2005 Pontiac GTP Grand Prix issues

**98-LS1** · 08-21-2006, 10:18 AM

I know this isn't related to LSX cars but is GM and some of you may know something to help me a little. My GTP has been acting slow lately, meaning at WOT close to 4,000 RPM's it cuts power almost like the computer is retarding the timing. It feels somewhat sluggish down low as well, the car runs smooth and gets great fuel mileage but doesn't feel like 265 supercharged HP to me. The monte carlo with the N/A 3.8L felt faster than this GTP, what could be the problem? Does the supercharger just make low boost from the factory or is it something else wrong? HP wise it shouldn't be much slower than the GXP with the 5.3L but I got a feeling a GXP would handle this car fairly well. I have under 9,000 miles on the car currently, all services have been on time and it was broken in correctly. Thanks

**SSpdDmon** · 08-21-2006, 11:14 AM

If it's detecting knock, it will pull timing. Peak torque is where you're most likely to see knock due to high cylinder pressures, which is roughly 4,000rpm for you I'd guess. Are you running premium gas? Is it blended with ethanol? If you got a couple tanks of bad gas, that'd be enough to do it. Try running a couple tanks of good premium gas without ethanol through it and see if that helps any. If there were other issues, you should have an SES light. Check the air filter too to make sure it isn't clogged with anything. If all else fails, take it in to the service department since it's still under warranty and complain about it. They can probably scan it with a Tech-II and see if anything comes up in the PCM. Also, keep in mind the warm temps of summer can slow down a car by a good .2 to .4 seconds in the quarter mile. If you got used to driving the car when it was cooler out, that could be a part of it as well.

**98-LS1** · 08-21-2006, 11:35 AM

Originally Posted by SSpdDmon

If it's detecting knock, it will pull timing. Peak torque is where you're most likely to see knock due to high cylinder pressures, which is roughly 4,000rpm for you I'd guess. Are you running premium gas? Is it blended with ethanol? If you got a couple tanks of bad gas, that'd be enough to do it. Try running a couple tanks of good premium gas without ethanol through it and see if that helps any. If there were other issues, you should have an SES light. Check the air filter too to make sure it isn't clogged with anything. If all else fails, take it in to the service department since it's still under warranty and complain about it. They can probably scan it with a Tech-II and see if anything comes up in the PCM. Also, keep in mind the warm temps of summer can slow down a car by a good .2 to .4 seconds in the quarter mile. If you got used to driving the car when it was cooler out, that could be a part of it as well.

Thanks for the help, I am running premium in the car like the Z-28 so that could be the problem. Do supercharged engines like higher or lower octane fuel? I was thinking of buying some fuel additives to clean the fuel up a little while adding a couple more points to the fuel grade. So far there has been no SES or check engine lights on, it runs great until I get in the throttle hard then it cuts out. Could be reading detonation and cutting the timing back, that's what it feels like because my Z-28 done the same thing when I tried running 87 to save money, the camaro hated that low grade fuel, it would knock and cut power way down but runs fine on 93. I'll try that and see what happens, thanks again.

**Fugiero** · 08-21-2006, 11:58 AM

Originally Posted by 98-LS1

Thanks for the help, I am running premium in the car like the Z-28 so that could be the problem. Do supercharged engines like higher or lower octane fuel? I was thinking of buying some fuel additives to clean the fuel up a little while adding a couple more points to the fuel grade. So far there has been no SES or check engine lights on, it runs great until I get in the throttle hard then it cuts out. Could be reading detonation and cutting the timing back, that's what it feels like because my Z-28 done the same thing when I tried running 87 to save money, the camaro hated that low grade fuel, it would knock and cut power way down but runs fine on 93. I'll try that and see what happens, thanks again.

Well SS covered about everything but one area. I owned a 99 GTP and try checking the supercharger belt for glaze, oil or if its loose.

**Hi-Po** · 08-21-2006, 09:03 PM

Originally Posted by 98-LS1

Thanks for the help, I am running premium in the car like the Z-28 so that could be the problem. Do supercharged engines like higher or lower octane fuel? I was thinking of buying some fuel additives to clean the fuel up a little while adding a couple more points to the fuel grade. So far there has been no SES or check engine lights on, it runs great until I get in the throttle hard then it cuts out. Could be reading detonation and cutting the timing back, that's what it feels like because my Z-28 done the same thing when I tried running 87 to save money, the camaro hated that low grade fuel, it would knock and cut power way down but runs fine on 93. I'll try that and see what happens, thanks again.

High.