Ky944TurboNewbie

I have been battling an issue for over a year and getting frustrated. Looking for some new ideas. Here is the link to my thread from the beginning.

https://rennlist.com/forums/944-turb...old-start.html

Right now, the issue is running at WOT. My mechanic says the car is cutting out around 4k to 5k rpm's. When I ask if it still has gas and spark after a cut out, he says it does. You can feather the gas and keep it running for a couple of miles. However, if you give it any gas at all, it kills the engine.

If you cycle the ignition, it starts up and works fine until you hit WOT at 4k to 5k rpm's again. ALSO.... If you don't cycle the ignition key, but shift into neutral and don't give any gas, the engine will die. WITHOUT turning off the key, you can start it up by popping the clutch and it works perfect again until the engine cut is triggered at 4k to 5k rpm's.

The car is completely stock. At this point we have swapped out the main computer with known good ones 3 times. We have swapped out the KLR with a second one. We have tried several different chips in each of the computers. My mechanic is convinced the problem is electrical, but cannot figure out anything that could possibly cause this behavior.

I don't have access to a different car to start swapping out parts until we find one that will work. I need your best troubleshooting ideas. What would cause my car to be fine by cycling the key or restarting. Thanks in advance for putting some brain power behind my problem.

Van

Replace TPS and AFM.

Both might measure up good, but if they've deviated from original specs enough, there will be a mis-match in the info they send the computer. For example, the TPM might not be reading full open, but the AFM is seeing full volume of air, so the computer will interpret that is too much air for the throttle opening (over boost).

I had that happen at the track... short-term fix? Wait for it.... Ziptie a rock to the throttle body.

MK_951

^lmao...that is actually pretty good idea for not hitting WOT. I had the same problem actually but it was the chips I had in the computer. I swapped them out with some Stage 2 Authority Chips I had and the car hasn't given me the problem since.

Ky944TurboNewbie

So if I assume there is a problem with the TPS or AFM, that would trigger the overboost. From there I would expect the car to do what? Does it cut ignition? Does it cut gas? It is my understanding it will be in effect for 60 seconds and reset, or it can be reset by the ignition switch. Is there any way to confirm the problem without swapping out parts?

I don't understand why the overboost behavior requires the engine to die before it resets. Any thoughts on that?

magcuda

I've seen the TPS do this many times

Van

Yes, when my car was doing it, it was cutting either spark or gas (I don't know which... for I was driving at speed...) - my hunch is that it cuts fuel when it hits the rev limiter and overboost. If it only cut spark, you be pumping raw fuel out the exhaust and could cause quite a backfire if the exhaust is warm (and isn't it always warm on a turbo??).

I'd have to turn the key off and then back on to reset the computer (a little unnerving on the race track).

Ky944TurboNewbie

Would disconnecting the TPS tell me anything about whether or not the TPS is working?

Oddjob

The DME overboost protection is a fuel cut.

Disconnecting the TPS will cause the car to have a high hunting idle, poor off throttle response and the KLR will go into "limp" mode, so no boost and retarded timing.

Can run a diagnostic check on the TPS using a multimeter per the factory service manual (turbo supplement). Looking for resistance values across pins and the full load signal to the KLR.



The different chips and DME/KLRs that you have tested, are they all factory/stock chips? Or have you tried any aftermarket DME chip?

Van

When I had this problem on my turbo, both the TPS and AFM measured within spec per a multimeter. I chased that problem for a year - replaced knock sensor, cycling valve, DME, chips, etc. Then I replaced the TPS and AFM and the problem went away.

reno808

VAN i like the creativity.

xsboost90

i had this problem on the track when temps outside were high and my intercooler was heat sinking. It would be ok until i started pushing the car and it would cut fuel usually during a long corner exit and throw the car around. I ended up cleaning the intercooler- lots of dirt between the fins- and after changing to a vitesse tune which deleted the fuel cut it went away, but i also agree the TPS sounds suspect.

