TT

That is correct. The aftermarket kits include a KLR chip that will not fault when overpressure is detected. Fuel cut overboost protection is in the DME code.

Yes. For your case the VAF is outputting >= 4.5V at 5500 rpm to 4.6V at 6240rpm+ for the triggering period. The time period is determined in the non-realtime part of the code, so it is dependent on which code is executed (simple counter; it increments faster for WOT and higher RPM).

I would go back over the KLR/CV/WG plumbing again. As smokey said, a leak problem can be so slight as to be nearly undetectable.

Oddjob

Ive had the head off the car, replaced the clutch, and replaced/rebuilt the turbo, so the intake and exhaust plumbing has been in and out several times, cleaned and checked. Problem is still there. But I understand and agree with the light bulb analogy and have my own similar first hand experiences. So Im going to spend much of my spare time this wknd retracing the intake and exhaust plumbing, and swapping out some of the components.

I got the fault code LED to work: from the socket schematic in the factory manual, the negative wire goes to the "LED" socket and the positive wire goes to the "A"socket, not the Ub as Ive seen shown.

I took the car out and got it to stumble (complete cutout at high rpm, WOT in 3rd). It continued to stumble at or above 3/4 throttle position until I slowed at a stop sign. After slowing, the car drove fine and when I got it home to check, there was no error blink code. I tried the whole driving test again with the same result.

So according to this, the KLR is not recognizing or storing an excessive boost condition. Also I didnt think of this before, if the KLR identifies a problem, it will run in safety mode (retarded timing, reduced boost) until the car is shut off. It did this when I disconnected and reconnected the TPS (to check for the 4-1 blink code); I had to shut it off and restart to get it to run any boost.

Looks like it could be worth checking the DME chip and/or the AFM, if it is the DME triggering a fuel cutoff. Or as others asked, is it something other than overboost cutout, that is dependent on stock vs aftermarket chips?

TT

I went back and checked the code, the counters are decremented in the timed loop so the overboost protection cuts in if the boost limit is exceeded for about 3 seconds. Overboost protection will remain in effect for up to 60 seconds unless you return to idle, then a lower value counter takes over and clears the condition. Fuel cut only occurs for overboost, redline, and coasting.

I have hit overboost with an aftermarket chip. In first I could get near the 7000 rpm revlimit, in 4th and 5th I would hit overboost at 6500 rpm. I currently have my overboost limits set to values well above those that APE and Dinan use and on occasion still get near overboosting.

I would check the chips again with different stock KLR and DME chips. You mentioned you had replaced the KLR. Hopefully it came with a stock KLR chip for your model car, not a later 88S/89.

Good luck.

MildMax951

TT, you're the MAN, thanks for finally putting me out of my misery, below is a previous post of mine in reply to a listers overboost cut question.

If I had known what I know now, I wouldn't have cut the pefectly intact clear line to the KLR to install a check/bleed valve as an experiment.

Obviously it failed miserably for the reasons you have outlined in great detail, but it's nice to finally know why! Thanks again.

smokey

Oddjob, it seems that on the basis of TT's advice, you need to get on the highway, trigger the overboost, and then continue driving for a while without dropping the rpm's to idle. Then pull off the highway, let the engine continue running, and check the blink code. That's what I did to check the blink code, and now (thanks, TT) I understand why that worked. It sounds like a bad DME or an intermittent vacuum leak that occurs only at WOT.
Have you checked your grounds? It's amazing how many problems occur due to bad grounds. I've had a problem for four years with my heater turning on full blast entering Turn 4 at Mosport. Tried four AC control units, had the system apart many times. Would stop for a while, then return. Think I got it now (knock on wood, this weekend at Mosport will tell): a rusted sheet metal screw on a ground strap that only acted up when everything got hot on the track. Anyway, just a thought.

Oddjob

>"I wouldn't have cut the pefectly intact clear line to the KLR to install a check/bleed valve as an experiment. "

Initially I was planning on doing the same thing (placing a pressure regulator set to 7 psi in the boost line tube to the KLR) based on my thought that the KLR identifies and controls the overboost protection. Glad I didn’t get that far.

>"That's what I did to check the blink code"

When you hit the overboost condition and were able to get the fault code from the KLR, did the car run in the safe mode (low boost) until shut off? Or did it continue to stumble hard at open throttle?

