pcar964

I'm having a strange problem with my '91 Turbo. The car is completely stock except for Tial WG with 1 bar spring, and cat delete pipe.

Lately, if I'm accelerating through the gears at full throttle, the fuel will cut off abruptly soon after I shift. In other words, everything is fine as I accelerate to 6k rpms in 2nd gear, but shortly after I shift to 3rd the fuel cuts off abruptly. Is this an overboost situation? If so, why does it only happen after I shift into the next gear?

Here's another strange thing - if I accelerate to 6k rpms in 2nd gear, shift to 3rd, but then wait for 2 seconds before getting back on the throttle, everything is fine. It seems to only happen if I'm back on the gas in the new gear quickly. Is this a wastegate problem? A Blow Off Valve problem? If it is an overboost situation, it would appear that the boost builds up between shifts, and if I'm back on the throttle too quickly it overboosts. I don't want to drive the car until I know what's possibly causing this, I don't want to damage it!

Thanks in advance to all the gurus!

turboholic

sounds like overboosting. the engine probably wont reach full boost in 2nd (revs climb too quick), also at 6k rpm the boost goes down, so when you shift into the next higher gear you may hit the boost limiter in mid rpms which results in a fuelcut. Try to drive the car at 2000rpm in 3rd or 4th gear and let a friend keep an eye on the boost gauge, you keep an eye on the street. Then give WOT and see what happens. If it cuts off between 3-4,5k rpm then you have a overboost problem. If thats the case, swap the 1bar spring against a 0,7bar spring and get yourself a boostcontroller.

If the problem only occurs while at WOT and after shifting through the gears quickly, there might be a problem with the BOV as you mentioned. Do you have the stock plastic unit?

Metal Guru

If you have a boost gauge you'll see a little spike when the throttle shuts but that wouldn't be enough to trip the cutoff switch (switch is made at approximately 1.4 bar).
On my car if I let off the accelerator abruptly while at full boost the deceleration can be violent. Is that what you are feeling?
Check the ground for the switch. It's the brown wire that's bolted to the drivers side of the intercooler with a 10mm hex bolt. Make sure it's tight.
Otherwise you might have a faulty BOV.

dholling13

Problem: Free spooling turbo created by no cat with crazy boost spikes in your intercooler where the over boost switch was moved from previous year turbos; In a 930 the switch was located in the intake, less crazy boost spikes.

Solutions: Buy another over boost switch ($68 at a dealer) and move it inline with your ez unit and your line that runs from the wastegate to the intercooler. You can use the brass brake line pieces from pep boys to make a "T"; connect them with teflon tape. The over boost switch will require a ground which you can connect to the one 10mm screw on the intercooler. Then leave your old switch in the intercooler and connect the wire to your new properly placed boost switch.

Metal Guru

Drew, if the turbo is freewheeling, then the bov is open (assuming it's not defective) and there is no boost to bother the switch, correct?

dholling13

In theory yes; but I installed a brand new 996 blow off valve and later an Evolution and still had the same problem.

pcar964

hmm, thanks to all for the ideas. I hope it is something as simple as a faulty BOV, it would seem to be the most straightforward diagnosis. If the boost cutoff shuts fuel down at 1.4 bar, I have a hard time believing I'm reaching that much boost at the intake - wouldn't the car feel noticeably faster? It doesn't feel (seat of the pants) like it's overboosting that much.

So do I understand this correctly - that the overboost switch is at the intercooler, which is a very conservative place to locate the switch, and overboost at the intercooler doesn't necessarily mean all that boost is getting to the engine? And also, which BOV do you all recommend for me to replace my stock unit?

Thanks!

dholling13

Evolution BOV; but you still might have the same issue...

Turbohead

Those over boost switches are flaky and can and usually do trip before a 1.4 bar spike. I had to change 3 of them until I finally got one that worked.
That said You can determine if the it is the switch easily.
Take the wire off it and ground it .Do the same routine with the shifts. if there is no fuel cut than you know for sure thats it.
Real boost gauge tapped at the throttle body is a good Idea to see what really going on.
An EBC is even a better idea.
here is mine.
Elliot

Colorado964turbo

Make sure the connection is tight, and that the wires are long enough. Since it is happening when you shift, the wire may be working it was loose, just a tad which is enought to cause the "overboost" function to kick in.

Bill

pcar964

Thanks for everyone's input. I think I'll go the route of replacing the overboost switch, and replacing the BOV. I will post the results.