2001 GT Miss under load

Hello !

I have 2001 Pontiac Grand Prix. 126000 miles. Maintained well but driven hard. Lately noticed reduction in power and mileage drop. No major components replaced other than plugs and wires. Diagnostics indicate misses on cylinder 1 and 5. Tech indicates more misses occur on cyl 5. It has gotten progressively worse over the last 4 weeks. Just today the engine would get to around 4000 RPM in 2nd and go no further. All the while at this RPM you could hear it missing. Normal driving is good, but sluggish.

So I am thinking coil pack, but the two cylinders that are showing miss are not on the same coil.

Check engine light flashes when the misses occur, but other than that only codes according to tech are the miss.

I am reluctant to put a bunch of money into this car, so I am trying to diagnose this on my own. I also figure it could be low fuel pressure, but if that were the case I would suspect I would have misses on other cylinders.

Any thoughts or ideas?

Spinman

sounds like possilbe bad cat

I agree with Troy.

Rent a vacuum guage if you dont have one from a place like autozone or advance. Tap into unported vacuum, and measure vacuum with the engine at idle, then rev it to 2000rpm's and watch the guage. If it dropps off, chances are you have a bad cat converter.

Since there is a known issue with the converters on the 01-02 w-bodies, I would agree that is your issue. There was an extended warranty offered on these models, but it was limited to 120k miles, which you are now over. I would suggest getting a quality aftermarket hi-flow cat since you're past the warranty period to get it fixed for free.