Oddjob

Ive seen several threads in the archives that touch on this issue, but I never quite found a solution:

Car is an 87 turbo running stock chips and open exhaust.

Im hitting overboost protection above 5k rpm in 4th gear. Secondary boost gage indicates between 10-11 psi peak and drops down to 9-10 at higher rpms, which is right in the factory spec range and shouldnt cause the shut down.

It only hits the overboost on the track (probably because I dont do a lot of driving at full throttle above 5k in 4th or 5th on the street), so it makes trouble shooting difficult.

Ive swapped the KLR box and that did not solve the problem.

It does not have the overboost symptoms when running APE stage I or II chips (which it shouldnt)

Any ideas or solutions?

TonyG

The answer is simple:

It's not your KLR box. It's your KLR chip.

You are hitting the stock overboost cut-off with the stock KLR chip.

With the APE chips, the overboost protection is not there, which is why you are not experiencing the overboost protection.

Solution: change your chips (but beware of running lean unless you have the proper setup to run the boost you are going to run).

TT

Overboost protection code is in the DME. IIRC a stock 87 951 with the K26/6 would normally be down to around 7-8psi at redline so you are exceeding the boost limit programmed into the stock DME chips at the higher rpms. You will not experience this with the APE chips since they set the overboost limit to the maximum value all across the board basically eliminating overboost protection. Run the APE stage I chips if you have them.

Oddjob

The car is setup to run PCA club racing in the stock class, which means stock power (stock chips). So I have to make it work as is. There is nothing done to the car to increase boost, except reduced back pressure with an open exhaust (standard 2.5"). Ive had two other turbos with the cat out and never had them hit overboost protection, plus there are a ton of guys in PCA Racing with the same setup.

With the boost the car is currently making, I don’t think it should be hitting overboost protection. I think the factory spec converts to around 8.5 psi above 5000rpm, but Ill check it again. Im seeing maybe 9psi at that rpm range. If it is running too much boost, the only thing I can do to reduce it, is put a cat back on.....

The other problem, I think unrelated, is the car is down on power. Nothing dramatic, but its not pulling like a car that is pushing too much boost. Down about 5-8mph at the end of the front straight and 200-400 rpm lower at known short shift points.

I tried the APE chips just as a trouble shooting test. I know they do not have the overboost protection mapped into them. If the car would have the same stumble/shut down with these chips, the problem would have been something other than the factory overboost protection (which is still a possibility). But the car ran fine with these chips, unfortunately, I cant use them for racing.

I swapped the KLR box (complete), to see if the boost pressure sensor in the original box was failing and reading too high; telling the KLR computer the car was overboosting even though its not. But again, that did not correct the problem.

I am not aware of any other component failure or error that will cause the car to trip into the overboost protection mode. I know the DME will cut the fuel in this mode, but it has to get the signal to do so from the KLR box, correct (?), thats where the brain reads the pressure.

Flight_951

Are you certain it's overboost protection?

Could it be that there's not enough fuel by 5000rpm in fourth gear? Fuel pump, fuel pressure, and injectors up to specs?

Oddjob

Well Ive learned never to be 100% certain of anything, especially with this car (came with a lot of gremlins). But Im pretty sure thats what is happening.

Symptoms: dramatic cutout under full throttle at high rpms in 4th gear. Once it hits the cutout, the car will stumble/cutout anytime the throttle is opened more than 3/4 while in midrange rpms in any gear. Car has to be shut off and turned back on to run normal again.

Since I dont do a lot of full throttle driving in 4th and 5th gear above 5k rpm on the street, Ive only encountered this problem on the track, which makes trouble shooting more difficult. And its does not always happen. Sometimes it will happen the very 1st time I hit those conditions (above 5k in 4th, full boost), other times I will get several laps or even sessions in before it cuts out. Probably weather/temp dependent.

I am making an assumption, that if it were a fuel delivery problem, it would happen with after market chips too, which it doesnt.

But Ive been dealing with this problem for two seasons now, and the weak power since the first event this year. So Im nearing the point of swapping every component until both problems are resolved.

Im usually up for a challenge, but this one has me stumped. Any and all suggestions appreciated.

HansB

I have got the exact same problem. Boost set to 0.9 bar with MBC and stock chips. Will change them and let you know what is the result.

Frank Holland

Same problem over here! Boost is set to almost 1 bar, no cat, 3.0 bar FPR, Lindsey dual port waste gate and stock chips.
I have a AFR meter in the car but wasn't paying attention to it before the problem occurred.
I think the car is running to lean and the computer (KLR/DME) stops the ignition. After that has occurred it keeps cutting ignition when the boost hits approx. 0.4 BAR.

It happened when I floored the pedal in fourth while driving @ approx. 2000 RPM. Perhaps the fuel pump isn't delivering enough fuel!?

Oddjob

Well if you have the boost dialed up with a mechanical pressure relief valve set higher than the stock boost curve, stock chips will definitely trigger the overboost protection.

I rechecked the manual: for the K26/6 cars (non S) the factory spec is 1.75 bar absolute pressure at peak (around 3500 to 4000 rpm) and then it drops to 1.52 +/- 0.03 bar by 5800 rpm. That converts to 7.2 to 8.0 psi overpressure (gage). So Im guessing the overboost will kick in somewhere above 8.0 psi at 5800 rpm. But Im not sure where it will shut the car down at lower rpm.

