Erroneous "overboost" situation... HELP!!!

[carolinatrophy]

I’m experiencing a problem with my 1986 944 Turbo, and I can’t for the life of me figure it out.

I think I am getting an erroneous “overboost” shutoff in the car. When I first bought the car last year, it felt smooth and quick, and I was getting a reading of about 1.8 bar on the factory gauge (0.8 bar boost) at peak. Now I am getting about 0.6 bar at peak (1.6 bar on the gauge), and the car obviously doesn’t feel as quick anymore. But what’s MUCH more irritating is that under prolonged full-boost conditions (when I am accelerating hard up a long hill in 4th gear, for example), the car will just shut off, as if it’s sensing an overboost condition.

The car is definitely NOT overboosting. In fact, as I said, I think it’s slightly down on boost from where it was. But the reason I’m pretty sure it’s an overboost cutoff is because I installed a set of Lindsey Racing chips, which defeat the overboost cutoff, and the problem immediately disappeared. Unfortunately, the Lindsey Racing chips were horrible in a lot of other ways, and I got all sorts of driveability problems and hiccups in the power delivery, so I took them back out. As soon as I put the stock chips back in, the car runs smoothly, but that overboost shutoff is happening again.

The car is stock except for a Lindsey Racing 3” exhaust and cat bypass. The bypass valve (blow-off valve?) is a brand new OEM part, and it has had no effect on the boost or the overboost problem.

Any idea what I should check? Is there an actual “overboost sensor” or is that simply a function of the circuitry of the KLR computer itself? Any thoughts or ideas?

Thanks so much in advance,

[rberry951]

Hey Paul,

Before when you were getting 1.8bar on the gauge and it dropped to 1.6bar could you 'feel' a difference, or did it feel about the same?

There is a MAP sensor in the KLR that the white vacuum line is connected to, sometimes these sensors go bad, but usually exhibit different symptoms than what you describe. Another possibility is a partial blockage of that line. You can take the line loose at the KLR and on the intake and blow air through from the KLR end to remove any oil/debri/gunk that may have found its way in there if it is simply an erroneous reading.

If you can indeed actually 'feel' a difference in boost then it is possible that you've developed a boost leak on the exhaust side as the overboost protection is incorporated in the DME over specific values on different RPM ranges.

And last, but certainly not least is the possibility you have a blown or leaking head gasket. There is a chemical you can put into your coolant tank that will turn colors if there is any exhaust gas in it, great head gasket troubleshooting tool, though I forget what it is called.

Regards,
Russell

[carolinatrophy]

Russell,

Thanks very much. I can definitely feel a drop in power when the boost dropped. The car is significantly less quick. I'll check the KLR line today, and I've checked the head gasket and exhaust and all seems okay.

The other thing I was thinking was a possible problem with the cycling valve. Maybe a vacuum leak there, or a failure of the nipple on the line that leads to the wastegate, or maybe even a failure of the valve itself...

But the KLR line is a much easier check than the cycling valve - that thing looks irritating to reach! Particularly for guys like me with huge hands... I'll keep you posted!

Thanks again,

[nynor]

mine was doing something similar when i first bought it: ran great up to about 5800 RPMs and then it was like the engine shut off until the RPMs dropped again, but only when it was under boost. if i took it to red-line gently (no boost, just controlled acceleration) it didn't have a problem. also, when it cut out, it would backfire, LOUDLY!

i ended up troubleshooting the fuel delivery and a whole bunch of stuff and then, on a whim, traded out the computer and KLR and the problem was gone.