Bob Krantz

I know this has been covered before, but I can't turn up the old topics with the search. My 1989 C4 runs great most of the time. It does have episodes, though, where the engine misses or almost seems to quit while cruising at light throttle at about 2500-3000 rpm. Symptoms are worst when coasting downhill, and when holding a real light throttle at constant speed. Acclerating or dropping rpm down restores normal running.

I have recently changed the temp head sensor, O2 sensor, and idle speed stabilizer. In the past year or so I have changed plug wires, caps, rotors, and cleaned and checked all electrical connectors. About 3 years ago, I had to replace one coil. The engine is all stock and original, with 109,000 miles.

Any advice on what to test/check next?
Bob K
Anchorage, Alaska

Anatol

Hi Bob,

I currently have the same problem. However, things are worsened due to my light RS flywheel and Roock chip. On a bad day the engine cuts out at almost every traffic light. Starting from cold can be painful too: smooth idle for approx. 10 sec. and afterwards revs rise and fall plus minus 500 rpm until cut out. But there are also days where the engine performs perfectly - usually when I show it to my mechanic. What an endearing little beast!

Sofar I have had the temp sensor changed to the updated version. However, since you have changed many more things this leaves me a bit hopeless. On the other hand my mechanic thinks it might be fuel supply. We will have a look at it next week.

I searched the archives yesterday and can recommend the following strings:

"total muscle failure" 09-02-2001
"engine hesitates from idle" 05-08-2002
"hesitation and low power" 11-20-2001

Hope this helps.

Anatol
C2 1991

Ade - C4 91

Hi Bob,

I think you have been through the same process that I’ve been going through for at least 18 months. In my case the car does this when the temperature changes.

As usual these are just guesses;

You mentioned one coil was replaced, It may be time to replace the other one now.

The airflow meter may be defective and giving unreliable reading, which fluctuates the mixture values. This can result in the engine running quite rough, lower power, inconsistent or snatchy,

If the car is surging under low throttle, then it could be dropping in and out if idle mode - the throttle stop plate maybe too close the idle switch and interfering or the vacuum hoses around the idle stabiliser maybe leaking.

What's your oil level like, do you ever overfill? This causes a lot of problems.

Have you replaced the DME relay? Worth a try.
Have you checked the Distributor belt?
What's your battery like?

Have you tried resetting the DME, if so what the effect? Does the problem go away for a while?

Funny really, was just sitting at my desk with a copy of 911 & Porsche world open on your running report and a picture of you leaning against your car. Looks like Bear country.

Good luck and if you need any more help just ask.

Ade.

joey bagadonuts

Good to hear from you, Bob! No suggestions, aside from making sure your air filter's clean, but I just wanted to say I've enjoyed your running reports and think it's great you're cruising around the Yukon in a C4!
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Bob Krantz

Thanks for the suggestions so far. A few things I forgot to mention: I replaced the DME relay about a year ago, and the battery is a new Optima. I have never over-filled the oil, but the previous owner did: soon after buying the car, I removed the entire intake system to get to an oil leak at the temp sensor plate. When I did that, I found oil pooled in the throttle body. That was nearly 5 years ago.

Any other ideas out there? This problem is getting really annoying.

Ade - C4 91

Hi

You said you removed the entire intake system, did you have it pressure tested afterwards? Should be able to hold 80Psi (I think) for 5mins. Small leaks play havoc with air/fuel mixture. If you just removed the throttle body, did you over or under tighten the large jubilee clips (Note that overtightening can deform the rubber and cause leaks as well). One way to look for leaks is using a small portable LPG bottle and (carefully) while the engine is idling, flow the gas around each seal - if the idle changes - you have a leak.

ISV also needs to be considered. If you have had Oil contamination, was this cleaned as well? lots on this in past threads.

Did you confim that the second coil was ok? Have you tried swapping them around?

Ade.

Ade - C4 91

Hi,

Forgot to mention, have you had the ignition leads checked for resistance? They start to degrade after 60K miles/10 years or so.

Ade.

Wsp964

Bob,

Someting else ...

There was a may 15 1990 TSB concerning erratic idle, loss of power, stalling.

Cause : air flow meter wiring harness protective cover contacts the ventilation hoses causing current flow interuptions on the plug terminals.

The 911 Carrera 2 model year 1990 and Carrera 4 model years 1989 and 1990 were to be modified, even if no engine performance problems were present.

Work procedure :

1. Remove air filter housing cover and filter
2. Push down locking spring on air flow meter wiring plug and disconnect plug
3. To gain acces to plug, pull wiring harness with plug between airflow meter and iddle stabilizer towards rear of vehicle
4. Pull back rubber boot and cut protective wire covering using side cutter or similar tool
5. Heat protective wire covering with hot air blower to max 100C or 230F. Pull off and discard protective wire covering
6. Reinstall rubber boot on plug housing. Loop wiring harness along side of plug and tie wrap.
7. Reconnect wiring plug.
8. Check clearance between wiring harness and vent hoses. Distance should be a least 15 mm. If necessary, bend bracket on right shock absorber mounting slightly forward. Make sure veent hoses are properly installed on bracket at shock absorber mounting and on bracket in front of air flow meter. The small diameter hose must be routed below the large diameter hose.

9 and last. Reinstall filter and filter housing cover.

Hope this helps.

WSP964

Wsp964

So Bob,

Any results ?

Fred, Long Island

I'm having a similar although more severe problem. It cuts out at idle after the enrichment flapper engages numerous times. It also cuts out at mid rpm under way. This all started with the warning lights doing the Christmas tree effect along with the buzzer.
I recently removed the engine for seals,(pardon my reduncance as many of you have heard this story)and the result is this symptom. The engine runs fine...at times...it stalls after doing a hunt at idle. The enrichment flapper is being told at idle it needs to activate. My mechanic is going through the processes and it is narrowing down.
Battery?
CPU?
Idle control?
We'll find out.
In previous posts, I ranted about all this ABDS as a curse as it seems to sense in error, and then start to control a unit that is functioning in the basic way it was designed some 30 years ago. The engine wants to run as it was intended.
I compare it to a nagging wife that just isn't happy letting you do what you want or intended to do.
Ghosts in the machine, indeed.........