Intermittent Stalling on a Hot Engine Issue
06-19-2014, 03:42 PM
#1
Burning Brakes
Thread Starter
Intermittent Stalling on a Hot Engine Issue
We have an issue with my 993 that is baffling. Following a total rebuild of the 3.6 engine, the engine starts immediately and runs well while COLD. Once the engine has run for 15-20 minutes and has heat soaked the engine will idle roughly and will stall if load is suddenly removed from the engine.
Another observed phenomena is that the ECU, for unknown reasons, will not allow an adaptation to occur.
In other words, you can be driving along at 30 mph, in an appropriate gear, and in preparing to stop, you depress the clutch. On about the fifth or sixth stop once the engine reaches temperature, when the clutch is depressed the engine speed will drop to a low enough point where it will stall. Conversely, on 5 out of 6 stops from speed, the engine speed will drop to the proper idle speed without incident.
The phenomena is more prevalent when engine speed is less then 2500 rpm when the clutch is depressed to unload the engine. If engine speed is above 3000 rpm or if you just "blip" the throttle while declutching, the engine will not stall. Once the engine reaches idle (assuming it does not stall) the idle is dead solid giving no indication of any instability. Further, the engine can sit there for 30 minutes with a solid idle at about 900 rpm.
If the motor stalls, it will immediately restart with the characteristic "flare" in rpm followed by a return to a good solid idle. A restart NEVER results in a stall. You must load the engine and then unload it to trigger a stall.
The idle stabilization valve was quickly changed out which made no difference in the symptoms mentioned above. We've also changed out all ignition components including wires, plugs, caps and rotors. Both ignition coils have been changed. Static tests of the MAF and the idle position sensor have been made (but not at temperature) and were both within range.
Finally, the ECU was removed from the car and sent off to a specialist in Bosch ECU's. Their test of the ECU showed nothing wrong. It should be noted that during the engine rebuild, the ECU was reconfigured to take advantage of a more aggressive RS hydraulic cam that was installed in the engine. I have been told that the ECU programming has nothing to do with my current issues.
This is a classic case of an intermittent problem that only materializes on a hot engine.
Though both the MAF and throttle position sensor "test" to be good, we are now going to change them out. Assuming no results from that, we are about out of things to do.
Has anyone ever encountered such an issue of intermittent stalling on a hot engine when load is suddenly removed?
____
Another observed phenomena is that the ECU, for unknown reasons, will not allow an adaptation to occur.
In other words, you can be driving along at 30 mph, in an appropriate gear, and in preparing to stop, you depress the clutch. On about the fifth or sixth stop once the engine reaches temperature, when the clutch is depressed the engine speed will drop to a low enough point where it will stall. Conversely, on 5 out of 6 stops from speed, the engine speed will drop to the proper idle speed without incident.
The phenomena is more prevalent when engine speed is less then 2500 rpm when the clutch is depressed to unload the engine. If engine speed is above 3000 rpm or if you just "blip" the throttle while declutching, the engine will not stall. Once the engine reaches idle (assuming it does not stall) the idle is dead solid giving no indication of any instability. Further, the engine can sit there for 30 minutes with a solid idle at about 900 rpm.
If the motor stalls, it will immediately restart with the characteristic "flare" in rpm followed by a return to a good solid idle. A restart NEVER results in a stall. You must load the engine and then unload it to trigger a stall.
The idle stabilization valve was quickly changed out which made no difference in the symptoms mentioned above. We've also changed out all ignition components including wires, plugs, caps and rotors. Both ignition coils have been changed. Static tests of the MAF and the idle position sensor have been made (but not at temperature) and were both within range.
Finally, the ECU was removed from the car and sent off to a specialist in Bosch ECU's. Their test of the ECU showed nothing wrong. It should be noted that during the engine rebuild, the ECU was reconfigured to take advantage of a more aggressive RS hydraulic cam that was installed in the engine. I have been told that the ECU programming has nothing to do with my current issues.
This is a classic case of an intermittent problem that only materializes on a hot engine.
Though both the MAF and throttle position sensor "test" to be good, we are now going to change them out. Assuming no results from that, we are about out of things to do.
Has anyone ever encountered such an issue of intermittent stalling on a hot engine when load is suddenly removed?
