New to the forum but I,ve got a real headache and I'm hopeing someone might be able to help. I've got a 92 Z28 305 TPI. It acts just like a clogged converter. Once it gets warmed up or closed loop you lose almost all power. It will rev to about 2500 and very slowly start to drop off and it sounds like it might pop a little through the intake. At idle when it is open loop it is a little rough but i think that has to do with weak injector. At idle when its warmed up it surges from about 750-1800 rpm. Things I've checked: dropped exhaust from manifolds back and inspected cat, checked fuel pressure and volume and replaced pump and filter, replaced plugs wires cap rotor, distributor was replaced as well as the computer before I bought the car. Verified the prom and the computer was correct and checked the coil and module in distributor and checked timing. Could not find any vaccuum leaks. It seams to somewhat of an intermittent problem, once while I was working on it I got a good 20 minutes of really hammering on it and it ran great. Sometimes when it is hot it won't even start, it only stalled once, and it has absolutely no engine codes. Maybe somebody knows something I havent thought of yet. Thanks in advance for any help given.

 

sounds to me like the timing is off. could also be a bad intake manifold gasket. check vacuum at idle with a gauge, it should be above 20 inches. rev it to 2500 and let off and it should read about 25-28 inches. anything lower may be an intake issue. if you visually checked the cat and it looked okay, then assume it is okay. if you want to be sure though, you can remove and o2 sensor and use a pressure gauge to check. anything higher than 5 psi and there is a blockage somewhere. did you run the car with just the open manifolds? the way it sounds though im more leaning to an ignition related issue.

 

The timing is dead on, and the cat we checked, also ran it with open exhaust right out of the manifolds. A good friend of mine has been going over the car trying to figure out what is wrong with it, but I will check with him on vaccuum. Everything in the ignition seems to be ok, last I heard he was thinking maybe in the wiring somewhere like a bad ground. I will talk to him tonight and keep you posted. It is a very odd problem that even the local Chevy dealers really have no idea. Thanks again.

 

the surging idle made me think of a problem that a co worker had on his girl's ram, it had a cracked intake manifold, and sometimes ran great, sometimes ran like crap, and the idle was sometimes all over the place. however, breaking up at high rpm and burping back sounds more like an ignition issue.

 

Piece of advice, for the most part, don't trust the chevy dealers diagnostics.

I brought my iroc in there with a problem. Car was surging like crazy on low RPM. They didn't charge me told me that from their view it was too much money and not worth my time to fix it.


Clearly they didn't even try, as it was as simple as the wrong computer. (Was the computer out of another car with a 350.)

10 bucks they just brought it in, looked at it, asked arouind if anyone knew TPI, noone did, and drove it out.