2002, 4.8L, has severe misfire under heavy load, at or near WOT. PCM shows P0101 and P0300 codes. All seem to check out fine. MAP, MAF are OK. Plugs, wires, coils all OK. Injectors clean. Any ideas

 

are you sure the plugs are ok? I mean the electrodes themselves? I had this problem after a cam install once and it turned out to be the electrode on one or two plugs had been pushed in all the way and it was causing the misfire.

 

Yes, even changed them to make certin no internal problems might be causing the issue.

 

my only other guess would be that the cam may be installed wrong

 

It's possibly the 2 Knock Sensors in the valley cover. Check those nest.

 

Did this problem just start? What work has been done to the truck since the problem surfaced? Do not rule out plugged cats if you still have them.

I need to know what you have done to the truck, custom tune? in order to attempt to diagnose.

 

Yes, problem just began. Went to pass on the interstate and when I hit the accelerator the engine began to miss, could hear the valves chattering and the check engine light began to flash. Tried it several times until the MIL light set. Codes were P0101, P0300, and P0302. Ran the recommended checks (from the GMC shop manual), can't pin point a faulty component. Only happens at high engine rpm and heavy load such as pulling a steep hill and attempting to pass at >70 mph. At normal highway speeds the engine runs fine.
Only one modification. Installed an Airaid intake system several years ago.

 

you need to check your backpressure...pluged kitties act like what you have described. find a gauge and test em

 

Have you checked your A/F and Spark at WOT? delete the codes and re-tune. Might wanna ask this over on tuning & diagnostics.

 

Tom,

Did you ever get to check the catalytic converters to see if one or both is clogged?