____
06-19-2014, 08:59 PM
#2
Maybe the clutch is dragging on disengagement.
During deceleration is it hard to remove the gearshift from 3rd or 2nd gear?
It's been reported that dirty transmission shaft splines can cause disengagement issues.
In some cases, the ECU will blip the throttle to prevent a stall.
Unfortunately, the effort to remove the transmission and clean the shaft is almost as much
work as a clutch replacement.
How many miles on the clutch? What type of clutch do you have? Have you seen any debris
on the air filter that appears it could have come from the clutch?
-bruce
During deceleration is it hard to remove the gearshift from 3rd or 2nd gear?
It's been reported that dirty transmission shaft splines can cause disengagement issues.
In some cases, the ECU will blip the throttle to prevent a stall.
Unfortunately, the effort to remove the transmission and clean the shaft is almost as much
work as a clutch replacement.
How many miles on the clutch? What type of clutch do you have? Have you seen any debris
on the air filter that appears it could have come from the clutch?
-bruce
06-19-2014, 10:44 PM
#3
The issue occurs at idle as well so it should not be related to the clutch. It could be the DMF if so equipped. Have you checked fuel pressure when the engine is hot? The trick will be to do this when the engine is running rough. I'm guessing that the fuel pump and pressure regulator are not the issue, but it would be good to rule them out.
06-19-2014, 10:53 PM
#4
Rennlist Member
I'd think this has vacuum leak written all over it. Especially, since the engine was just rebuilt. If you could put it on a smoke tester or a sniffer, you may find out real quick if this is the issue.
06-20-2014, 12:01 AM
#5
Intermittent DME/fuel relay issue?
I'd try swapping the relay first. Maybe a cracked solder joint causing the intermittent issue?
I'm thinking vacuum leaks may not manifest themselves during hot engine only; but a sniffer test is not all that expensive and should be examined as a possibility..
I'd try swapping the relay first. Maybe a cracked solder joint causing the intermittent issue?
I'm thinking vacuum leaks may not manifest themselves during hot engine only; but a sniffer test is not all that expensive and should be examined as a possibility..
Last edited by nine9six; 06-20-2014 at 10:18 AM.
06-20-2014, 11:09 AM
#6
Rennlist Member
My experience with my 3.2 was that the higher idle when cold would overcome a vac leak. When the engine heated, and the idle settled down, that's when the leak would potentially stall the motor, when load or throttle is cut off.
06-29-2014, 10:47 PM
#7
Burning Brakes
Thread Starter
Update
Thanks for all the interest and responses to this post. I want to take a few minutes to update you. However, let me start by saying that after two months in my mechanic's shop, we have not solved the issue.
The car is still in the shop. My mechanic is now really curious as to the cause of the stalling; but, to date has not solved the problem. He has kindly stopped running the clock on my car and is chalking up the work to being "educational" since this is one of the toughest problems he has ever come across.
The problem is exacerbated by the fact that it only occurs on a hot engine, and is somewhat "intermittent", so trying to work with it is a PITA since it does not always show itself.
The shop has tried swapping out most of the engine management system components. They also pulled the ECU and sent it to a Bosch ECU specialty shop in Florida, where it tested out fine.
The latest trail that they are on is thinking that I may have an intermittent issue with the head temperature sensor. As you may be aware, the sensor is screwed into the #3 cylinder head making it a pain to get to. But, Sol (my mechanic) thinks that since this issue is temperature related, that that sensor may be part of the issue. It could be related to some corrosion on the threads of the sensor (I found the sensor bright and shiny when I took it off the motor, so did not chase the threads on the sensor). Sol is going to play around with loosening the sensor and seeing if he can get it to malfunction and then tightening it to get it to change. Then he wants to pull it and clean up the threads and try his test all over again. If he is unsuccessful in getting the temp sensor to cooperate, he will just replace it with a new sensor, and hopefully, my issues will go away.
That is the latest theory. Sol runs a Porsche race shop here in Chicago and specializes in the air cooled motors, so he has taken my issue on personally.
Hopefully, he will solve the issue and then I will report back on what happened.
Since the engine management system was working correctly when I pulled the engine, I elected to not change out most of the management components. In retrospect, I think that I may have been penny wise but pound foolish. The components were all original so I should have changed them out wholesale since the car is 17 years old. But, I did not. But, I did send the distributor out for rebuild, and the injectors were serviced and flow tested.
