Wondering if someone can throw some suggestions my way. Iv got a problem with my H2 missfiring. 6.0l engine with 75,000 miles.

Bought the truck a few months back, its been faultless. Few weeks ago i accidently filled it up with a 99 ron fuel, (force of habbit from running few of my other cars). After a few miles it developed a missfire. It then clicked id filled it with the wrong fuel as usa fuel is around 91 ron if im right? and thought it may have killed a plug.

The missfire occured quite randomly, it would miss on one cylinder, but if you accelerated hard it would fire on all eight. Then it started missing on two cylinders and constantly. I order'd some new plugs and they came this morning, i put them in and all was fine for a few miles, now the missfire is back. Not as bad as before or as consistent but still there. The car has been through 3-4 tanks of fuel since i accidently filled it with 99ron.

Can you suggest something else for me to check? Im going to order some new ignition wires tomorow and start checking the coils but is there anything else to look at? Theres only one hummer dealer in the UK and its a good 4 and a half hour drive away.

Any help is appreciated.

Kind Regards

Jamie

 

Im new to the hummer scene but have owned many GM vehicles. Like Corvettes and Yukons. Basically the same although most of my other vehicles were HEI ignition and none were computer controlled like this one is. Hummer system is very reliable and is used in all GM SUV's. I am still trying to figure out if its trottle body injection or tune port. I havent got the book yet and the engine runs flawless so not really in the market for a book yet.

As for your misfire, I would suggest popping off the plug wires and lightly replace them on the plug. Start the vehicle and remove and replace one plug wire at a time. Every time the wire is removed, the engine shoudl begin to misfire. When you pull a wire and there is no change, this is the cylinder that is misfiring. Of course you would then check that cylinders combustion family IE: Plug, Wire, Coil, Injector if applicable, valves/ compression. Sounds to me like a bad wire / coil or plug. These engines are tanks. Wish i could help more but I havent even taken that black cap off the engine to see whats under it. Anyone else chime in? Let us know the disposition. It would help others.

 

Plug wires or clogged injectors are what I would think. Have you replaced the fuel filter? A plugged fuel filter can lower the fuel pressure and cause a missfire. Keep us posted.

 

Put a diagnostic on it today and it said missfire problem with coil E. Which if im using common sense i would suggest is number 5. May be wrong common sense doesnt always rule lol.

I also put a fuel system cleaner through her today and the misfire seems alot better. I just drove it about 50 miles and it ran smooth as silk. Il monitor it and see how it goes. I've order'd some new ignition wires last night and i think im going to replace the coils regardless because iv been offer'd a good price on them.

 

What type of cleaner did you use? I hear some eat away at internals. Specifically Seafoam and some of those additives and engine detergents that oil change shops use. Ive been hesitant to add any. Not sure what the fuel line is made of.

 

What did the old plugs look like when you pulled them out? Did they all look the same???

 

Yea they all looked the same. They looked ok in fairness. Fitted new leads, didnt really help. Awaiting new coils now as the diag said coil 5 is playing up so hopefully new coils should sort it. Just delivery to the old united kingdom is dragging.

 

well, to cause a missfire, you can have manythings go bad. If you are highly suspicious of your recent refueling, then it is likely caused from an injector, or a plug. The plug wires shouldn't really be the culprit.

There is one cylinder that is causing the problem, when you hammer the gas, other cylinders are missfireing due to that one, not really a significant issue. You can check out which one is the problem by the miss-fire counts in your diagnosic tool..

In any event, with the diagnostic tool hooked up (tech II or alike) see what cylinder is misfireing. Swap plugs with acylinder that is working good, no change? swap wires, no change? swap coil packs, no change swap injectors etc.... if it STILL doesn't change and is the same cylinder after all of those swaps then you need to dive into the valve train....

I recently had to do this on an Alero - which I found to be a rocker arm was over torqued after the tech installed new head gaskets.... I had to re tap and heli-coil the rocker arm and then eveything was good to go....

 

Phazer is right on, the other ways that were suggested your flying blind. You need to find out waht cylinder it is and the procede. You can go to a autozone and have them scan it, should come up as a misfire code for that cylinder. I own a shop and we would get $94.50 to get you to that point.The from there we would do a pin point diagnostic test to find out what the actual failure is. My guess would be a coil.......but the again it could be an injector wire or plug, I'd very much doubt a mechanical issue valve train etc.

 

"There is one cylinder that is causing the problem, when you hammer the gas, other cylinders are missfireing due to that one, not really a significant issue."

How would other cylinders misfire due to one cylinder misfiring? Or they seem to misfire? Just curious. And yes I am old school when it comes to trouble shooting.