Odd Misfiring Issue

hockeypro1411 · May 12, 2011

The other morning I was driving to work (55 mph, cruise set, nice weather) and my CEL starts flashing and the motor revs to about 2500. I took it easy for the last few miles to work and tried shutting it down and re-starting the motor. Even at idle the light still flashes. The code showing was the good ole' P0300 Random Cylinder Misfire, so the first thing I tried was plugs and wires and that did absolutely nothing to help. Next was pulling the MAF sensor off and cleaning which didn't help either. I kind of ruled that out because I pulled the plug on it and the motor sputtered a little so I figured the MAF was doing it's job. As I was taking off the air intake tube, it was looking like a storm was rolling in so I put everything back together really fast and drove to a buddy's house with a bigger garage. The light stayed off for the whole ~5 miles or so and the truck ran great. The next time I started it in the garage, the light is flashing and has been ever since. The truck has absolutely no balls and is sucking fuel like crazy. So far I've tried:

New plugs and wires (AC Delco plugs and Belden wires)

Cleaned MAF sensor with MAF cleaner

Pulled throttle body off and cleaned

Checked fuel pressure at rail (55-56 lbs)

Checked vacuum (18-19 lbs)

Dumped SeaFoam into the gas tank on a whim

Fuel filter is maybe 6 months old

Engine is a stock 6.0 LQ4

I was going to try a new intake manifold gasket, but the vacuum seemed OK or JUST slightly low. My next guess was a camshaft position sensor, but I'm not sure. I'm leaning more toward it being a failed sensor of some sort simply because it showed no signs of trouble right up until the light came on and the motor started running like junk. Any insight would be appreciated..

hockeypro1411 · May 13, 2011

Add a crankshaft position sensor to the list of things that didn't fix it.

Prokopchuk_9 · May 13, 2011

i would place bets on CAT converter bud...the whole not getting any rpms says enough to me that you had a CAT grenade on ya..or coil pack...but if it was a coil pack it would tell you what cylinder it is i think eh?? somebody chime in on that im not sure if you get a random missfire or if it tells you what cylinder #...

but i would get a heat gun, or find one somehwere let it idle. check how hot your cats are gettin. i forget the running temp but im sure somebody will know. it sounds alot like that man...

hockeypro1411 · May 13, 2011

I read on here that a bad cat will cause this, but in theory both cats would have had to fail because I have true dual exhaust. If a cat failed I'd think it would only affect 4 cylinders. I'll investigate more into the cats today for sure though.

Prokopchuk_9 · May 14, 2011

I read on here that a bad cat will cause this, but in theory both cats would have had to fail because I have true dual exhaust. If a cat failed I'd think it would only affect 4 cylinders. I'll investigate more into the cats today for sure though.

man i had true duals on my 02 before i swapped motors, i had a cat fail. just one...and it ran like SHIT

hockeypro1411 · May 14, 2011

Tried a camshaft position sensor and an intake manifold gasket to no avail. Finally I decided to pull the spark plugs off the ignition coils one more time and noticed that the #1 cylinder had really weak or no spark. I replaced the ignition coil and the motor runs stronger than ever. I'm still kind of wondering why I got the p0300 instead of the p0301 because it was only the #1 cylinder that was messed up..