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944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Porsche Crest 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:*https://www.youtube.com/watch?v=y8oERuwWA8Y

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?
Old 07-11-2019, 12:14 AM
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Garage
vacuum leak....faulty idle control valve, possibly??? had you tried investigating those already?
Old 07-11-2019, 09:03 AM
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Are the plugs wet after it stalls?
If so check/replace the cap, rotor, coil wire.
If no help, consider hotwiring the coil as a test.

Still no help? Try hooking up an inductive timing light and a NOID light so you can video them with the tachometer and see if one or the other drops out when the tach dives.

Need more? As above but monitor fuel pressure as well with a mechanical fuel pressure gauge.

If that's not easy hook a can of starting fluid to a vacuum line and give it a shot while the car is dying.
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1987 928S4
1992 968 cabrio
1994 968 cabrio - Supercharged
Old 07-11-2019, 09:50 AM
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Also check the FPR and damper to see if they are passing fuel out the vacuum side.
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1987 928S4
1992 968 cabrio
1994 968 cabrio - Supercharged
Old 07-11-2019, 09:53 AM
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Vacuum leak most likely culprit:
- measure intake-manifold vacuum at idle with warmed up engine
- measure AFM output voltage at idle and 2500rpms in neutral

Measure fuel-rail pressure at idle with vacuum hose connected and disconectes
Old 07-12-2019, 12:00 AM
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944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by PHillary View Post
vacuum leak....faulty idle control valve, possibly??? had you tried investigating those already?
Hi PHillary


Thank you for helping!

I have not yet.
I'm rather sure it's not vacuum related.
Car runs absolutely perfect - no loss in power, running weird or anything.

I will look into the idle control valve though.
Are there any ways of testing it without buying a new one?
Old 07-14-2019, 02:07 AM
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944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by Jfrahm View Post
Are the plugs wet after it stalls?
If so check/replace the cap, rotor, coil wire.
If no help, consider hotwiring the coil as a test.

Still no help? Try hooking up an inductive timing light and a NOID light so you can video them with the tachometer and see if one or the other drops out when the tach dives.

Need more? As above but monitor fuel pressure as well with a mechanical fuel pressure gauge.

If that's not easy hook a can of starting fluid to a vacuum line and give it a shot while the car is dying.
Hi Jfrahm


Thank you for taking the time. - Much appreciated!

Will definately look at the plugs. - I haven't yet looked to see if they get wet.
Timing and NOID lights sound like a great way to find the source of the problem too. - Will look into that.

It is well above my mechanical level, but I will try and look into it and see if someone will assist me. Thanks!
Old 07-14-2019, 02:10 AM
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944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by DannoXYZ View Post
Vacuum leak most likely culprit:
- measure intake-manifold vacuum at idle with warmed up engine
- measure AFM output voltage at idle and 2500rpms in neutral

Measure fuel-rail pressure at idle with vacuum hose connected and disconectes
Hi DannoXYZ


Thank you for replying.

I'm pretty sure vacuum is not the problem.
Car runs absolutely perfect with no loss in power, sputtering or anything.

It stalls as if you turned the key.

Will consider measuring anyway. Thanks!
Old 07-14-2019, 02:13 AM
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Try a working computer- or send computer for repair.... meaning, check the signals and spark, etc.

I put a thread on Pelican with details as to how I arrived at this solution. Iíll try to find it.

Here it is :

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/1032929-solved-panic-yet-another-my-car-dies-while-driving-post-suspects-fpr-fuel-new-post.html

... ignition coil ok? Spark? I donít know how to check signals from computer.

Hope that helps

Last edited by Bukowski; 07-14-2019 at 05:14 PM..
Old 07-14-2019, 05:11 PM
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944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by Bukowski View Post
Try a working computer- or send computer for repair.... meaning, check the signals and spark, etc.

I put a thread on Pelican with details as to how I arrived at this solution. I’ll try to find it.

Here it is :

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/1032929-solved-panic-yet-another-my-car-dies-while-driving-post-suspects-fpr-fuel-new-post.html

... ignition coil ok? Spark? I don’t know how to check signals from computer.

Hope that helps
Hi Bukowski


Thank you for helping out.
I read your thread. - Very interesting, but I think the source of my car's stalling is different.

I have already tried a known working computer (DME/ECU) and ignition coil. - It made no difference.


I think, I'll have to put some kind of measurement gear on and wait for the stall, to see weather it's spark og fuel missing.
Old 07-15-2019, 06:22 AM
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Ah, now I see the computer in your list. I took it to mean the DME relay.

How about fuel pressure? Test per Clarkís.

How about moisture or water in the fuel? Could it be just enough water to let the car run but stall if itís made to work. That actually happened with a yard machine I have.
Old 07-15-2019, 06:30 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by Bukowski View Post
Ah, now I see the computer in your list. I took it to mean the DME relay.

How about fuel pressure? Test per Clarkís.

How about moisture or water in the fuel? Could it be just enough water to let the car run but stall if itís made to work. That actually happened with a yard machine I have.
Fuel pressure is good. Measure 2.5 bar yesterday.
I can now rule out fuel or vacuum. - It does not get spark when it stalls. See update below.


