3.2 Engine Stalling - Stumped

[wnsgc]

Greetings folks,

Here's the latest. Over the weekend, we replaced the ignition switch and ignition coil, but the car still stalls.
@John Walker: We did check the ground wires above the #1 cylinder - thanks.
@Chuck H.: We did by-pass the DME and we also replaced the DME with another unit, however I'm beginning to wonder if both DME's could be bad
Today, I'm going to send my DME out for a bench test to see if there are problems with the solders (cold or cracked). At this point, I believe we've tried pretty much everything related to the electrical system related to the DME and fuel system. The car gives the appearance it's running out of gas (even though we have a new fuel pump, fuel pump relay and the tank is filled).
The mystery continues and it gets more frustrating.
Thanks for the support.

[MMiller]

Feel your pain brother....just went thru this with my 88.

I did not see where you checked/addresed the TPS. Sorry if I missed that in the thread. You probably did since you have worked all around it but thought I would mention it as it was the mystery problem in my car. My stumble was at a higher RPM but might be worth a look

Pelican Parts - Product Information: 944-606-113-01-M14

Best of luck

Mike

[wnsgc]

Mike,

I had to look-up what TPS meant... Ha!!!

Yes, I did replace the throttle switch last year. When I installed a Steve Wong chip, I was reading thru his pre-installation tips, which included the throttle body & switch, to ensure I was getting WOT (Wide Open Throttle). That's when I discovered my TPS was dead, so I did replace it.

Thanks.

[Chuck.H]

I read once about the resistance strip wearing out on the 3.2 flapper style airbox, maybe you can check into that? If it was bad in certain spots the engine could be starved for fuel.

Sorry if you mentioned checking that already.

Chuck.H
'89 TurboLookTarga, 332k miles

[zarl911]

i had a similiar issue last week after picking my car up from the bodyshop. soon as i drove off a good mile up the road it would sputter and backfire around 3k rpms then stall out. i was told to try to reset the computer. now how do i do that? i am running a cone air filter....once in awhile it will act up still not as bad as it did when i first picked it up.

[EMJ]

Originally Posted by wnsgc

Greetings folks,

Here's the latest. Over the weekend, we replaced the ignition switch and ignition coil, but the car still stalls.
@John Walker: We did check the ground wires above the #1 cylinder - thanks.
@Chuck H.: We did by-pass the DME and we also replaced the DME with another unit, however I'm beginning to wonder if both DME's could be bad
Today, I'm going to send my DME out for a bench test to see if there are problems with the solders (cold or cracked). At this point, I believe we've tried pretty much everything related to the electrical system related to the DME and fuel system. The car gives the appearance it's running out of gas (even though we have a new fuel pump, fuel pump relay and the tank is filled).
The mystery continues and it gets more frustrating.
Thanks for the support.

Get a magnifying glass, pull the DME, and check for cracked joints. I had the same problem 5 years ago and no one could figure it out. Luckily, I only paid for labor diagnosis and didn't replace a bunch of parts. The problem was minute cracks in the DME joints. A couple of fellow pelicanites diagnosed the problem for me on this board. Soldered up the cracked joints in 10 minutes and the problem never occurred again.

[wnsgc]

Ed,

That's basically what I'm going, or I should say I'm having someone else do it. I've heard enough about the cold & cracked solders to figure it's time to go down that path. I spoke to Steve Wong today (who's chip I have installed and who provides a bench test and repair service) and shipped my DME to him this afternoon. I do hope he finds a problem, and one that would cause this issue, as it's been driving me crazy for seven months now. Once I get his report, I'll report back to everyone.

Thanks.

[wnsgc]

Greetings folks,

Here's an update on my stalling problem. I just got off the phone with Steve Wong. He bench tested my DME and was able to get the unit to shut down. He found the circuitry pertaining to the ignition coil was bad and he found a few cracked solders. After making the repairs, he bench tested it again and then put my DME in one of his cars and drove it for 20+ miles, without a problem. He shipped it back to me today and I hope to have it early next week. I can't say for sure this will solve the problem, but I do hope so. At least if this problem continues, I know it's not the DME. Steve did say the problems he found would lead to the stalling problem. So I do hope this is it. The weather in central NJ is getting nice and I'm ready to take the roof off and drive.

More to report next week - hopefully good news!!!

[KNS]

Thanks for reporting back - I hope that's the end of it for you. As these cars get older I imagine we'll run into this problem with more frequency.

[Tippy]

You're in good hands then. It'll be fine I'm sure.

