Thread: Low boost, stumble at low RPM, and backfireing.

I have a 02 Grand Prix GTP. I had rebuilt the lower end of the motor and replaced all the gaskets except the head gasket. I am having problems with the car. When the car is at low RPM's and I only give it slight throttle, it wants to stumble and loose some RPM. I am having a problem with the boost. When I first got it back together, I didnt seem to have a problem. I am nearing the 500 mile mark on it and I had slowly lost all boost. I did get some of it back. after messing with the supercharger bypass actuator. It does appear to be sticking. I got it somewhat freeded up and when I get past the spot it binds it does give me some boost, bt only half boost at most. Backfires on heavy deceleration. Their is also a exhuast leak somewhere, but theirs always been one since I got the car before the rebuild. Vacuum seems good and I get a smooth idle. No codes. I did do the relearn process with a tech 2 we had at work. But this was done after I lost all boost. It did say that it learned correctly. Maybe I did something wrong or the Tech 2 wasn't properly updated for my car? I do remember when I did the learn procedure their was some discrepancy about RPS.
Anyways, what are your thoughts on the cause and fix? I don't have very much experience or background with supercharges

Hopefully you had a machine shop do the proper lower end work and not just install new bearings and call it good.

That said, what spark plugs are you running? Assume you're running 91+ octane fuel. Sounds like a vac leak to me. I'd also fix that exhaust leak as it could be causing an issue. How long ago have the spark plug wires been changed?

Actually I replaced the crank, the journals looked pretty bad. Plugs and wires are new. Cant remember what plugs I used, it was whatever was required for it, I remember they were expensive. As for the fuel goes, no I been running 87. Wasn't aware I was supposed to be running 91+. Never had any drivability issues before I rebuilt the lower end.
The throttle response has really been getting bad now. At lower RPMS it does take some pushing on the throttle before the car wants to go instead of hesitation/stumbling.

Definetly need to run 91 or greater octane at all times or you risk tearing up the motor, especially under boost. As long as the plugs are NOT Platinum plugs then you should be fine. Stock irridium or copper only.

I'd check for vac leaks even though you say its not showing any signs of one. Often the "T" under the supercharger snout likes to crack and cause issues. I'd fire it up and hit all the vac line connections with some carb cleaner and see if the motor stumbles/changes tone. If so then there's a leak.

When you replaced the crank and main bearings, did you have the block align bored or just stick a new crank in it?