Hello, I need some help. I have a 1999 Monte Carlo V6 3.8L with about 198K miles. I am having a major issue with the car. It is complicated. It is doing many things from stalling out while driving, to bad acceleration sometimes leading to stalling, to rough idle, etc. It just seems like it loses power after driving it for a little and dies. Things like using AC/Heat makes it run worse. I just changed the fuel pump/sensor and fuel filter. It has recently new spark plugs and wires. I had the ignition module and alternator tested and both are good. I unplugged the MAF and took a test spin and seemed to run a little better, but then died when the put the car in park. Can someone shed some light to me? Also, my check engine light is NOT on. Sometimes the car will run fine (usually first ride of the day). But I am lost. I have read so many possibilities from O2 sensor, to Throttle Body, etc and would see if anyone could help me out.

 

Sounds like you could have an issue with the crankshaft position sensor

 

Current mpgs?

 

My gas mileage is not too great at all.

 

Check coilpacks, had some of the same issues on my l67

 

When is the last time you did/had a tuneup?

I'm also leaning towards battery and alternator...
You said it runs worse when you turn on the AC/Heat... Sounds like its a loss of power... That's where I would start... Did they do just 1 test on the alternator? Probably should have it tested multiple times... You could have a failing diode in it...

Whens the last time you put a battery or an alternator in it? or is it all original equiptment?

I'm thinking its a multiple of things..
tuneup, alternator.battery and or possibly a failing fuel pump...have you ever changed the fuel filter? Have you run it on empty for any length of time... could have sucked up the residue in the fuel pump thus causing it to start to fail... Check the fuel pressure at the fuel rail...that's usually a good indicator of a fuel pressure problem/pump failure.

It could also be dirt clogged injectors...

I say start with the tuneup/ and battery and alternator...
especially with that amount of miles on it...
Maintenance is key to keep our Montes running in tip top condition!

 

Catalytic converter is my vote. Had same symptoms on my 03, particularly loss of power and stalling/dying after driving a few minutes. Got better when I unplugged MAF for a day or two but then it didn't matter if MAF was plugged in or not. It started slowly then over the course of a week or a little longer the problem got so bad I barely got the car to the shop 2 miles away.

 

I agree..