Engine shutting off on spindown. Please Help :(

[x0054]

I have posted this on Clark's Garage, but no one knows what this might be. Perhaps some one here might. Please help

Here is the problem as it is right now: I have a 1988 944 Turbo. The engine shuts off if while under load (accelerating, but does not have to be hard) I let go off the gas and hit the clutch at the same time, such as you do when braking. In this case the engine slows to about 1000rpm stumbles slightly and dies This happens EVEN if at the time of pressing in the clutch and getting off gas the engine is ONLY going 2000RPM. This happens EVERY time. However if I do the same wile the engine is not under load, like when it's just freewheeling in gear or if I rev it up to any speed in idle and let go, the engine DOES not die Also if under load I let go of gas and then tap it slightly just before it reaches 1000RPM the engine also does NOT shut off. The main problem is while accelerating if I linger in neutral for more then 2 seconds I end up with dead engine. It restarts just fine if you pop the clutch while coasting. And again this does not happen if I was to rev the engine up with no load and then let go.

When the engine dies (split second before it dies) it has about 10:1 mixture of fuel and about 10 mercury inches of vacuum (usually it idles at 18). So it has the fuel and air to burn. I do not know about spark on the other hand.

In trying to pin the problem down I disabled idle stabilizer by setting the TPS so the microswitch would not engage. I even after that set the idle to be at 2100RPM, yet that still did not solve the problem

I also have a light clutch and flywheel on this car however I now ruled out that as a problem because this problem happens even at very low engine RPM, as low as 2000rpm. However when my mechanic installed the new clutch he happened to snap the plastic end on the cycling valve, that was JB Veldted back together. I looked at it recently and it was leaking slightly on the waistgate house side. Sinse I have a manual boost controller that controls the boos pressure I bypassed the cycling valve (it was not doing anything anyway) and now I have a direct, gated system. That, however, did not fix the problem. My mechanic has a nasty message awaiting him on monday. I also checked all of the vacuum line, they are only few years old and are in good condition, no leaks.

This problem has became worse very gradually. It first appeared about a year ago (first I experienced it). But it only happened under emergency braking conditions so I did not care. After the clutch job it has became slightly worse but not to much so. But then it gradually became worse. Now it is really bad I am running out of things to check...

Any other ideas, something I can look at??? Sorry for the long message. Please help!

- Bogdan

[AFJuvat]

I would venture to guess that you have a bad vacuum leak and\or your idle stabilizer valve is shot. readjust the throttle body so the TPS contacts when you move the throttle, then disconnect the ICV and try again.

AFJuvat

[x0054]

I tried that today. No luck I disconnected the ICV, plugged up both holes and readjusted the idle. The car runs fine but dies just the same then coming off load. Absolutely no change

I looked at all of the line, including the vacuum brake boost and the stabilizer lines. I see no vacuum leak. Any other ideas?

- Bogdan

[fast924S]

What about the AFM are there any worn spots on the slides?? What other aftermarket products are you running??

[ae1969]

Ok.......

Unplug TPS.

Adjust idle to approximately 1500 rpm.

Not too much lower.

You will run off the partial throttle maps. No idle maps or WOT maps in this manner.

Let me know if the problem still persists.

[x0054]

Just tried that, unplugging the TPS. I adjusted the idle to about 1700RPM. The problem has NOT changed

I also did several logs of my MAF system. I have a Lindsey Racing MAF with Mafterburner, it works fine and has been on the car for about 1.5 years now, so I doubt that's the case. I also have an independent Wide Band O2 on the car, so it runs in Open Loop all the time and is tuned by the MAF. From the log it looks like the engine has anywhere from 10:1 to 12:1 A/F to burn at the time it shuts off.

When doing the log I pulled out the foot wall cover for the CPUs. Everything looks normal, however at one point wen the car shut off again and I came to a stop I saw just a hint of smock from that area, but nothing is hot, and I can not see nor smell anything burned.

Considering that the all of the idle programming was turned off when I unplugged the TPS I am at a loss as to what is doing this. The engine runs grate under load, accelerates fast and feels grate. But unloading it rapidly stalls it.

Also my battery box is good, no rust at all, so the CPU never saw water.

I don't know what else to check. Any ideas?

- Bogdan

[fast924S]

Run it a wiggle all the wires you can find, Dme wires everything and see if it stalls also check grounds and relays, I wonder if the sudden decel shakes the engine hard enought to shakr something loose for a second causing it to stall. Totally sounds like a wiring problem somewhere

[ae1969]

Well the idea of hint of smoke from the DME is never encouraging.......

Do you have access to another DME?

[x0054]

I just came back from my mechanic, and with no luck. He stared at the car for about an hour, we did disconnect the vacuum lines to the fuel pressure regulator and the damper. That did not change a thing. We also changed the BOV to vent to atmosphere, that also head zero effect. Unfortunately the only 2 951s he head on the lot at the moment ended up having after market brains, one was tec3 and the other something else. So we did not swap the brains. He offered to get friends 951 with stock DME and swap that, but at that point he would start to bill me and it would probably run about $200 to do that. I figure for that, perhaps a bit more I can by a used DME and KLR and swap it my self, and have a spare if DME is ok.

Actually at the moment I suspect that there is something wrong with the KLR. As far as I know this car's KLR controls the spark, and It almost seems like the KLR does not fire the cylinders right. My friend also suggested that it could be the clutch switch. He is a Dodge mechanic and apparently the SRT4s have the same problem and it is because of the clutch pedal switch. So I am going to check that out right now.

If you guys have any other ideas, please post them. At this moment I just checking anything and everything that is remotely related to the engine running.

Thanks,

- Bogdan

[x0054]

Well I checked the clutch switch, and it works fine. I also did several dozen runs with the car and logged them in to my laptop. In looking through all of the logs it looks like every time the engine stalls it registers significantly more air coming in vs. the air coming in at same RPM at the times when it does not stall. Here is how it look. Vin is MAF signal in volts x 1000. A/F mixture is X:1 and the value is times a 100, for the purposes of the range of the graph.

First graph is of what happens when it dies or almost dies. The engine does slow down below 560RPM but my MAF can only record >560RPM.


This one is of a normal slowdown, as you can see the engine does not try to die here.


The only difference is the first graph depicts engine spindown from about 2800 RPM on Boost, the second, however, depicts my engine spinning down from about 4500 RPM (so that the air flow in the MAF is bout the same) with no Load nor boost before hand. As you can see for some reason the engine sucks way more air at 1500RPM in the first then it does in the second graph. I think that is one of the reasons for the stalling, however I can not figure out if the reason for this is a MAF sensor problem or something else. The MAF sensor works fine, as far as I can tell, does not jump or anything like that.

In addition to all this a few weeks ago I head a problem were after standing in the sun all day I started the car and it refused to go, it would lean out the more I pressed on gas and would not want to go past 2000RPM. After shutting off the car and then immediately restarting it everything fixed it's self... Can this be related and what can this be? This has not happen since.

Sorry for such a long message and thanks for all the help,

- Bogdan