Having said all that, Sol has swapped out just about every component on the engine chasing this issue. He did find one vacuum leak, and he did find two bad acting O2 sensors, which was interesting since I DID install all new O2 sensors. And, two of the four turned out to be problematic. Go figure.
Prior to bringing the car to Sol, I did have a vacuum test performed at another shop. They did a smoke test and did not find any vacuum leaks. So, it is interesting that Sol found a vacuum line disconnected when they first started working on my car. But, I obviously have multiple issues on the engine.
To answer some of the questions posted earlier in this thread, I DO have a LWF on the car. And I do have the RS clutch package installed with the LWF. These components are relatively new, having been installed by the PO several years ago. But, the car is a 1998 model, and the stalling issue squawked on the 1995 cars never showed up on my car. Having said that, there is no doubt that the LWF exacerbates my stalling. If I were to go back in with the DWF, the increase in mass would probably curb my stalling issues. But, I do not think that the situation is caused by the LWF. I drove the car for a year on the LWF, and stalling was never an issue. Something has changed.
When the stalling first showed up, I first pulled and cleaned the idle stabilization valve. When that did not fix the problem, I replaced the ISV with a new part. And, that did not fix the issue either.
The other areas that have been explored are the throttle position sensor and the MAF sensor. Neither has been changed out by Sol; but, I suspect that if the test of the temperature sensor fails, we may move to changing out either the MAF sensor or the throttle position sensor (or both), even though both "test" OK right now.
Stay tuned.
The car is still in the shop. My mechanic is now really curious as to the cause of the stalling; but, to date has not solved the problem. He has kindly stopped running the clock on my car and is chalking up the work to being "educational" since this is one of the toughest problems he has ever come across.
The problem is exacerbated by the fact that it only occurs on a hot engine, and is somewhat "intermittent", so trying to work with it is a PITA since it does not always show itself.
The shop has tried swapping out most of the engine management system components. They also pulled the ECU and sent it to a Bosch ECU specialty shop in Florida, where it tested out fine.
The latest trail that they are on is thinking that I may have an intermittent issue with the head temperature sensor. As you may be aware, the sensor is screwed into the #3 cylinder head making it a pain to get to. But, Sol (my mechanic) thinks that since this issue is temperature related, that that sensor may be part of the issue. It could be related to some corrosion on the threads of the sensor (I found the sensor bright and shiny when I took it off the motor, so did not chase the threads on the sensor). Sol is going to play around with loosening the sensor and seeing if he can get it to malfunction and then tightening it to get it to change. Then he wants to pull it and clean up the threads and try his test all over again. If he is unsuccessful in getting the temp sensor to cooperate, he will just replace it with a new sensor, and hopefully, my issues will go away.
That is the latest theory. Sol runs a Porsche race shop here in Chicago and specializes in the air cooled motors, so he has taken my issue on personally.
Hopefully, he will solve the issue and then I will report back on what happened.
Since the engine management system was working correctly when I pulled the engine, I elected to not change out most of the management components. In retrospect, I think that I may have been penny wise but pound foolish. The components were all original so I should have changed them out wholesale since the car is 17 years old. But, I did not. But, I did send the distributor out for rebuild, and the injectors were serviced and flow tested.
Having said all that, Sol has swapped out just about every component on the engine chasing this issue. He did find one vacuum leak, and he did find two bad acting O2 sensors, which was interesting since I DID install all new O2 sensors. And, two of the four turned out to be problematic. Go figure.
Prior to bringing the car to Sol, I did have a vacuum test performed at another shop. They did a smoke test and did not find any vacuum leaks. So, it is interesting that Sol found a vacuum line disconnected when they first started working on my car. But, I obviously have multiple issues on the engine.
To answer some of the questions posted earlier in this thread, I DO have a LWF on the car. And I do have the RS clutch package installed with the LWF. These components are relatively new, having been installed by the PO several years ago. But, the car is a 1998 model, and the stalling issue squawked on the 1995 cars never showed up on my car. Having said that, there is no doubt that the LWF exacerbates my stalling. If I were to go back in with the DWF, the increase in mass would probably curb my stalling issues. But, I do not think that the situation is caused by the LWF. I drove the car for a year on the LWF, and stalling was never an issue. Something has changed.