The problem Im having is the overboost protection is cutting the power out and I have not done anything to the car to increase the boost.

Ive done some more trouble shooting and I can get it to kick in, in 3rd gear. Probably about every other run it will cut the power, usually just above 5000 rpm. At that point my boost gage is showing about 9.5 psi.

Im trying to extract the fault code from the KLR memory but havent got a good blink code. Has anyone tried the home made LED tester shown either on Lindsey Racing's web page or Dan's Racer X? The photo in Lindsey's page does not match the connection points shown on the Racer X page. Which is correct?

jimbo1111

Could it be that you detecting knock instead of boost protection? If so check the fuel delivery. pump,fuel filter, vac leaks at the fpr. (that hose always pops off)

TT

The DME handles the overboost protection itself with no input from the KLR. The 87 stock chip overboost curve goes from 0 psi at 1500 rpm to 10-11psi from 3000 - 5000 rpm and drops to around 7-8psi at 6240 rpm. Overboost protection (aka fuel cut) goes into effect when boost exceeds the limits by ~1psi for a specific period of time (the period changes with rpm).

The KLR is programmed to keep the boost level below the overboost protection levels, so you would want to check that it is operating properly. Make sure you have a stock KLR chip, check the vacuum/boost line going from the manifold to the KLR, all the lines going to/from the cycling valve & wastegate, and the cycling valve itself.

Oddjob

For various reasons and maintenance, Ive replaced the fuel filter, fuel press reg and damper, cycling valve, KLR-manifold boost tubing, and KLR box in the past 2 years. All did not have an effect on the overboost symptoms. Fuel press is at 2.6-2.7 bar at idle w/ the vac line off, 2.3 bar w/ it on, which is ok.

>"Could it be that you detecting knock instead of boost protection?"

If the KLR detects knock, it should retard the timing (6 deg if I recall) and reduce the boost to approx 0.4 bar (1.4 absolute). Which is different than the symptoms Im having. But I am still trying to retrieve the error/fault blink code from the KLR, which can identify either a knock or overboost problem.

>"The DME handles the overboost protection itself with no input from the KLR. "

So if there is a fault in the control system causing a premature trigger of the overboost protection, it could be in the DME/DME chip? If the DME alone controls the overboost protection, then for testing: replacing the DME chip with an aftermarket chip will eliminate the cutoff, not the KLR chip?

Pin 3, 31, and 32 on the DME are signals from the KLR, and pin 5 on the KLR is the pressure indicator output. So the DME is taking the pressure output signal from the KLR, determining an overboost condition exists, then cuts the fuel; rather than the KLR telling the DME to cut the fuel because the KLR measured and identified the overboost? The KLR does store the overboost fault code.

The other possibility/probability, is that the car is actually building too much boost (which I had originally dismissed because its down on power). And the cause could be a sticking waste gate, a CV staying closed or blockage/leak in the line to and from, or a KLR fault. Since Ive replaced the CV and KLR and inspected the lines, Im leaning toward the aging waste gate, but it seems like an unusual symptom for a waste gate failure.

Anyone had a waste gate go bad where it held boost 1 psi or so too high at high rpms but actuated normally under other conditions?

TT

You did replace the stock DME chip with an aftermarket one and reported that no overboost cutoff occurred so it would seem the boost control system (KLR/CV/WG) is allowing more boost than stock. If you are sure about the banjo bolt, lines, KLR and CV, that only leaves the WG. I agree that seems unusual but you never know. One other item to check before pulling the WG is swapping in another stock DME chip just to be sure the one you have is 100% OK.

Pin 3 on the DME is the WOT signal that normally comes from the TPS on a 944, but comes from the KLR on the 951. DME pins 31 and 32 are the timing signals, out/in, that are used for ignition retard during knock detection. Pin 5 on the KLR is for driving the boost guage on the instrument cluster and never makes it to the DME. The DME looks at load to determine overboost. Load is air flow/rpm and is related to intake manifold pressure. The KLR's overpressure detection is separate from the DME's overboost protection, so do try to get a code from the KLR to verify the overboost (jumper an LED between the 12v and LED pins on the test connector).

smokey

I had similar symptoms in my car about two years ago. It turned out to be a cracked spigot on the cycling valve, causing an intermittent vacuum leak. The cycling valve was working (you could hear it click). Check the vacuum line from the cycling valve to the wastegate, and all the connections. As to the blink codes, get a diode from Radio Shack, and plug it into the appropriate test points when the problem occurs (after coming off the track), as per the workshop manual. You have to have the correct polarity. In my case, the blink code showed an overboost condition, but it took a while to figure out why. On an unrelated note, I once checked out all my house wiring after a light wouldn't work after I put in a new light bulb. Turned out the new light bulb was faulty. Try a new cycling valve with a new vacuum hose.

Oddjob

>"You did replace the stock DME chip with an aftermarket one and reported that no overboost cutoff occurred"

I had replaced both the DME and KLR chip, assuming the protection mapping was in the KLR chip, not the DME chip. For my own understanding, I was clarifying that for this test, there was no need to replace the KLR chip because the protection is controlled only by the DME chip, correct?

Unfortunately, an 86-87 stock DME box/chip is about the only thing I dont have a spare for.

>"The DME looks at load to determine overboost. Load is air flow/rpm and is related to intake manifold pressure.

So the protection mode is triggered by exceeding a certain air flow level (measured by the Air Flow Meter) over a set period of time, through a given rpm range?