple test ISV, just by putting 12V on it and ground and listening it moves. (no visual inspection)
- No obvious vacuum leaks, but not gone it through meticulously.

I hope you can help if some good advice or if you tried the similar problem before.
Thanks in advance.

BR
Chris

[Jfrahm]

Run it for 10-30 seconds and let it die. Check the plugs. Are they black? Wet? Smell like gas?

Will starting fluid keep it alive?

[walfreyydo]

A couple things to check (in order of priority), suggested by Clarks Garage (https://www.clarks-garage.com/shop-manual/ts-01.htm)

-DME temp sensor (thats the blue top one) - controls cold/warm fueling
-Vacuum Leaks - a large leak will cause very rough running, especially at startup/idle
-Throttle Position Sensor (TPS) - tells the DME the throttle plate is closed/idle
-AFM (did you reposition the arms? Test voltage across the whole potentiometer range?)
-Clogged Fuel Filter and/or Tank Screen
-possible IACV/ICV issue (I think your ICV is variable. Remove, clean and bench test. It should move freely with no obstruction with any gunk or carbon)

Also, what was the actual fuel pressure reading in psi? This will help determine if your FPR is bad (high) or you have a blockage (low). Just testing for pressure isnt enough, need to know actual pressure

[PorscheCollecto]

Hi Jfrahm and walfreydo,

Thanks alot for your suggestions. I'll be spending my upcoming weekend trying out your good ideas.

The things I can answer already now, I'll do now:
1) Fuel pressures:
* 2.5 bar with engine not turn on and jumper instead of relay.
* 2.0 bar with engine running in idle.

2) Check the plugs:
* Yes, did check the plugs. And those are black (not really wet), just very black. But I'm assuming it's because we've tried so much with the engine now and turn it one several times. Also it's tricky to let it run for 30 seconds, because it so fast dies, but with high throttle it ought to be possible. So I'll try it in the weekend. I don't remeber them smelling alot of gas, but it might be because I didn't really think about put them under the nose.
Have tried with starting fluid, so will also try this in the weekend.

3) AFM
* Yes, I did re-position the arm so it was not in the same groove. So I'm pretty sure the AFM is working fine. 99% sure at least.

That's the things I could answer upon now....more investigation seems to be needed. I'll try to do it in the weekend.
More inputs are for sure very welcome. Thanks in advance.

Cheers
Chris

[djnolan]

check the large vacuum lines from the venturi to the ICV and AOS.

[walfreyydo]

Fuel pressure looks good, we can assume there are no fuel related issues with FPR, fuel filter/pump/tank screen. Ill assume the AFM is also good, but you could probably easily check the voltage to verify no other issues (see clarks AFM testing procedure). We cannot yet eliminate injectors though, but before going there, lets look at some other possibilties.

I would proceed looking for vacuum leaks (still a strong possibility, check your brake booster line and the line under intake manifold, near the brake master cylinder), test (or just change out) the DME temp sensor, they are cheap. Test the TPS (see clarks) and run 12V through the IACV making sure its clicking and working correctly (again see clarks).

Report back

Also this may seem obvious (and probably not your issue) but ensure the plug wire order is correct on the distributor (as well as checking the cap and rotor). Running on 3 cylinders can mimic the symptoms you describe. Putting a drop of water on each exhaust manifold pipe can also help determine if you are running on less than 4 cylinders (assuming the car runs long enough to heat up).

[Jfrahm]

Originally Posted by walfreyydo

Fuel pressure looks good, we can assume there are no fuel related issues with FPR, fuel filter/pump/tank screen.

I would not assume the FPR could not leak fuel into the vacuum system and adjust fuel pressure, it's worth checking for a leak to vacuum. But maybe it'd be more likely that the damper is leaking fuel to vacuum.

It sounds very rich, IMO no need to try starting fluid to see if that keeps it running. It's either leaking fuel into the engine (injector or FPR or damper) or the DME is adding the extra fuel. I am a little surprised disconnecting one injector did not help matters but maybe that's the leaking injector.

If the DME is adding the extra fuel it may be a bad temp sensor giving it bad info to work off of.

Backprobing the temp sensor inputs at the DME is a good test, expected values are in Clark's.

-Joel.

[Monkey Wrench]

if you clean or replace the plugs then you might do it again and see if the carbon was from earlier on or from the current conditions.

you can do a leakdown test to check if any injectors are leaking or if the fuel rail can maintain pressure, there might be a check valve involved near the pump.

I had that issue in my volvo and a symptom was that when I did start it I got white smoke for a block. and then it cleared the excess gas and ran pretty normal for the trip but it was causing all kinds of poor running and starting issues. basically it runs on 3 until it can dry enough to become not flooded. another symptom is one plug being blackened more than others.

Ive been watching some videos on checking injectors and am attempting to build a little tester so I can observe the spray pattern and shutoff

air leak might be found by a smoke test. Its worth looking into maybe.
i agree with al the other advice. its just a process,

I was thinking to check for a manifold leak, someone suggested putting oil near the manifold gasket , theory is it will then blow blue smoke if oil gets sucked in, Ive heard of using propane , carb cleaner and seeing if idle speed changes. wont hurt to check if any nuts are loosened off.

thinking the others are on the track that its running rich, a leaky manifold might cause a lean mix so based on that , leaky gasket may not be a very correct assumption.