>"...trigger the overboost, and then continue driving for a while without dropping the rpm's to idle. Then pull off the highway, let the engine continue running, and check the blink code."

I could run long wires and route the LED into the passenger compartment to check for a blink series when the stumbling is actually occurring. Otherwise it would be near impossible to check within a 60 second window and w/o letting the car drop to idle (hard to keep the rpms up when stopping and getting out of the car). Or do I need to maintain the overboost stumbling condition for over 60 seconds, in order for the KLR to store the fault?

What Im understanding/guessing, is that the DME is triggering the overboost protection, but the KLR has not identified excessive boost pressure (so the car does not run in the safety mode, nor is a fault stored in the KLR memory). So no matter what I did with my testing, I would not be able to retrieve an error message.

Another odd symptom: when the car drops to idle after the stumbling (at the first stop I come to), it wants to die (had to rerun the test a couple times because it died on its own). It would do this also when I came off the track and slowed in the pit lane. After restart, it runs/idles fine.

>"I've had a problem for four years with my heater turning on full blast entering Turn 4 at Mosport. "

That’s pretty funny. At least you didn’t have your Airbags go off under braking. Blows the sunroof out onto the track and makes an expedient chicane/obstacle to slow down the guys chasing you.

smokey

My problem occured first on the track, and I would lose all power, which I assume was safe mode. The car did not stumble, I just couldn't catch the 914 I had just lapped. I don't think stumbling is part of the safe mode. I went in, shut down the engine, and then took it out on the highway for a test without pushing it, and it worked fine. I could repeat the problem by pushing it (WOT) on the highway, and on the basis of TT's information, it may be that if you keep the revs above idle for 60 seconds, the error message should be stored. (Is that right, TT?) Did you stay above idle for 60 seconds after the problem started? I think I have a record of the actual blink code I had, and I'll check that, but it didn't help much with the diagnosis. Has anyone else ever here gone into safe mode with standard chips and an open exhaust?
I had a major stumbling problem last year, lack of power, black smoke, and back-firing that turned out to be a bad (new) spark plug. Put in a new spark plug, but still down on power. Turned out to be a blocked cat caused by the misfiring. At least I don't have air bags.

TT

Remember the DME and KLR both operate separately except for the WOT and ignition signals. If the KLR registers overpressure, it will hold the code until you shut off the car.

Let me also clarify what I posted earlier, the DME will hold the overboost indication that is set with the fuel cut for the full 60 seconds. The fuel cut itself will clear at set intervals. For those 60 seconds, the DME will immediately reapply the fuel cut if you exceed a lower load limit that equates to about 5psi below the overboost limit for rpms > 3000. The problem that you have seen is that this lower trigger value equates to about -10inHg at idle which the car will likely hit if falling back quickly to idle. Cycling the ignition is the quickest way to recover. I think I will reprogram my DME to clear the overboost indication below 1500 rpm if the load is below 0 bar.

Oddjob

Update:

I swapped the AFM and TPS from another car and the overboost protection cutout problem disappeared.

Not a good test because I changed two items, but Im attributing the problem to the AFM, sending a faulty flow signal (too high) to the DME.

bdellis

Like tony said its the chip.Or if you have a good chip that allows overboosting the you have a vac leak somewhere thats showing its ugly face in 4th and 5th gears at the highest boost/tq range.

I had a bad intercooler boot and some other vac line problems that exhibited the same problems.

Chipster

I had the same problem on the track at a recent DE. My symtpoms were the same. Since the DE was in conjunction with a PCA club race, there was no shortage of Porsche certified mechanic types available. Everyone that looked at the car came away scratching their heads. I even had a mechanic from Fred Baker Porsche fiddle with it - this guy actually did work on the first 951 to come to the US. He was able to make the car hit the over boost by holding one foot on the brake and one on the gas in third gear (boost builds quickly) while out on a local county road. I had recently replaced the TPS and Idle Speed regulater and the wastegate was holding pressure fine. The car also has stock chips and a cat by-pass. After a short time, he diagnosed the problem as a probable vacuum leak - but we were unabe to find it at the track.

I also have a home made blink code tester wich did show an overboost code - so there was no doubt what the problem was.

