Stalls and back fires
 

I have a 2003 monte carlo ss with 3.8 series 2. The motor locked up so I had a shop put a series 3 upgrade kit in and then do a top swap to add a M90 Eaton supercharger. The motor was bored 30 over and added 32 lb fuel injectors. The shop said they flashed the computer to the motor add ons so there was no need to replace the stock computer. The car idles fine and runs but if I apply more than half throttle the car stalls out and back fires. I've called the shop but they are now out of business and everyone else says they will not touch the car not knowing all the specs of what was done with the computer. Someone advised me to go to a hotter spark plug so I went to the ngk 55's. No change. It's like the car gets too much fuel and not enough air or fire. I'm at a loss here. Has anyone else experienced anything like this or have any ideas? Please I need help.

It was a mistake to bore it over, from what I've read.

I'd suggest getting a tune from somebody who knows about these cars. You could start with a vendor like ZZP or Milzy or Overkill, but be sure that you know everything that has been done for a fact.

Your issue is likely tune related, especially if no CEL is on.

I agree with Nighthawk. And make sure you tell them that you trans is still stock! The SC-MC has a different final gear.

at what gap did you set for your plugs?

What is a series 3 upgrade kit? I could see doing a s3 uim, but obviously that wouldn't work with a supercharger.

Has it ever run correctly?

What part of the country are you in?

Do you have any way to post a scan of the pcm data?


If it has never run right, id lean towards a tuning issue -maybe the tune is so far out of whack that over half throttle, the pcm just can't adjust anymore and it starts going lean/rich.

If it did run right at some point, but now doesnt, id think it was a mechanical failure, maybe a sensor not reading right or a failing fuel pump.

Hard to diagnose without some sort of data though, a pcm scan of the major sensors would be the most helpful.

I'm also not buying the spark plug thing. If you think at all that it could be that, put stock spark plugs in it gapped just like stock. That thing should be able to run more than half throttle with any normal replacement spark plug unless gaps are insanely out of whack or you're not even on the right planet for heat range. It sounds like it's just a stockish l67, so stock replacement plugs are just fine.