Good to hear!

[wnsgc]

Greetings again,

Here's another update. We installed the newly fixed DME, hooked up the new battery, and took a test drive. The car pulls very nice, with more power than previously experienced. However....

That's where it ends, as the car stalled out again, in the same manner as it always had!!! I now have a good working DME, not to mention all the other parts I've changed and/or tested, and I'm back to where I started. I have no clue what to do next.

[ischmitz]

Scott,

just reading through all your stuff. One thought: Is the pattern of occurance of the stalling the same after having done something to the DME? How sure are you that the DME is really good/fixed? FYI, when I work on these I use a high-powered stereo microscope to inspect the PCB. There is more than solder joints that can cause intermittent problems.

Next, have you been able to determine if you lack fuel, spark or both when it happens? It wasn't clear whether the stall is followed by a period of no-start? If so a test light could tell if you get fuel signals. Also, I would look at fuel pressure to rule out any issue with the supply.

Can you explain a little more about the 3000RPM? Is it always stalling at that 3k or only when under full load? Any time dependance? How soon does it come back?

My next step would be to drive the car with a break-out box (or the DME open) and monitor key signals (fuel pulse, ignition output, DME relay output, supply power) with an oscilloscope (2 person job) or high-speed data logger to see what is happening during the stall.

Ingo

[rusnak]

I dont follow the term "stalls out". What exactly is happening? Loses power? Dies completely? Won't restart? Needs a lot more info to complete the picture.

[wnsgc]

A lot of good questions, so I decided to write-up everything I know to date (sorry, but this is long):

Car:
1985 Porsche 911 Carrera Targa, with 127k miles, stock engine; Steve Wong Chip added to the DME

Issue:
Engine shuts down (i.e. stalls) under heavy acceleration, over 3k RPM’s, consistently.

Note: The engine does not shut off when the car is stationary and the engine RPM’s are pushed past 3k RPMs, nor does the engine shut off when driving slowly under 3k RPM’s.

I have not tried to slowly accelerate above 3k RPM’s (i.e. not under heavy acceleration).

When the engine shuts off:
- The RPM’s do not immediately drop to 0, they slowly decrease with the speed of the car.
- The engine does not buck, sputter, or back-fire

Temporary Resolution:
When this problem initially began, I was able to replace the DME relay and the car would start immediately. However, over time, the DME relay replacement would only work until the next heavy acceleration. At this point, after all of the work completed (see below), the engine only re-starts after a period of time, say 15 minutes. One more note, DME relays which were removed, were re-used and would allow the car to start.

Work Completed to date:
- Inspected and cleaned all grounds
- Inspected and cleaned the battery cables
- Replaced the following:
o Top Dead Center (TDC) sensor (insulation around wires deteriorated)
o Cylinder Head Temp (CHT) sensor (original 1 wire, went to two wire w/ground)
o RPM Reference Sensor (insulation around wires deteriorated)
o Fuel Pump (original 27 yr old part, which was drawing heavy amps)
o Fuel Pump relay
o Fuel filter (the current filter was replaced 2k miles ago, but it was replaced again, just for completeness).
o Wiring Harness (from the DME to the engine). Found a number of open wires (i.e. shorts), as well as a number of vacuum leaks in tubes near the rear of the engine compartment.
o Battery (it finally died after 11 years (Interstate), so I can’t complain about it)
- Temporarily by-passed the anti-theft system
- Temporarily replaced:
o DME
o Ignition Coil
o Ignition Switch
o Air Flow Meter
o Fuel Injector Harness
- Previously replaced the throttle body and switch.
- Replaced the DME with a known good DME. However for completeness, I sent my DME to Steve Wong, for a bench test. Steve found the circuitry that controls the ignition coil. He also found some cracked solders. He then installed my DME in one of his 911’s and drove it for 20+ spirited miles, and it performed well.

Known and unknown:
- When the engine shuts off, I can hear the fuel pump. The fuel pressure was checked and it had between 50 – 70lbs of pressure.
- I also have spark at the engine. We hooked up a light meter to the distributor and it was lighting up, however the engine would not start.
- What causes the car to stall and re-start. What problem subsides 15 minutes after the engine has shut off, that now allows it to re-start?
- We get the impression (only a guess) that something is telling the fuel injectors to turn off, but I can’t substantiate this.

As always, any and all suggestions are welcome.
Thanks.

[Lorenfb]

"How sure are you that the DME is really good/fixed?"

