Rock Chip

I am new, and apparently not that bright when it comes to sending myself links with important information and things I intend to use later. I’ve had a problem for a long time with my 1994 968 6-speed. It’s running great, but won’t idle. I’ve been throwing parts at it to no avail. There was a pretty exhaustive discussion/sticky on the subject. I found here, but I cannot locate it. Any assistance would be most appreciated. I have a very good local mechanic (although a generalist) lined up to help me this weekend & troubleshoot it runs great above idle, but absolutely refuses to behave once you let off the pedal and it dies. I replace the idle control valve, had the MAF meter recalibrated by that shop that does it in Boulder, Colorado, that the temp sensor, checked for leaks, smoked the system, etc. really frustrating and it just seems like it should be something simple I missing but no joy so far. Thank you for your patience and again... Much appreciated.

Jfrahm

What do the plugs look like? If you let it die are they wet with gas?
Will it idle if given shots of starting fluid or fuel into a vacuum line?

It's possible the ECU isn't trying to idle, that might be a problem with the TPS. Check that and check the injector signal with a noid light or good DMM that has Hz and duty cycle. Check for the spark going away when it's supposed to be idling.
Check for a strong spark with a spark checker.

Some other possibilities:
Issue with the vacuum line to the brake booster, cap it off as a test.
Broken vacuum fitting under the throttle body

-Joel.

Rock Chip

[QUOTE=Jfrahm;18874362]

What do the plugs look like? and If you let it die are they wet with gas?
- Like it’s running rich, a bit black but even and dry. Less than 300 miles on them. Pulled the spark plug leads individually, which made it run worse (missing) on each of the 4 cylinders

Will it idle if given shots of starting fluid or fuel into a vacuum line?
- Negative

It's possible the ECU isn't trying to idle,
- Just so we’re on the same page here, is the DME the same thing as the ECU? Under the passenger floorboard/firewall? … Assuming so, we traced the wires from the ICV to the wiring ECU/DME wiring harness and successfully load tested connection with noid light.

Might be a problem with the TPS. Check that
how exactly do I do that? I actually have 2, a new one and very good condition Bosch OEM unit that wasn’t defective after all. We put a power probe to them both directly, one while installed one held in hand. They both reacted identically to the power probe, a crisp snap open, or shut based on which ground was engaged. When hooked up & powered by the OEM plug.however, they both do a high speed wobbly shaking dance, when the Key is on and I believe when it’s running, but I don’t know that it ever stays fully closed when connected to factory plug. What is the DME supposed to be telling it to do?

check the injector signal with a noid light or good DMM that has Hz and duty cycle. Check for the spark going away when it's supposed to be idling.Check for a strong spark with a spark checker.
- Ran out of daylight and patience, so above TBD

Some other possibilities:
Issue with the vacuum line to the brake booster, cap it off as a test.
- No change when capped/tested.

Broken vacuum fitting under the throttle body
- inspected & intact

Additional questions, could oil cap be a source of vacuum leak? EGR Valve? It appears that the main wire to the alternator is exposed and constantly “Hot”, could that be part of the problem and what is the remedy?

Thank you for your generosity and patience with your recommendations and time. This is mind numbingly frustrating. It drives GREAT off idle, and you can tell it’s just a little stupid thing that’s probably pretty easy to resolve once found, but the German Haystack is large, and the needle, apparently quite small… I never thought I’d yearn for the simplicity of troubleshooting and working on my Lotus Esprit!

walfreyydo

Vacuum leak or DME temp sensor would be the next things to check. DME temp sensor in particular because it tells the DME what the engine temp is, and when cold, richens the mixture. If the temp sensor is faulty the cold/warm idle mixture will be off. Also, good news, its only around $15 and extremely easy to access.

If that doesnt solve it, I would go back to the basics. Check your fuel pressure, make sure its within spec. Review Clarks Garage, follow the steps, test each sensor/system, especially fuel. As youve found out, guessing and throwing parts hasnt gotten you far and probably dented your wallet. https://www.clarks-garage.com/shop-manual/ts-01.htm

Yes DME = ECU. These cars dont have an EGR, but they do have an AOS, which is similar to a PCV system.

Jfrahm

Sounds rich to me and fuel doesn't help it idle so I would check the FPR to see if it's leaking fuel out the vacuum port. A fuel pressure test would be good particularly to see if it bleeds down fast when shut off but that's harder.

Pull the vacuum line to the FPR and if you find gas, that's bad. If not put a scrap of line from the FPR vacuum port to a bottle and crank it. Be ready to fight a small fire if things go badly.
It's safer to use a DME relay jumper to build fuel pressure and that's not a bad thing to have and to know how to use. But cranking will do it too.