I have a 2004 GP GT with 104k. At 40-60 the engine will stumble when accelerating. It gets worse when driving long distances. Last month I replaced the trans filter and put in synthetic fluid. I have replaced the ignition coil packs this week but still have the same problem. I have also recently cleaned the throttle body and had a coolant flush. I replaced plugs and wires about 10k ago. I have not thrown a code yet. Any help on this problem would be appreciated.
In most cases a stumble under load would have me looking at the coils. Generally this is caused by weak spark, so that'* coil, plug wires, and plugs. Check them all. You can also check the fuel pressure.
I checked the fuel pressure today. Key on, engine not running about 45 psi, with car at idle 55-60 psi. The gauge needle did not level out on this but was bouncing while running. The haynes manual I have says the pressure should be 50-62 psi with key on engine off and 3-10 psi less when running. Not sure if this is a problem having less pressure with car off.
I am going to pull the plugs tomorrow to inspect them.