ok im hoping i can get some help with whats going on with my 2003 Hummer h2 6.0 lq4. It is missfiring like crazy, it is throwing P0300 Random cylinder missfire, it has New AC delco iridium plugs, new wires, heads are freshly rebuilt. New Maf Sensor, Fuel pressure is correct, compression tests is ok. After going by coil by coil disconnecting them to see which ones wernt working i found that only 1 cylinder on passenger side head is working. I removed oil cap where oil is poured into engine and steam is coming out and smells like pure fuel as if passenger bank is dumping way too much fuel. i took injectors off to take a look at them they seemed ok. Really hoping someone can help me out. I disconnected both o2 sensors while running and no change. Any suggestions?!?

 

what was done to the truck prior to it starting this? Had it ran correctly prior to the work done to it (heads rebuilt)?

 

Could be a clogged up catalytic converter on that side. Try physically removing both upstream O2 sensors from the exhaust and see if the misfiring goes way. If so that is the problem. Removing them might be a bear, you will probably want to soak the treads with some penetrating oil for a bit first, then run it for a couple of minutes to heat things up just a little (not blistering hot).

Other things - possible if you had the heads reworked that the head gasket on that side got damaged or is in some way defective. Possible the intake manifold is tilted on that side and not sealing right. That could easily happen if the knock sensor harness under the intake got out of position when setting the intake back down.

Possible bad injectors or coil packs on that side.

 

Did all of this just happen after the new heads and other parts?

How did you test to see which cylinders were working?

Have you made sure the cam sensor in the rear is connected? No vacuum leaks?

Check the MAF and make sure it is installed correctly. Try to disconnect the wires and see if that changes how it runs. I found out the hard way that the rubber connection at the throttle body tends to curl under on the bottom and not seal correctly and that will cause what you are going thru. Make sure on the filter side that you have properly lined up the MAF. It does make a difference.

Tim

 

Ran the exact same way as before the head work was done, it changed nothing atall after rendoing heads and head gaskets.

 

Have you ran a compression test?

Tim

 

I would have a crank relearn done to make sure the crank sensor has not gotten out of sync. If a crank relearn is needed it will also make the SEL light flash above 60 MPH most of the time but not all the time, the crank sensor can cause the P0300 code and its used to detect the misfires, its also used for the injector firing.

The symptoms your explaining look like possibly a crank sensor

Neal

 

removing the O2 sensors while running and it made no change would help eliminate plugged cats as the cause.

 

a bad ground or bad wire on the crank sensor harness would also be something to look for but with a scanner you should be able to trouble shoot most of this.