P0102, P0300

I have a 2002 Silverado pickup with a 4.8 litre engine. The truck seems to idle ok, but has no power going down the road. It takes a lot of throttle to get the engine to accelerate. Accordingly, the transmission shifts late too due to the increased throttle opening. Accessing with the software "PMScan" shows a random misfire P0300. Fuel pressure is normal at 49 to 60 psi no load and full load respectively which seems normal and is the same as my son's 1999 silverado. Engine does seem to run lean, and will try to backfire if too much throttle on road test is given. O2 sensors seem to be ok, ranging from 600mv to 100mv actively switching. MAF shows 6.5g at idle and I seen a reading on the road test as high as 85g. We switched MAF's from the 1999 to the 2002 to see if the MAF was bad with no change in preformance. My gut feeling is that the exhaust is restricted (possibly by Cat converter). Does any other one have an idea what this could be? It really is puzzeling to me as I am currently a diesel mechanic also having experience with OBD2 since it come out. I was a light duty mechanic from 1970 on up, having never really quit working on cars and pickups. Could a MAF show 85g with a restricted exhaust still?
Thanks, Dennis

Problem is believe solved. I took an old O2 sensor and gutted the fitting. I then brazed in a 3/16 pipe to hook up to a low pressure guage 1-10lbs. I found this guage as a combination vacuum and pressure guage at Autozone used for testing old fuel pumps and current vacuum systems ($20). I hooked the fittings into the exhaust system and took a pressure reading of exhaust back pressure at idle to 4000 rpm. I found these readings:

1. pressure was 1 lb at an idle.
2. pressure went to 3-5 lbs at 4000 rpm with open throttle in neutral.
3. pressure was widely fluctuating from 1-4 lbs when I power jacked it while sitting.

then hooked it up to a known good 1999 chevrolet the same way.

1. pressure was 1lb or less at idle
2. pressure stayed about 1 lb even at 4000 rpm
3. pressure never went above 1 lb even when power jacking at an idle.

Conclusion: catalytic converter is plugged on the 2002 Chevrolet 4.8 with codes P0102 and P0300. P0102 is logged because of poor engine breathing and P0300 logged because of misfires occuring because of poor engine breathing.

This diagnosis is left for other people with similar problems to diagnose with.
Thanks for being able to post this repair.
Keep in mind that exhaust backpressure has to be interpeted from vehicle to vehicle and engine exhaust system etc. There is no preset figures to say good or bad. You have to diagnose what would seem to be normal and possibly compare with another vehicle.

Congrats.

Thanks for posting back the repair.

Good job.