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?
Old 07-15-2019, 10:48 PM
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Check Wires to fan resistors - helpful to have a volt meter that plugs into the cigarette lighter : this is based on a late 944 NA, so YMMV :

Puzzler : yet another ďmy car died while drivingĒ post
Old 07-16-2019, 02:32 AM
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Quote:
Originally Posted by Stentoft View Post
Fuel pressure is good. Measure 2.5 bar yesterday.
I can now rule out fuel or vacuum. - It does not get spark when it stalls. See update below.
Is that 2.5-bar with or without vacuum-hose connected to FPR?


Quote:
Originally Posted by Stentoft View Post
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?
Where are you measuring ground on engine? What are you using as ground for meter? Unless you mean measuring voltage at coil (red probe) and using engine as ground (black probe).

There is no ground on coil. Only power and when coil is open, power appears at both terminals. Power goes into one terminal, across primary coil's wires and appears on other terminal. That's correct behavior for electronics. Voltage drop is V=IR and when there's no flow, there's no voltage-drop and exact same voltage should appear on both terminals of coil. It's the DME that grounds coil to fire and it's only momentary. You need oscilloscope to measure coil. There is always voltage on both coil terminals (high) and it only is grounded (low) by DME in pulses.



Test for spark at plugs:

1. pull plug wire
2. insert spare plug
3. place plug on engine to ground case
4. crank engine

If you see spark jumping across plugs, then everything related to ignition is fine: speed/ref sensors, DME, coil, power to coil, coil wire, distributor, rotor, plug wires, plugs.

Again, measure intake-manifold vacuum. That number will tell you everything you need to know to fix this problem 100%
Also measure AFM output-voltage at DME connector.

Last edited by DannoXYZ; 07-17-2019 at 01:04 PM..
Old 07-17-2019, 12:39 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
944 1982 N/A
 
Join Date: Jul 2019
Location: Copenhagen, Denmark
Posts: 7
Quote:
Originally Posted by DannoXYZ View Post
Is that 2.5-bar with or without vacuum-hose connected to FPR?


Where are you measuring ground on engine? What are you using as ground for meter? Unless you mean measuring voltage at coil (red probe) and using engine as ground (black probe).

There is no ground on coil. Only power and when coil is open, power appears at both terminals. Power goes into one terminal, across primary coil's wires and appears on other terminal. That's correct behavior for electronics. Voltage drop is V=IR and when there's no flow, there's no voltage-drop and exact same voltage should appear on both terminals of coil. It's the DME that grounds coil to fire and it's only momentary. You need oscilloscope to measure coil. There is always voltage on both coil terminals (high) and it only is grounded (low) by DME in pulses.



Test for spark at plugs:

1. pull plug wire
2. insert spare plug
3. place plug on engine to ground case
4. crank engine

If you see spark jumping across plugs, then everything related to ignition is fine: speed/ref sensors, DME, coil, power to coil, coil wire, distributor, rotor, plug wires, plugs.

Again, measure intake-manifold vacuum. That number will tell you everything you need to know to fix this problem 100%
Also measure AFM output-voltage at DME connector.
Thank you for helping!
Sorry Iím at bit late - car is parked at mechanic and his shop is closed a few weeks.

Great info on the coil. - That makes sense.
We did test for spark at the plugs. Here is how:

Ran the car warm, let it stall, would not restart immediately.
We used a timing light (tested and working beforehand). - No spark at plug.

We did have fuel pressure (jumped dme relay) and power at coil. We also tried two other ignition coils. - No difference.

It would only get spark after cooling down.

It seems to be a combination of the engine being warm AND the fans pulling som power.
Thats why I consider trying another alternator. - Maybe it simply doesnít produce power enough...

Keep in mind itís very consistent (car running until itís warm, let it idle, fans come on and boom, stall). Itís not exactly at the moment, that the fans turn on (most times they turn on, off, on and THEN stall - as in video in original post in this thread.)

How do I test intake-manifold vacuum and why would that tell me what I need to do?
Sorry if Iím asking something stupid. Iím a complete newbie and never worked on a car before.
Old 07-28-2019, 02:41 AM
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What about the wires to the cooling fan resistors? I donít know what early cars look like, but on mine, the wires were shorting out on the blower motor housing. The post I linked shows pics of this. I open the hood, look under the cows near the battery.
Old 07-28-2019, 03:01 AM
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The car should run for 5 minutes or more with no alternator and the radiator fans on full blast. Batteries are like 50 Amp hours so even 80 amps for a few minutes is no big deal.

Next I suggest testing with a proper DME relay jumper in place then I would try hotwiring the coil and then maybe hotwire the DME to a different circuit, I see you had some sort of DME relay socket wiring problem. Powering the DME and the fuel pump with a switched, fused wire from the battery might be interesting.

Also check immediately when it stalls to see if the plugs are wet (fuel but no spark) or if they are dry (normal shutdown). I'm guessing you have a wiring issue shutting the DME down, old alarm system or kill switch type problem, or failing that there is a problem with previous testing or assumptions.
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1987 928S4
1992 968 cabrio
1994 968 cabrio - Supercharged
Old 07-29-2019, 05:27 AM
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Also measure temperature of DME when car dies.
Old 07-29-2019, 01:03 PM
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I chased a similar problem for months, replaced a ton of stuff, etc.

Turned out the likely culprit was either (fixed both at once, so not sure which):

- loose reference sensor connector (unplugged it and plugged it in "better")
- TPS (replaced with new one, car idles much better and doesn't die now).
Old 08-03-2019, 07:19 PM
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