Thread: 2004 Grand Prix GT Shifting/Acceleration Issues

I am experiencing an issue with my 04 GT. When the car is driven off a cold start the issue is nonexistent but as it warms up it wants to shift early when at low throttle input. I can be going 30-40mph and it seems to shift into overdrive, my vacuum drops to around 3.8-4.3 and there seems to be a large dead zone in the pedal response. If I tap the pedal or apply more than half throttle it will downshift/rev up and take off as normal. It almost feels like I've lost all power until it downshifts and seems to want to shift into OD earlier as the car gets warmer. I have a hardwired radiator fan, temps stay around 195-200. The car has thrown an intermittent P0420 Catalyst Below Efficiency code but it doesn't always come back after being cleared. I never smell any fumes or Sulphur smell but upon changing my rotors in the rear noticed both mufflers have 5-7 holes on top of them. The car was purchased for $800 with 160k from a friend so I don't want to throw much money into it. So far all I have done is a general tuneup : plugs, wires, air filter, MAF, MAP, PCV valve and oil change. I will be doing transmission filter and fluid this coming week. I can't hear any intake leaks or find any spraying cleaner around.

Another weird symptom is the car seems to rev as I let off the throttle or brake while cruise control is on. Ex: Driving/Cruise set and at 1200 rpm, let off throttle, rpms shoot to 1400-1500 and then drop, is this just a symptom of the TC being locked up?

Any help is greatly appreciated before I throw anymore money at this thing or take it in.

Being a 04, I'd remove the throttle body and clean the entire thing inside and out. Likely a bunch of carbon built up on it and it's been known to cause strange issues. You mentioned the MAF in your tune-up. Did you just clean it or replace it? If the latter then hope it was a genuine Delco MAF as these cars seem picky towards aftermarket ones. Most people that have to replace a MAF just pick one up in a junkyard they're that picky.

I cleaned the original and kept it after it didn't seem to change anything. I then pulled one from an 04 at the local junkyard and cleaned that before installing, that one is currently on the car. At first I suspected water in the trans fluid or contamination but it doesn't seem anywhere near a pink color. I'll upload pics of the fluid, just waiting on a filter and fluid to arrive.

Cleaned out the throttle body today along with as much of the intake as I could get for now until I replace the gasket and deep clean the intake as well. So far it still seems to be locking up the TC early at 25-30 mph at low throttle/cruise inputs. After the throttle body cleaning it seemed to have a single long start and a few delayed shifts first time driving. Is this normal or unrelated to the cleaning?

Trans filter and fluid will be arriving tomorrow so hoping that will fix the issue. If not I will look into the pedal sensor and see about replacing with a low mileage junkyard one before ordering a new one for full price. Still around 1200 invested after buying for 850 and doing the tuneup, sensors, full brake job and oil/trans fluid. That's around what I wanted to have invested total so hopefully I can get this issue solved asap.

Well I was driving over some tracks this morning and the resonator busted off its mounting point and fell to the ground. I had to remove the whole exhaust assembly behind cat including mufflers and leave it to pickup later. The torque converter lockup seems to be gone under 45+ mph, even with cruise control on and so far I believe its functioning as it should. Drove around as long as I could stand the droning and the P0420 code has yet to return as well.