911 Carrera 3.2 stalling problem solved

[Sils1968]

Hi, I just want to share my experience with a stalling problem that seems to be quite common across 911 community.

I have a '84 911 Carrera coupe in pretty good shape, 150t miles on the clock. Got the car three years ago, repainted, changed the seals and whetherstripping, done. Looks like new. Was happy with the car until the last summer.
After a long city drive on a hot summer day parked the car in mu yard. After a couple of hours started the car and it run very poorly. Almost no idle. Somehow managed to get it in the garage and decided to look in to it the next day.

A year before the car already had idling problem that was solved by replacing the ICV. I suspected that the ICV might be the cause again, so I checked and cleaned it. The car started good and idled a couple of minutes then shut off and resumed a few times before finally stalling. It looked like fuel supply was cut off. And starting again was harder and harder. Above 1600 rpm the engine run fine.

First thing, I searched for answers in different forums. Found similar problems and many suggestions for the probable cause and solutions.

I checked everything according to Bentley service manual. Sensors, vacuum lines, grounds, AMF, throttle position switches, fuel pressure, ignition coil and wires etc. No problems found. Suspected Motronic. Found a guy in Germany who repairs these things. He suggested that speed and position sensors might be bad, even if they pass the tests. Changed both sensors, the same result. Replaced the head temperature sensor, no change for the better. In my desperation I even tested and cleaned the fuel injectors. And all the time the car started perfectly when cold and then stalled again after a few sharp rpm drops.

Despite that all the test results were OK, one small thing was wrong. I noticed voltage spikes at the AMF power supply pin. Normally there are 5v, but just for the moment when engine stalled there were more than 50v. The 5v are supplied by motronic, where does it get the 50v? Somehow I got the idea that this has something to do with ignition coil. Changed it with a new one and everything is fine since. By the way, the old coil was fine according to Bentley test results and had all the same resistances as the new one.

Hope that this might help to someone with similar symptoms.

[kodioneill]

When you say AMF do are you referring to the air flow sensor?

[DRACO A5OG]

Interesting, please keep us posted if this fix sustained itself.

[Sils1968]

By AMF I ment Air Flow Meter (AFM). My mistake.

[jlex]

Does the coil actually supply power to the AFM??? Is there some kind of connection according to the wiring diagram?

[hcoles]

sub - interesting

[boyt911sc]

Originally Posted by jlex

Does the coil actually supply power to the AFM??? Is there some kind of connection according to the wiring diagram?

I am glad the OP was able to fix his problem but not totally convinced that the ignition coil was the culprit (?). If the old ignition coil passed the tests and considered good, would it be prudent to re-test the old ignition coil back on the car again? Just curious.

Tony

[DRACO A5OG]

^^^ that would be most logical and empirical. Hmm?

[CCM911]

Tony:

How about he tests the coil cold, and then repeats the test on the coil when hot?

[Sils1968]

I will put back the old coil and see what happens.

[Sils1968]

Originally Posted by jlex

Does the coil actually supply power to the AFM??? Is there some kind of connection according to the wiring diagram?

The coil is not connected to AFM. The power supply 5V is coming from Motronic.
The AFM was not the only place where the short voltage spikes were noticed.
Each time when the engine tried to stall and came back, the DME relay clicked. So, I thought that there might be a problem of power supply to the primary circuit. Attached a digital voltmeter and there were more than 12v in the short moments of stalling.
Sometimes electricity is a mistery.

[GH85Carrera]

I had a similar running problem. The car ran 100% normal and all of a sudden it was down on power and I just barely got back home. I did all the same sensor swaps and no luck. I finally swapped the coil since I had one on hand that I carry on road trips. Bingo, back to normal running.

[Sils1968]

Following your suggestions I put back the old coil. By the way, I was curious too, to confirm that the coil is the cause. I tested and cleaned the injectors and replaced the coil at the same time. It was possible that one of these to components were causing the stalling.
Today I started the engine with the new coil inside. It ran fine. Then put back the old one and after a few seconds it tried to stall three times and finally stalled. Changed to the new coil and everything is OK. All the resistance measurements are the same for both coils.
The only remark from this test is that this time the engine was warm and it stalled almost immediately in comparison with previous times, when the engine was started cold and ran a few minutes before stalling.

[mnez]

What coil was removed, and what did you replace it with?

[wwest]

Poor DME ground return?

I think the power resistor that provides the PWM of the coil primary is on that panel with the coil, while the DME that provides the drive for that power transistor is under the driver's seat.

If that panel is not well grounded to the chassis the DME ground will be providing the coil ground return.

50 volt spikes co-incident with plug firing?

[Sils1968]

Originally Posted by mnez

What coil was removed, and what did you replace it with?

I just installed back the old ignition coil to see if the problem returns.

[wwest]

Bad coil HV flashover from the HV side, internal or external, to the 12 volt source side just might result in serious voltage spikes in the long lead distance to the front fuse panel.


Even worse if the flashover is to the PWM driver's DME terminal.

DME ground return appears to be directly to the engine.

[pmax]

Lightning has a way of finding the path of least resistance.

How's the weather over there ?

[Sils1968]

Originally Posted by pmax

Lightning has a way of finding the path of least resistance.

How's the weather over there ?

At the time the stalling started it was hot and dry.