Mike951

'86 951

so i've been trying to figure out this problem for a while now...when getting on the car hard, I am experiencing a violent stumble under boost, it almost feels like what I would imagine overboost cutout to be. today for the first time I experienced a cloud of smoke, unsure of the color as it was kinda dark out...didnt seem to be too black though. this stumble is very random, sometimes the car is fine, it seems to only happen if i am under heavy acceleration...will not happen at the same RPM with light acceleration. The car has new plugs, wires, cap, and rotor, so I dont think those are the problem. Also, recently, my fuel pump has been getting very loud, a very loud electrical sound, as if it is workng too hard, i dont know if this could have anything to do with this stumble problem?

any help would be good

thanks
Mike

black944 turbo

Are you running chips? Are you running an AFM or Maf?

RKD in OKC

Is the full throttle stumble like you turned the car off then back on real quick?

Overoost protection keeps you from accelerating for a full 30 seconds.

If it is like turning the car off then back on check your engine ground strap. I had a violent stumble only on full throttle acceleration after a new clutch. The engine ground strap didn't have enough slack. The torque of full throttle acceleration was moving the motor enough the pull on the ground strap causing a loss of connection. It would also stumble in "at the limit," high G right hand turns as I applied throttle on exit. The stumble in turns was more repeatable than the full throttle stumble.

I had to take my mechanic that did my clutch out to show him the stumble as he wouldn't push the car hard enough either under acceleration or in a turn to get the stumble. The stumble in a turn upset the balance causing the car to move out in the turn about 3 feet. Unnerved the mechanic enough that he says he will never ride with me again.

Mike951

No chips or maf, doesnt feel like car is shutting off, it stumbles then when i get off the throttle it comes back.

Maybe I should clean up all the grounds? Could that be causing this?

Does anyone have a diagram of where all the major ground connections are that would be good to clean?

Thanks
Mike

testarossa_td

TPS
http://www.arnnworx.com/tps-repair.htm

black944 turbo

Could be TPS or check vacum lines. How much boost are you running? If more than say 13 than probably overboost protection. How much vacume are you pulling at idle?

TRWright

Check all of your vac lines. My car showed the same symptoms when it was pretty much stock. The hard plastic line that went into the KLR had started to break right before it went into the KLR. The car stumbled like it was only running on 3cyl and threw out lots of black smoke under mid-hard acceleration.

Tech575

Since you said the plugs, wires, cap, and rotor are new....could be the TPS as mentioned above, or possibly a dead spot in your AFM. I had a similar problem and it turned out to be my AFM. There is a procedure on Clark's Garage for testing it.

reno808

could be ur coil.

testarossa_td

For AFM info

http://www.frwilk.com/944dme/afm.htm

Jimbo951

Check the pressure pipes between the turbo, intercooler & throttle. I had a hose split, and whenever I would get above 1.2 bar or so, the hose would open spill out a bunch of air, and then the CPU would think something bad had happen, turn off the engine. Once the pressure had stabilzed (withing 1-2 seconds) the engine would try to develop power again. This resulted in very, very violet bucking.

This happened at the track, and within a few minutes of trying to run WOT, the car was undrivable. Not sure how this problem would progress on a street car.