Hi, im new here and i registered cause i have this problem and can't seem to figure it out. I have a 2006 Hummer H3 with a 3.5 engine with a misfire, but no misfire code. I checked the fuel pressure and it is 60 psi. (air filter, spark plugs and wires are all good)

When i hook up the fuel pressure gauge, it fluctuates. Anyone have any ideas to what it might be? Do the injectors go bad on these trucks? I would appreciate the help, thanx

 

Bad head most likely.... this was problem with alot od 06's

 

Thank you for the response. Would a bad head cause the fuel pressure to fluctuate? cause that seems to be the main problem..Would there be a code atleast if the head was shot?

 

The fuel pressure should be relatively constant. You could have a fuel pump issue??

Injectors can get dirtyif you are running some bad gas. Might try 5-10 gals of a high detergent gasoline (mobil) or Marathon with STP. I am not a big beleiver that gumout and the like do much, but you could always try a can of fuel injector cleaner and see if it improves.

 

This is what i did yesterday so far. I did a compression test and a leak down test. The head is solid. I replaced all 5 injectors from another cylinder head i have laying around. The injectors were clean. I adjusted the position of the coil and changed the spark plugs. I still get a miss only in idle on cylinder #5. I also tested the fuel pump. The fuel pump seems fine. I would think if the fuel pump was no good, the truck would atleast hard start or have a stronger misfire.

I tried calling 3 hummer dealerships, the techs are horrible and seem like they dont want to help me.

I am about to pull the head, what usually goes wrong with these heads? like what should i look for if anyone knows?

 

Change the #5 coil on plug packwith #3. See if you still have a mis and where. If the misfire does not follow the change, you know your coils are good and then look at thewire/connecter to the misfiring #5 plug for a bad connection.

When the fuel pump first gos, you usually do get a hard start, but not every time. If it is the head, pulling the head will not do it for you, the early 06s had bad heads, and
the heads were replaced with hardened head valve seats. Unless your head laying around is for the 3.5L and a good one??

Look at the H3 Frequently Asked Questions sub forum: Head and Valve Problems.

 

I thought you weren't getting any misfire codes? If you are getting a consistent misfire code on #5 then you just have to check the basics..suck, squeeze, bang, blow. Granted, there are roughly a million things that could be causing a problem with any of those, but it sounds like you know what you are doing. It cant be fuel pressure because that would affect all cylinders, not just #5. The TSB for the head problem advises the tech to do an injector cleaning, then if the problem persists to do a leakdown test on the head. If you already did a leakdown test, and it was good, then you shouldn't be dealing with a head issue either. Doc olds is right, the best way to diag your problem is to move things around and see if the miss follows it. Its the easiest, and surest, way.

 

Thank you for all your replies, i really appreciate it. I did what doc suggested yesterday and the miss still stayed in cylinder number 5. It wasnt showing any codes, but when u pull the coils for each cylinder, nothing happens with #5.

I inspected the head after i pulled it, all the valves are seated 100%. While i was pulling out the valves, i noticed the springs on #5 cylinder didn't feel like the other 4. It was a bit too soft. Not wanting to take a chance with used ones that i have, i purchased new Valve springs and hardned HP valve seats,put the head back on and bang, problem solved. Cylinder #5 is not missing anymore. Thanx again for all your responses, you guys have been helpful.

 

Sweet! Glad we all could help. Good to have another member here not affraid to turn a wrench!!!!

Please feel free to read the questions of others and pitch in when you can to help out. KUDOS on a job well done.

 

Impressive you did all that yourself...Good job