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944 sudden stall in idle - makes no sense! :(

 
Old 07-11-2019, 05:04 AM
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Martin Stentoft
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Default 944 sudden stall in idle - makes no sense! :(

Hi guys


I have had three Porsche specialists help me, but none of them has seen or heard of this before.

Car: 1982 944 N/A

Problem
Car cuts out (as if you turned the key) when it's warm and idling.
See the last seconds of video here:

I will drive the car for 15 minutes, let it idle and the car dies after about two minutes.
It's consistently happening every single time and it's very sudden. As a result, I'll end up stalling at red lights etc...

I can't always start it right after, and it's definately heat related, I would say.

Already tried
  • Known working DME computer
  • Known working AFM
  • Known working ignition coil
  • New ignition switch
  • New DME relay
  • Fixing DME relay socket
  • New speed and reference sensors
  • New temp sensor

I'm at my wits end...
What should I try and how should I proceed?
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Old 07-12-2019, 04:27 PM
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edfishjr
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I'm thinking on it.

I think you are you saying that you put a known-good DME in and it still stalled? and you put a known good AFM in and it still stalled?

No other symptoms? meaning that engine runs good except for the stall?
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Old 07-12-2019, 09:46 PM
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Originally Posted by edfishjr View Post
I'm thinking on it.

I think you are you saying that you put a known-good DME in and it still stalled? and you put a known good AFM in and it still stalled?

No other symptoms? meaning that engine runs good except for the stall?
Also, this is an 82? A euro car? Sorry, I just realized you're in Denmark!

Last edited by edfishjr; 07-13-2019 at 08:46 AM.
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Old 07-13-2019, 10:28 AM
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I think this problem is probably electrical, not fuel related and is related to heat as you suggest. My thinking:

DME will shut down engine if:

-DME loses power- check/clean/tighten power wire from battery to DME at positive battery terminal.

-loss of airflow signal from AFM (I think, but maybe not)- replaced AFM with known good, but could be short in wiring. Check connector at AFM. Check AFM output voltage at DME input after car has stalled. When moving the air flap the voltage should get higher as air flap moves more.

-loss of power from DME relay- you put in new relay. Check for relay block fault. Also remove DME relay and use 3-wire jumper to run car to absolutely eliminate relay as the source of the problem.

-loss of speed/reference sensor input- Clean DME grounds

This assumes DME is good. Internal DME fault can cause this problem. Heat can cause bad solder joint to open up or other components to fail.
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Old 07-14-2019, 06:58 AM
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Martin Stentoft
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Originally Posted by edfishjr View Post
I think this problem is probably electrical, not fuel related and is related to heat as you suggest. My thinking:

DME will shut down engine if:

-DME loses power- check/clean/tighten power wire from battery to DME at positive battery terminal.

-loss of airflow signal from AFM (I think, but maybe not)- replaced AFM with known good, but could be short in wiring. Check connector at AFM. Check AFM output voltage at DME input after car has stalled. When moving the air flap the voltage should get higher as air flap moves more.

-loss of power from DME relay- you put in new relay. Check for relay block fault. Also remove DME relay and use 3-wire jumper to run car to absolutely eliminate relay as the source of the problem.

-loss of speed/reference sensor input- Clean DME grounds

This assumes DME is good. Internal DME fault can cause this problem. Heat can cause bad solder joint to open up or other components to fail.
Hi edfishjr


Thank you for taking the time. Much appreciated!

Yes, this is a '82 euro.
Are there any differences in this regard?

Yes, I have tried switching both DME computer, AFM and Ignition coil for known working ones.
It makes no difference what so ever. I thought it would be a bad solder joint, but no luck.

Absolutely no symptons. It runs perfectly, which makes me believe, that it's not fuel og vacuum related.
It must be electrical. It happens bretty much as if you turned the key. No sputtering og anything.


________________________________________________________________________ _________

-DME loses power- check/clean/tighten power wire from battery to DME at positive battery terminal.
Power wire from battery to DME? How do I locate that? - Wouldn't it just be one of the pins on the DME (ECU) connector?