Oddjob

Are you thinking the TPS has an intermittent fault where it interrupts the full load signal to the KLR, so the DME abruptly cuts fuel incorrectly thinking the throttle plate is closed? And its not actually triggering the overboost protection function? Because I am not sure where any of the TPS outputs have a direct input to the DME overboost function (?). Thats just a DME function measured thru the AFM.

Ky944TurboNewbie

To be honest, I am just trying to figure out how the TPS fits in as well since it was part of the resolution for Van. I don't think it is an intermittent problem as it will consistently happen at about 4k or 5k rpm's under boost according to the mechanic. My mechanic is convinced the problem is electrical based on his 40 plus years working on Porsche's and other European cars. If I follow his assumption that it is electrical, I need to look at what triggers the overload protection. If I understand correctly, overload protection is a DME computer function only based on the voltage from the AFM and RPM's. It is triggered when the threshold is exceeded for a certain amount of time.

The main computer has been swapped with 3 known good computers and different sets of stock chips. With no change. For good measure we have swapped the KLR computer and multiple KLR stock chips. Everything behaves exactly the same. At this point, I am assuming I don't have a problem with the computer.

What could be the other electronic reason's it would fail. The signal from the AFM could be the problem. Based on Van's suggestion, I was also considering the TPS. I assume the speed and reference sensors could also be the problem.

Speed and Reference Sensors- I have never had an issue with the tachometer, even during an overload episode. Does this rule out the Speed and Reference Sensors? My mechanic has visually inspected the flywheel and the gap for the sensor. (I hope I am passing on what he said correctly.)

Air Flow Meter - AFM might be bad. However, the multimeter tests from clarks-garage show that it is working correctly.

I have also read threads that claim a weak spark is causing overload protection. Is this possible?

I have read a thread where ingition wires were shorting to the fuel injector harness causing the engine to die under load. Is this more than a remote possibility?

Mechanical reasons that could be causing the overboost are likely be a line to/from the wastegate or cycling valve is leaking, a stuck wastegate, some other vacuum / boost / exhaust leak.

I am hoping that my case is different than most and it might point to a likely place to test. When the overload protection is triggered, the engine will die if the clutch is pressed in and no gas is given. However, you can feather the gas a little and keep it going at speed for a couple of miles. (Longer than the 30 or 60 seconds that I keep reading is the time for the overload protection to reset.) The overload protection never resets UNLESS.... Edit - I have since learned the system resets after 60 seconds.

When you cycle the key, the car starts easily and works great until the overload is triggered again.

Additionally, once triggered, you can push in the clutch and let the engine die. Without cycling the key, if you let the clutch out and start the engine that way, it will take off as if nothing ever happened with the overload protection reset.

I am trying to think outside the box to determine what the likely issues could be. I have been reading everything I can about the overboost / overload protection and have run to the end of my imagination.

I am working on a budget and don't want to start throwing parts at the problem until one fixes it. That can be a pretty deep hole. Especially now that I have an independent Porsche wrench looking at it.


Thanks for looking and giving it some thought.

Michael

fast951

Which exhaust are you using? Which wastegate? Does the cutoff kick in when in higher gears only (3rd, 4th and 5th)?

Oddjob

I didnt read your other thread - have you tested the boost level with a confirmed/accurate gage to see how much boost the car is acutally making? Maybe it is overboosting?

Can you get access to a spare AFM and test the car w/ that?

Weak spark would not cause overboost to kick in. Weak spark would cause a high end, high load miss.

Overboost should not cause the car to stall. It will run fine at everything up to 3/4 throttle position or so, and anything above that the DME will cut the fuel again. It will do this for a couple minutes or so after the iniitial overboost condition was detected. Once that preset time period has passed, the DME will reset itself and allow full throttle again.

Do you have a set of aftermarket chips you can try? Aftermarket DME chips do not have the overboost limit on them. If you test a set in your car, and it doesnt stumble anymore, than its an overboost problem. If the car still stumbles w/ an aftermarket chip, then you have another fuel/ignition problem to search for.