Constant Misfire

hockeypro1411 · August 6, 2009

My 2003 2500HD with the 6.0 has been giving me problems lately. Since the day I bought it, the truck has flashed a Check Engine Light when I went above 65 on the highway or when I would accelerate hard from 40-45 MPH up to around 60. At that point it was only doing it when the truck was at 2500+ RPM's for over 20 seconds. This past week the truck will drop into OD doing about 2000 RPM's at 65 MPH and flash the CEL. Even though the RPM's are down, it still won't go over 65 without misfiring. I was originally thinking it was plugs n wires, but the fact that it does it no matter the RPM's has me thinking twice. My next thought was that it has something to do with the bigger tires im running. According to my GPS the speedo is about 6-7 MPH lower at 55 MPH. Could the light be a result of the computer being confused? I'm stumped.

Edit: I've tried gas from different stations, and all octanes from 83 to 92.

doverarjim · August 6, 2009

My truck will miss as I go down the road, had it checked out and something had chewed on the wires to one of the O2 sensors. They had to order the sensor and I have not been able to get the truck back to them for the repair, hope to get it done first of next week and I will let you know if it helped.

txab · August 6, 2009

I'd definitely readjust for the speed difference when you put a tune on it.

What codes do you get?

hockeypro1411 · August 6, 2009

I'd definitely readjust for the speed difference when you put a tune on it.

What codes do you get?

P0300- Random Cylinder Misfire. Always the same. Well, so far anyway.

Jcol · August 11, 2009

hockeypro1411 - Do you feel the misfires or does the code just set? When I bought my truck that radom misfire code would set everytime I drove above 65 mph and occationally under hard acceleration. I never felt the misfire, the code just set. After trying everything I finally took it to a dealer who figured out that there really wasn't a physical misfire but there was a problem with the crankshaft position sensor and the stored crank positions in the trucks memory. They ended up fixing it by having the truck relearn the crankshaft positions at idle and WOT. This solved the problem. Good luck with yours.

hockeypro1411 · August 11, 2009

Thanks for the suggestion! I'll have to try that. I pulled a 6600# Bobcat this past weekend and it was "misfiring" even at low speeds, so it leaves me to believe it's anytime the engine is under a load. I'll definitely look into getting the crank position sensor checked out!

Jcol · August 11, 2009

The dealer actually drove the truck with a laptop connected and tried to get the code to set. When it did he was able to see what was happening before and after the code set and which cylinder. It usually showed hundreds of misfires from two different cylinders. This is after all the plugs, wires, injectors, O2 sensors and intake manifold gasket had been changed. When he realized it was not an actual misfire he put it in some learn mode and ran the engine at idle and WOT. Problem solved. He explained that the computer knows when the spark is fired and expects to see a pulse from the crankshaft senser within a certain period of time. If the computer doesn't see the pulse it assumes the crankshaft slowed down because of a misfire. Since there was no misfire, relearning the pulse times solved the problem. Had the truck now for 6 months and no code since the dealer visit.

Prokopchuk_9 · August 12, 2009

The dealer actually drove the truck with a laptop connected and tried to get the code to set. When it did he was able to see what was happening before and after the code set and which cylinder. It usually showed hundreds of misfires from two different cylinders. This is after all the plugs, wires, injectors, O2 sensors and intake manifold gasket had been changed. When he realized it was not an actual misfire he put it in some learn mode and ran the engine at idle and WOT. Problem solved. He explained that the computer knows when the spark is fired and expects to see a pulse from the crankshaft senser within a certain period of time. If the computer doesn't see the pulse it assumes the crankshaft slowed down because of a misfire. Since there was no misfire, relearning the pulse times solved the problem. Had the truck now for 6 months and no code since the dealer visit.

oh yeah..ive had this problem on my old 2002 4.8....all the dealer did was a crank relearn process...sounds like thats what they did for ya

hockeypro1411 · August 12, 2009

Excellent. Thanks guys! Today was my last day of work for a while, so I'll have some time to bring the truck in. Sounds like a crank relearn is what's in order!