2002 pontiac grand prix hesitation/ stumble on moderate acceleration and/or up hill . wide open throttle it goes just fine. no codes. idles just fine. no other symptoms. just bought car a couple weeks ago seems to be increasing in frequency and intensity.

SOURCE: Why is 2002 Pontiac Grand Prix sluggish when trying to accelerat

CHECK TRANSMISSION FLUID IT COULD BE LOW OR YOU NEED A FLUID AND FILTER CHANGE.YOUR TRANSMISSION IS SLIPPING REASON FOR SLUGGLISH TAKE OFF.

SOURCE: 1997 f150 4.6ltr engine has

thank you for responding, I went back to basics. New plugs began corroding on#3 & #7 cyl, causing a short(missfires).Tested the coil packs,& I recalled #3 cyl head was cracked, it was replaced with rebuilt head. As for the oil gauge flucuating, crank shaft bearings pitted and worn on #3 & #7cyl, due to the coolant(an apperantcommon problem..I took the truck in for an oil change,a ctc rookie put 5w30 instead of 10w30 as requested,in which caused the oil pressure to fluctuate. I dumped the the oil put 10w30 in and some lucas. 2 weeks later no ticking noises and no oil pressure changes.thankgod for lucus. its too cold to roll a set of bearings in the driveway. thanks again for your input. p s a 1997 f150 4.6ltr come with 2 sizes of serpentine belts, be careful of which one you install, we blew the bearings out of 2 new waterpumps,and for the last tip, the dealership uses a pair of vicegrips and an extra helping hand to set up the timing chains,that is what i was suggested to do by there top engine tech. prove me wrong. works like a charm.

SOURCE: no acceleration

I had the same problem with one exception. It would rev higher than 3500 but the higher it revved the more it bogged. I had the catalytic converter replaced. I thought I bought a new car. Acceleration is just like new. It was easy enough to test. My mechanic cut a hole in the exhaust pipe in front of the catalytic converter. It sounded like a truck but the acceleration difference was immediate. Easy diagnosis.

The car is hard to shift because the more you press on the accelerator the more the computer for the transmission won't let it shift. Which in turn causes the car to bog more sending more unburned fuel into the catalytic converter. Which in turn causes more carbon build up and more back pressure. Vicious cycle. It will only get worse.

My thermostat caused the entire problem. Thermostat was stuck open, engine never got to operating temp most days. Car burned rich. Bad gas mileage. A lot of unspent fuel being dumped into the converter. Probably have some underlying cause that needs to be addressed. Bad plugs, wires, coil, fuel filter.

SOURCE: Car stumbles,hesitates,loses power on Hwy, randomly

Did you check earth connection of electric system?

SOURCE: PO 420 and PO 455 codes

There are three things that can cause this that is unrelated to the cap, if the valve on the carbon canister is defective it would cause this it would also cause a vacuum leak making the car run lean so check the carbon canister check valve first. Next your EGR valve could be dirty or not functioning, depending on the model and engine size it may have two EGR valves one a mechanical and the other the digital EGR and it to may be dirty, if it's dirty it can be taken apart and cleaned with carburetor cleaner and clean the pintal. And lube the pintal valve. If you’re careful and not tear the gasket you can reuse it. If this does not seam to fix the problem then you have a vacuum leak or the device that pumps pressure into the gas tank may be defective.