-loss of power from DME relay- you put in new relay. Check for relay block fault. Also remove DME relay and use 3-wire jumper to run car to absolutely eliminate relay as the source of the problem.
Will look into block fault. Never heard of that before. I have tried jumping the relay - it still results in a stall. I can hear the pump still running, so fuel pump gets power.



Current plan:
Check for spark. - Sometimes I can't start the car right away after a stall.
Check fuel pressure with a tester. - Just to rule it out 100%.

I guess it should be possible to then move back through the electrical systems? - But in what order?
Spark plug, rotor cap, ignition coil etc?
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Old 07-15-2019, 10:57 PM
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Originally Posted by Martin Stentoft View Post
Hi edfishjr


Thank you for taking the time. Much appreciated!

Yes, this is a '82 euro.
Are there any differences in this regard?

It is unclear what differences there might be. As far as I know, it is mechanically and electrically extremely similar to the 1983 U.S. car, except without catalytic converter and oxygen sensor and with higher compression pistons. This means the DME, AFM, fuel pump, fuel injectors, etc. all work the same way. A person in my town has a 1982 Euro 944 that he imported, but I don't know how to contact him.

Yes, I have tried switching both DME computer, AFM and Ignition coil for known working ones.
It makes no difference what so ever. I thought it would be a bad solder joint, but no luck.

Ok, so you have proven that the problem is not one of these major components.

Absolutely no symptons. It runs perfectly, which makes me believe, that it's not fuel og vacuum related.
It must be electrical. It happens bretty much as if you turned the key. No sputtering og anything.

This points to some sort of fault with the wiring harness, not a major component, probably heat related as you suspect.


________________________________________________________________________ _________

-DME loses power- check/clean/tighten power wire from battery to DME at positive battery terminal.
Power wire from battery to DME? How do I locate that? - Wouldn't it just be one of the pins on the DME (ECU) connector?

At the positive battery terminal there is one very thick cable and one or more smaller wires that come off the connector. I think that one of these smaller wires is a direct supply of power to the DME. Make sure they are all well-connected.

-loss of power from DME relay- you put in new relay. Check for relay block fault. Also remove DME relay and use 3-wire jumper to run car to absolutely eliminate relay as the source of the problem.
Will look into block fault. Never heard of that before. I have tried jumping the relay - it still results in a stall. I can hear the pump still running, so fuel pump gets power.

I think this proves that the problem is not at this location.

Current plan:
Check for spark. - Sometimes I can't start the car right away after a stall.
Check fuel pressure with a tester. - Just to rule it out 100%.

I guess it should be possible to then move back through the electrical systems? - But in what order?
Spark plug, rotor cap, ignition coil etc?
Can't be spark plug. All 4 cannot fail at the same time and then resume perfect function.
Can't be ignition coil. (You proved that.)
Theoretically could be rotor or cap but unlikely. I would inspect or replace with new components.
Check the high-voltage wire from the coil to the cap. This is a single-point of failure that could fail with heat and cause shutdown. If the wire or connectors are bad they could get hot due to excessive resistance to current flow.
Also, when the engine has stalled but ignition still on you can verify the AFM signal at the DME connector (I don't know which pin) by removing the connector. The voltage should be non-zero and should vary while someone pushes on the AFM flap door with a rod inserted from the airbox. If no voltage is read at that pin then the problem is somewhere between the connector at the AFM and the DME connector. Good luck!

Be sure to find and clean the grounds I mentioned if not done already. Bad grounds can cause intermittent problems.
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Old 07-16-2019, 03:47 AM
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Martin Stentoft
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Originally Posted by edfishjr View Post
Can't be spark plug. All 4 cannot fail at the same time and then resume perfect function.
Can't be ignition coil. (You proved that.)
Theoretically could be rotor or cap but unlikely. I would inspect or replace with new components.
Check the high-voltage wire from the coil to the cap. This is a single-point of failure that could fail with heat and cause shutdown. If the wire or connectors are bad they could get hot due to excessive resistance to current flow.
Also, when the engine has stalled but ignition still on you can verify the AFM signal at the DME connector (I don't know which pin) by removing the connector. The voltage should be non-zero and should vary while someone pushes on the AFM flap door with a rod inserted from the airbox. If no voltage is read at that pin then the problem is somewhere between the connector at the AFM and the DME connector. Good luck!