When the stalling first showed up, I first pulled and cleaned the idle stabilization valve. When that did not fix the problem, I replaced the ISV with a new part. And, that did not fix the issue either.
The other areas that have been explored are the throttle position sensor and the MAF sensor. Neither has been changed out by Sol; but, I suspect that if the test of the temperature sensor fails, we may move to changing out either the MAF sensor or the throttle position sensor (or both), even though both "test" OK right now.
Stay tuned.
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06-30-2014, 12:24 AM
#8
First of all, good luck with this. I hope this gets solved soon.
I can think of one way to check the CHT sensor without pulling it.
Hook up an OBDII scanning tool that can give you live data.
[Have a passenger] Monitor the engine temperature parameter as the engine warms up and when the engine stalls.
You might also want to monitor the MAF and Throttle Position sensor values.
Send me a PM if you would like to try my original MAF. I bought a new one last year in an attempt to 'fix' my erratic idle. No difference. Switching on the AC does fix the idle though.
If you don't mind, What are your long term fuel trims for each bank?
I can think of one way to check the CHT sensor without pulling it.
Hook up an OBDII scanning tool that can give you live data.
[Have a passenger] Monitor the engine temperature parameter as the engine warms up and when the engine stalls.
You might also want to monitor the MAF and Throttle Position sensor values.
Send me a PM if you would like to try my original MAF. I bought a new one last year in an attempt to 'fix' my erratic idle. No difference. Switching on the AC does fix the idle though.
If you don't mind, What are your long term fuel trims for each bank?
08-31-2014, 11:27 PM
#11
Burning Brakes
Thread Starter
It's Been A while since I posted
It's been a couple of months since I posted anything related to the stalling issues with my car. Unfortunately, my mechanic has not been able to sort out the cause of the periodic stalling.
In the process of troubleshooting the issues, here are the significant things that have been looked at and resolved:
1. Vacuum leaks. For all practical purposes, the slightly rough idle and the intermittent stalling suggest there is a vacuum leak. The car has been smoke tested. All vacuum lines have been renewed and checked. Vacuum has been checked at idle. The motor produces and holds "good vacuum".
2. The ragged running that seems to be in evidence when the stalling occurs comes on as soon as the engine shuts down the air injection pump (about 20-30 seconds into a start). When this occurs, the only issue noted is that the CO readings will diverge on both banks. One bank goes to about 1% CO while the other bank goes to about 4% CO. I am told that both readings are within spec, but that there is quite a divergence that has not been explained.
3. I don't have any data on fuel trims for the car.
4. All injectors were pulled (one at a time) and a new injector substituted to determine any change in engine running. This test produced no changes.
5. Throttle Position Sensor has been checked using a resistance meter, and found to be operating within specs.
6. Cylinder head temp gauge was changed out with no impact on engine running.
7. ECU was pulled and sent to ECU Doctors in Florida. Their operational test on the unit showed no issues. Unfortunately, we have not been able to source another ECU to prove out the original box.
8. We are to re-check the air gap between the engine speed sensor and the flywheel. This sensor was correctly installed, but has not been verified for its 1.0 mm gap.
9. It has been suggested that we change out the DME relay; but, that has not been done yet. We will change it, through deem it unlikely to be the issue.
10. The ICV was replaced with a new unit when the stalling first occurred, and the new ICV did not make any difference in operation.
11. During the build, new spark plugs, engine wiring harness, spark plug wires, and distributor caps were fitted.
So, the issue is still there. In observing the "stalling phenomena", we now notice that normally on shutting the throttle, the RPM of the motor will "slowly" drift down to idle and then a solid and steady idle is established. On a stall situation, the first indication that something is amiss on shutting the throttle is that the RPM will drop very rapidly and the stall occurs as engine speed drops below the idle set point.
As noted earlier, my mechanic no longer charges any time to my car, and is really interested in resolving the issue. But, now that every component has been functionally checked or actually changed out for a known working part, he is now "rethinking" next steps.
In the process of troubleshooting the issues, here are the significant things that have been looked at and resolved:
1. Vacuum leaks. For all practical purposes, the slightly rough idle and the intermittent stalling suggest there is a vacuum leak. The car has been smoke tested. All vacuum lines have been renewed and checked. Vacuum has been checked at idle. The motor produces and holds "good vacuum".