Well stated! So the DME ECM still may be an issue, and was a waste of money.
The speed & ref have not been replaced, right? If that's the case, then
this consistent failure could be attributed to the speed sensor. A simple
test for this is to disconnect the ref sensor after the engine is started.
If the engine stalls or misses, the sensor is weak which can cause stalling
especially when hot.

Bottom line: The acid test for a DME ECM is to replace it with another.

[wnsgc]

As an FYI, and as noted above, I initially replaced my current DME (before sending it out for a bench test) with two known working DME's. One was a direct replacement, the other was the European DME, both worked in other 911's. So putting aside the DME issue and the repair of mine, I'm no closer to solving the stalling problem.

Does anyone have suggestions, thoughts, etc.?

Thanks.

[wnsgc]

Also has noted above, I have replace the RPM reference sensor. I'm not familiar with the speed sensor, where is it and what task does it perform?

As for the comment about it being a waste of money, maybe, maybe not, but in the end, I'm still no closer to solving this problem. Oh and thanks for reminding me how much money I've wasted to date, attempting to fix my stalling problem - it's appreciated...

[ischmitz]

Scott,

we understand how frustrating this must be. Just to clarify:

The 3.2 uses TWO sensors for the DME and both need to work. They are identical and both are mounted to the bell housing and look at the flywheel. One senses the toothed flywheel to determine engine speed and the other looks for a single pin in the flywheel to determine TDC. Make sure you (or your mechanic) replaced both. These can fail with heat and at certain RPM.

My other comment was regarding the load and RPM. You said you can take the engine above 3k when parked but not under heavy load. I'd be curious if you can try to get the car up to speed and then down-shift so you are at above 4k and then floor it. I want to understand if well beyond the 3k it can function under load.

The one thing that seems important is that the fault seems to get progressively worse. You said initially you could restart right away while more recently you need to have it cool down. This would lead me toward the sensors.

FWIW: I had a similar failure (can not start engine on ML430 in a window of 5 -15 minutes after shutdown on hot day" It was a flywheel sensor on its way out. The heat spike after engine shutdown caused an intermittent open in the sensor. Once the car sat for 20 minutes all was well and it never (yet) hicked up during regular driving. However, getting gas on a hot day turned into an embarrassment.

Ingo

[Lorenfb]

"You said initially you could restart right away while more recently you need to have it cool down. This would lead me toward the sensors."

Mostly likely the problem, i.e. since only the reference sensor was replaced and
the RPM/speed sensor most commonly causes running problems with age and
temp. So if the that resolves the problem, the refund you'll get on the DME ECM
should more than cover the cost of a new sensor and your headaches.

As I posted earlier:

"A simple test for this is to disconnect the ref sensor after the engine is started.
If the engine stalls or misses, the sensor is weak which can cause stalling
especially when hot."

This may help verify the problem. The speed sensor is next to the ref sensor and will
shutdown the engine if disconnected, whereas the ref sensor won't. In many cases
the car doesn't even need to be driven, and engine revs will cause a misfire/shutdown.

Check here for more insights under 'Intermittent Running': http://www.systemsc.com/diagnostic.htm

[ischmitz]

- When the engine shuts off, I can hear the fuel pump. The fuel pressure was checked and it had between 50 – 70lbs of pressure.

is this while cranking and no-start after the shut-down? Normally, the fuel pump only runs if one of the two conditions is met: (1) key in START position (cranking) (2) flywheel rotation detected and both sensors functioning. You only need (1) or (2). If the engine has stalled completely the fuel pump must be off. Not sure when you heard it. Please clarify.

- I also have spark at the engine. We hooked up a light meter to the distributor and it was lighting up, however the engine would not start.

And did you check for fuel pulses? Is the spark at the right time? Have you tried to introduce fuel (starter fluid into the air filter) to verify you are missing fuel but have good spark when in that condition?

-What causes the car to stall and re-start. What problem subsides 15 minutes after the engine has shut off, that now allows it to re-start? We get the impression (only a guess) that something is telling the fuel injectors to turn off, but I can’t substantiate this.

Think heat !!! it can explain the time effect
- can temporarely disable one of the sensors (but then no spark and no fuel)
- How about bad injectors (drawing too much current) causing the fuel injector stage in the DME to overheat fail to produce enough current to drive the inectors.

Next, you need to verify if you miss fuel but have proper spark. Use starter fluid or break cleaner straight into the intake when cranking. If the engine fires up you either miss fuel pulses or have fuel starvation for other reasons.