When I got the car home, I replaced all of the vacuum lines with the Lindsey Racing kit and replaced the BOV - which was leaking badly.

Since then, I have not been able to put the car into over-boost - even using the gas/brake method mentioned above.

I don't know if it was a bad vacuum line or a bad Blow Off Valve, but the problem appears to be solved. I'll be on the same track again in October, so that will be the real test. I'll post the results.

Chip

competition

Were you guys able to sort it out? I am having the same sort of issue. I am running strong for about 20 laps and then all of a sudden cars I could pull on (G class) start pulling on me. It seems to be either some sort of timing, overboost issue very similar to that of Oddjob. Just switched the afm. Any ideas?

Oddjob

I never did solve my overboosting problem for sure. Since that time, I have another engine and almost the entire exhaust and electrical system from a parts car, so I will never really be absolutely sure what the problem was.

Originally, I had first assumed that it was some error/fault with the control system because I did not believe the car was actually overboosting (since it was down on power). But since, I think it may have been running a little too much boost, but the compression/leak down was bad enough, that it still felt down on power. The original waste gate that I checked (just recently) does have a leak in the diaphram, which keeps it from opening fully - so that could have been the source of the overboosting problem.

There are quite a few variations of boost and power loss problems on a 944T. Im not sure what your exact symptom(s) or problem is. The overboost cutout is a dramatic boat anchor type stumble. If your car is running fine, then seems to just be down on power - I dont think that is the overboost protection. Is the car down on boost (running about 1.2-1.4 bar instead of 1.7-1.8)? If so, that sounds like the limp home mode, in which case you can read the fault code (blink code) to determine what the computer identifies as the problem. It also could be the KLR detecting knock (or incorrectly thinking its detecting knock), and retarding the timing and cutting the boost off a little. The loss of power would be somewhat similar to the limp home mode, but probably not quite as dramtic of a power loss (unless it thinks the car is continuing to knock on all cylinders, so it would end up dramatically retarding the timing and cutting boost).

First thing, outside of checking for various vacuum and exhaust leak issues, I would try to get the car to repeat the symptom, and then try to read the KLR fault/blink code. If there is no fault recorded by the KLR, you could try to test some of the other components/systems like the knock sensor, KLR/DME, cylcing valve, Waste Gate, AFM, TPS, etc.

competition

Oddjob. thank you for the reply. I was hoping you would respond since I did a search and it appears you had a very similar problem.
BACKGROUND
The engine has approx 2 hours on it since an entire rebuild. I am running a cat bypass otherwise it is stock including chips, to conform to PCA rules. The symptom is that after about 20 min at limerock a short track on the uphill in 4th under full boost, the car lost power. If I had to guess I was down 30 hp. Originally, I thought one of the tubes had blown off. Not the case. There was no stumble or bucking just down on power. I came into the pit and put in some 100 octane fuel. Next session I went out and car was fine again until the end of the session, then the same thing again.
DIAGNOSIS
I was thinking it had to be some sort of leak but why would it only happen at the end of the session? (except maybe heat it was about 75 degrees). That is why I thought it was overboost protection which reset itself when I turned the engine off. I will check the wastegate, cycling valve, knock sensor. I truly appreciate any of your ideas. Thanks again.

Oddjob

Im assuming you did not have the problem before the rebuild? Any electrical component or wiring replaced during the rebuild? I would certainly look hard at the entire intake/vacuum and exhaust systems for leaks since all/much of it would have been removed for the rebuild.

But since it sounds like your problem shows up after running hard, it may be related to something getting heat soaked - after it cools off, then it runs ok (?). Until it gets hot again...

Next time it happens, you should pull off the track, leave the car running, check for the KLR blink/fault code. Then shut the car off, and immediately restart it and go back on track to see if the problem either is gone or is still there. If its still there, then its likely something getting hot and causing some problem/error. If its gone - its a fault detected by the KLR (which should also show up with the blink code test light), which gets reset after shutting the car off.

Unfortunately, I know how difficult it can be to perform any trouble shooting on the street (when the problem typically shows up only during track speeds/driving) - which means you may have to wait until the next track event. Try to have a good source of spares and diagnostic tools with you at the track, and maybe you will be able to find and fix the problem. Another option would be to spend an afternoon at a dyno shop and running the car under loads - maybe that would be capable of reproducing the problem.