2. The ragged running that seems to be in evidence when the stalling occurs comes on as soon as the engine shuts down the air injection pump (about 20-30 seconds into a start). When this occurs, the only issue noted is that the CO readings will diverge on both banks. One bank goes to about 1% CO while the other bank goes to about 4% CO. I am told that both readings are within spec, but that there is quite a divergence that has not been explained.
3. I don't have any data on fuel trims for the car.
4. All injectors were pulled (one at a time) and a new injector substituted to determine any change in engine running. This test produced no changes.
5. Throttle Position Sensor has been checked using a resistance meter, and found to be operating within specs.
6. Cylinder head temp gauge was changed out with no impact on engine running.
7. ECU was pulled and sent to ECU Doctors in Florida. Their operational test on the unit showed no issues. Unfortunately, we have not been able to source another ECU to prove out the original box.
8. We are to re-check the air gap between the engine speed sensor and the flywheel. This sensor was correctly installed, but has not been verified for its 1.0 mm gap.
9. It has been suggested that we change out the DME relay; but, that has not been done yet. We will change it, through deem it unlikely to be the issue.
10. The ICV was replaced with a new unit when the stalling first occurred, and the new ICV did not make any difference in operation.
11. During the build, new spark plugs, engine wiring harness, spark plug wires, and distributor caps were fitted.
So, the issue is still there. In observing the "stalling phenomena", we now notice that normally on shutting the throttle, the RPM of the motor will "slowly" drift down to idle and then a solid and steady idle is established. On a stall situation, the first indication that something is amiss on shutting the throttle is that the RPM will drop very rapidly and the stall occurs as engine speed drops below the idle set point.
As noted earlier, my mechanic no longer charges any time to my car, and is really interested in resolving the issue. But, now that every component has been functionally checked or actually changed out for a known working part, he is now "rethinking" next steps.
09-01-2014, 07:19 PM
#12
Burning Brakes
I had a similar problem after my rebuild. Car would run fine and then stall after about 20 minutes and I could not re-start until the engine was cold.
As you are doing, I tried all kinds of things and then someone here suggested the flywheel sensor. I put in a new one and that solved it.
You do have some different symptoms however so I am just throwing this out there as an idea.
Good luck and keep us posted! You WILL figure it out!
As you are doing, I tried all kinds of things and then someone here suggested the flywheel sensor. I put in a new one and that solved it.
You do have some different symptoms however so I am just throwing this out there as an idea.
Good luck and keep us posted! You WILL figure it out!
09-01-2014, 08:20 PM
#13
Banned
Just like to put this out there....DME Relay First.
I had just bought my car and was runnin from Kentucky to New York on the I-90. Car Died.
Slowing for a Toll Booth, In Gear, Off the Gas, RPM were Low, Slowing Down - No warning - Car died doing 42 MPH.
I was going to just slide in the Clutch but she Stalled Prior to the Toll Booth ???? STRANDED !!
Different circumstances but a few similarities - Car was Hot.
I Waited 30 Minutes and she Re-Started. Drove to New York.
Let the car sit for 2-3 Hours and it started again.
Took Off, made 3 stops and she died again.
Changed the DME Relay and she ran again - No Issues since.
Now without gettin into the Confrontation Story with the Dealership Mechanic - Oops - I sent my DME to ECU Doctors and Found No Fault.
Mechanic Tested Feul Pump - All OK.
ECU DR - Recomendation - Change DME Relay and if no start then # 2 - Test First
# 2) Crankshaft Reference Sensor.
Test it first using an Oscilloscope - Verify that the wave Pattern that Outputs to the DME is the correct one.
Now I am not a Mechanic, so just sharing Experiences.
Cheapest Fix Going - DME Relay.
Best of Luck - Hang In There! You will find it - u may have already overlooked it ? Said Nah Thats Not It Hmmmm
Tom
I had just bought my car and was runnin from Kentucky to New York on the I-90. Car Died.
Slowing for a Toll Booth, In Gear, Off the Gas, RPM were Low, Slowing Down - No warning - Car died doing 42 MPH.
I was going to just slide in the Clutch but she Stalled Prior to the Toll Booth ???? STRANDED !!