Be sure to find and clean the grounds I mentioned if not done already. Bad grounds can cause intermittent problems.
Hi again


Solid advice - thank you!

Update
Yesterday we spent five hours meassuring, and we got closer, but what we found out is pretty weird...
The issue is ingition/spark related (ruling out fuel pressure, which tested out perfect).

Voltage at ignition coil measures around 12.7 volts, when fans are running. Note, my battery is rather new and strong.
There is constant power to coil (also when the car stalls and won't start).

What baffled us is, that measuring ground on coil AND ground on engine gave us 11.xx volts...
Could this be the alternator messing things up? - Getting warm to the point where it can't deliver power enough and for some reasons maybe sends power the wrong way?


I will try another alternator today and see if that changes anything.
It's worth noting, that the stalling occurs only when engine is warm AND fans are on (using maximum amount of power).

Any other possible solutions?
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Old 07-16-2019, 10:19 AM
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See comments in red, below.







Originally Posted by Martin Stentoft View Post
Hi again


What baffled us is, that measuring ground on coil AND ground on engine gave us 11.xx volts...

I'm not sure what you tested but I think this is normal. The negative side of the coil primary circuit is not a ground. It only gets grounded (to allow current to flow in the primary circuit and thus build energy in the secondary circuit) when the DME commands. When the engine is not running the primary circuit is not grounded or it would short-circuit the battery.

Could this be the alternator messing things up? - Getting warm to the point where it can't deliver power enough and for some reasons maybe sends power the wrong way?

I'm not sure.


I will try another alternator today and see if that changes anything.
It's worth noting, that the stalling occurs only when engine is warm AND fans are on (using maximum amount of power).

Interesting, but I don't know what it means. The fans will come on as the engine gets hot. So, the shutdown may not be related to the fans coming on. Both the shut down and the fans coming on may be because the engine is hot but these two things may not be related.

Any other possible solutions?
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Old 07-18-2019, 10:33 AM
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This sounds like a similar issue I have chased for 2 years.

1. Test the fuel injectors by removing the fuel rail and placing each injector in a jar of equal volume. Crank the engine for several seconds. Determine the fuel volume from each injector. If all 4 have equal volume, then injectors are OK. If one or more has less volume, then switch injectors low to high and rerun test to determine if that injector is still low volume at a different position, If yes, then get the injectors clean and test at SouthBay Fuel Injectors 516.442.4707. If No, then the fuel harness is the issue. Replace it with a Lindsey Racing product.
2. If injectors are OK, then remove the boot from the inlet of the AFM. This test takes 2 people. One starts the engine with the throttle about 1/2 open and the other squirts fuel into the inlet of the AFM..The squirt bottle most atomize the fuel into a fine spray. Start the engine and if it fires, keep squirting fuel for a few more seconds while the other person maintains the rpm above 2500 and oscillates it back a forth above 2500. If the engine sounds like is running OK then try it at idle. If it stalls, try to restart without spraying in fuel. If no restart after several attempts, then stop trying.
3. Next test that the engine has a good ground by taking a jumper cable and connecting one cable terminal to the negative post of the battery and the other end of the cable to a secure clean place on the engine block. Sometimes the grounds between the engine, located on the bell housing and the firewall of the body and the negative battery connection to the body develop corrosion resistances that can affect the reference sensors signal.
4. Rerun tests in item 2. If he engine starts, repeat the starting several times, If OK, then clean the grounds at the bell housing and firewall.
5. If there is no engine start or the problem comes back, then there is a high probability the reference sensor harness from the reference sensor plug to the ECU is bad.
6. Disclaimer: I have a Lindsey reference sensor harness scheduled for delivery July 20th. Post outcome shortly after that.

Last edited by T&T Racing; 07-18-2019 at 04:36 PM. Reason: needed to change misinformation and conclusioln
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