Misfire
 

I have a 2003 Hummer H2. It misfires on cylinder 1, 5, 6, and 8. However it doesn't start the misfire until 2500 rpm and 1 and 6 are the worst. I have changed the plugs, the plug wires, the coils, the injectors, the intakegaskets (update P/N 89017589), the intake and exhaust manifolds and gaskets, and removed the aftermarket alarm system. I for the life of me can not think of what else could be causing this misfire, so any suggestions would be greatly appreciated.

Thanks!

RE: Misfire
 

Hello Friend,
Make sure that your spark plug wires are connected to the correct spark plug.
#1 to #1, #2 to #2, #3 to #3....etc.
Or Check your timing.
I know the old way to check the timing was to use a timing light.
I'm not sure how it is for these newer engines.
You may need to replace the timing belt or chain.
I don't know which of the two (belt or chain) is used in this engine.
Or re-align the timing belt or chain with the cam shaft and the crank shaft.
If it skipped a notch, this may cause misfires.
This is a very basic test sequence but you may need to have it professionally checked since using your vehicle like this may cause severe internal engine damage in the long term.
Safari Hummer H2

RE: Misfire
 

I had a misfire and it was a cracked plug, if you changed the plugs, especially number 8 and the issue still is coming then not sure, what type of plugs and wires did you change to OEM for? I have heard that there are issues with different types of wires and plugs....

I used NGK Plugs and Taylor ThunderVolt Wires, I have no issues anymore, love them, I also have a K&N Air intake, no issues there either with an aftermarket Compustar Pro Alarm/starter P2W9000FM-AS, no issues there either but stock exhaust.

Hope this helps, not sure if it will, by the way, Mine didn;t misfire till around 2000 to 2500 RPM either and the check engine light would flash on and off at first then just stay on

RE: Misfire
 

dwaxman1,

Your problem sounds pretty much identical to mine. I did change all of the plugs, to the original GM Ac Delco ones. I didn't try anything aftermarket. I also changed the wires, but they are also GM stock. Maybe I'll have to try something aftermarket. Thanks so much for your help!

Transit

RE: Misfire
 

give it a try, My 7 and 8 were cracked and that screwed everything up, the 8 plug is a pain in the a$$ to get to, most people crack it, maybe the wires aren't pushed on tight too. I have read that if one misfires, sometimes you will have wrong or false signals about others too, maybe you really only have an issue with the one plug, if it is so then it would be 8.
Did the dealer or anyone do any compression test?

RE: Misfire
 

do you have any stored engine codes? we have had these in the past and some of them ended up being a fuel pressure regulator. how many miles are on your hummer? you can have somebody with a tech2 hook it up to you vehicle and see what is going on.

RE: Misfire
 

i just looked up your problem on dealerworld and it said that it could be anything from the o2 sensors or small debre on the mass air flow sensor. it also said use the factory spark plugs.

RE: Misfire
 

I dont think you got the plug wires wrong as its almost impossible to do on the 03 H2 as I belive it has coilpacks with no distributor? You would have to seriously to it on purpost to screw that up. I would assume you have the wires not completely "SNAPPED" on. You should feel and hear them snap when putting them on.

RE: Misfire
 

Hieveryone,

I just wanted to thank everyone for their suggestions, I am definatly going to give some of these a try.

I really appreciate it!

Mike

I have a 2003 H@ with 100K miles and it has the same problem, which of the prior ideas helped you with this??