Different circumstances but a few similarities - Car was Hot.
I Waited 30 Minutes and she Re-Started. Drove to New York.
Let the car sit for 2-3 Hours and it started again.
Took Off, made 3 stops and she died again.
Changed the DME Relay and she ran again - No Issues since.
Now without gettin into the Confrontation Story with the Dealership Mechanic - Oops - I sent my DME to ECU Doctors and Found No Fault.
Mechanic Tested Feul Pump - All OK.
ECU DR - Recomendation - Change DME Relay and if no start then # 2 - Test First
# 2) Crankshaft Reference Sensor.
Test it first using an Oscilloscope - Verify that the wave Pattern that Outputs to the DME is the correct one.
Now I am not a Mechanic, so just sharing Experiences.
Cheapest Fix Going - DME Relay.
Best of Luck - Hang In There! You will find it - u may have already overlooked it ? Said Nah Thats Not It Hmmmm
Tom
Last edited by TJ993; 09-01-2014 at 08:25 PM. Reason: typo
09-22-2014, 12:55 AM
#14
Burning Brakes
Thread Starter
Update
First off, thanks to all for all the ideas regarding my stalling issue. I especially want to thank those who continue to encourage me to not give up. It has been a bit demoralizing. I truly love this car, and now that I have fitted it with the Motorsports gearbox it is a total joy to drive. The close ratio box makes the car feel as though it has a large boost in power.
Talking to the shop last week, they are going to have the mechanic assigned to the car backtrack over everything he did in pursuit of fixing the issue. If that does not disclose anything, they will assign the car to a second mechanic who will be a set of "cold eyes" to begin researching issues.
Several of you have questioned long term fuel trim values. I need to pursue that with Sol. Not sure if they have checked them, but I'll pursue that with them.
Secondly, I will request that the speed sensor and DME relay be replaced. To the best of my knowledge, they are original on the car which now has 107K miles on the odo.
Going back to the observed phenomena that when the stall is about to occur, engine speed falls very rapidly when the engine is declutched, which to me indicates that fuel has been cutoff.
And, whenever I do get an engine stall, the motor always fires up immediately whether cold or hot.
As noted before, the ECU got a clean bill of health from ECU Doctors in Florida. But, the ECU is one of the items that underwent modifications during my rebuild. The ECU was sent off to have it modified to adjust for the slightly more aggressive cams installed. And, at the same time, I had them deactivate the immobilizer function. Deactivation of the immobilizer removed a huge PITA issue for me. The interlocks on starting the car were always a huge annoyance to me. As now configured, the car starts without having to activate the system with the key FOB. And, I no longer need to depress the clutch pedal before I can do a permissive start.
So, keep the ideas coming. And, thanks for the support.
Talking to the shop last week, they are going to have the mechanic assigned to the car backtrack over everything he did in pursuit of fixing the issue. If that does not disclose anything, they will assign the car to a second mechanic who will be a set of "cold eyes" to begin researching issues.
Several of you have questioned long term fuel trim values. I need to pursue that with Sol. Not sure if they have checked them, but I'll pursue that with them.
Secondly, I will request that the speed sensor and DME relay be replaced. To the best of my knowledge, they are original on the car which now has 107K miles on the odo.
Going back to the observed phenomena that when the stall is about to occur, engine speed falls very rapidly when the engine is declutched, which to me indicates that fuel has been cutoff.
And, whenever I do get an engine stall, the motor always fires up immediately whether cold or hot.
As noted before, the ECU got a clean bill of health from ECU Doctors in Florida. But, the ECU is one of the items that underwent modifications during my rebuild. The ECU was sent off to have it modified to adjust for the slightly more aggressive cams installed. And, at the same time, I had them deactivate the immobilizer function. Deactivation of the immobilizer removed a huge PITA issue for me. The interlocks on starting the car were always a huge annoyance to me. As now configured, the car starts without having to activate the system with the key FOB. And, I no longer need to depress the clutch pedal before I can do a permissive start.
So, keep the ideas coming. And, thanks for the support.
09-22-2014, 10:22 AM
#15
Rennlist Member
If the DME hasn't been changed yet that would be the first suggestion I'd have. After that I would suspect the ECU. Who did the reprogramming and has it been tested by others? Really need to find another ECU to try.