Good Morning Fellow H2 Owners,

I really need your help. Long story short:

2 months ago I purchased a 2007 Hummer H2 SUT sight unseen from a dealership (sadly I didn't have a mechanic inspect it beforehand). The dealership told me that it was in perfect running order and in fact they had recently had it serviced at a local shop which included replacing the catalytic converter, and two fuel injectors (I'll attach a picture of the work done to it). When the H2 arrived at my house for delivery the check engine light was on with a code reading P0300 (I'll attached a picture too).

Symptoms - truck idles fine...but it hesitates when you hit the gas. Once she's up and running it seem to run fine but there is a hesitation.

I dropped it off at my mechanic 6 weeks ago and he's slowly been working on it. Last week he called and said there are two bad exhaust valves on Cyl 1 and Cyl 3. They pulled apart the motor, had the heads machined and all of the work was done. When it was put back together the truck is still hesitating and he's still getting a misfire. When I talked to him yesterday he was retesting everything but said it's a hog when you hit the gas and said you'd be in trouble pulling out onto a street with it. This guy is the best mechanic in town and I really don't want to have to have it towed to the GMC Dealership to have them start from scratch.

Any ideas of what I can have him look for?

 

One other thing I forgot to mention. If you notice on the invoice where the catalytic converter was replaced the H2 had 79,722 miles. When I dropped it off at the mechanic it had a total of 79,772 miles. In those 50 miles which was probably driven from the Auto Shop to the dealership and maybe test drives, it completely burnt up the brand new catalytic converter.

 

what do the long term and short term fuel trims look like? have they cleaned the MAF sensor and throttle body. but maybe need to recheck both cats again. or maybe the pieces of cat got shot into the mufflers.

 

I went over the shop to visit with him today and see if he had made some progress and here is what I learned. The H2 is no longer misfiring...so that issue is fixed. He said when you go to drive it, there is no power so you wouldn't want to pull out into traffic.

I'm not sure what the long term/short term fuel trims are, but it's something I can ask him.

Yes, he cleaned the MAF and Throttle body and tested them. They all seem to be testing good.

Last week they installed a brand new catalytic converter. His head was where your head was on the exhaust being clogged, so he removed the exhaust system and check in the pipes and the mufflers for a clog and couldn't find anything.

 

7/20/2021 Update -- Today I went over and brought him a sheet of fixes that I've found on this website that he's going to go through and make sure he's done as well. He said at a quick glance he's done 90% of them, but would spend time going through each one individually. The H2 is no longer misfiring so that issue is resolved. Right now he said the truck is just running really slow and has no power. He pulled the o2 sensors and tested them, they seem to be okay. I asked to see if perhaps there was a programmer on the truck previously and his computer is showing that the software on the H2 is the factory software that should be on there and doesn't appear that there was ever any custom tunes added to it.

 

no power can be misleading. has he checked the transmission fluid to see if its past its life or mixed with coolant and or the transfer case? maybe the engine is making power but it's not getting to the wheels.

 

I would find a local tuner and have them look into the ECM programming, looking specifically if any of the SES triggers have been turned off.

Unplug the MAF sensor and force it into Speed Density mode, see if it drives any better.

Screw a pressure gauge into the downstream O2 sensor bungs and see what the exhaust pressure is, should be below 1 psi.

It sounds as if the truck is in REP mode and if certain triggers are turned off it will not set any indicators

Has he tried a Crank Relearn Procedure on it?

Neal

 

Grab your H2 and take it to a good GM dealer, the mileage you have on your H2 should not be having these kind of problems unless it was totally abused. Misfires, no power, codes, bad cats, this all make no sense. Short of having a couple of bad coils or deteriorated wires misfiresare uncommon with the mileage you have.

The LS/LQ9 is nearly a bullet proof engine. Bad 02 sensors will have more then 2 cylinders showing misfires as well as throttle body issues. It is true bad fuel injectors can cause burnt valves but a flow test can on those injectors will support that finding. 4L60 trans problems usually have shift flare issues or loss of ranges. I hate to sound judgemental but based on what you have said I think your mechanic is a parts changer and doesn't have a clue.

You need a pro to help you sort this out. A /GM dealer with a Tech 2/MDI 2 can give you a great deal of additional information your mechanic may be missing and give you a road to recovery that won't cost you a fortune.

 

Unlikely. Likely the (bet it was not a GM) dealer lied and no cats were replaced.
The 2007 plug up cats fast. Mine is an 07 also. Then throws random cylinder codes.
Check the pressure at the 02 sensor before and after each of the 2 cats. That differential in pressure will tell you if the cat is OK or plugged.

Make sure the 2nd guy actually replaced the 2 cats and y-pipe assembly (all one unit), that he billed you for.

2 dealers told me my valves were toast. There was NOTHING wrong with the heads. But their stupid junky OBDII sensors or Tech2 suggested it was a valve problem. It was. The valves could not breathe. But that was due to plugged cat. Only on one side, but both banks were throwing codes. Plugged side throwing more codes.

I bet there was nothing wrong with your valve train, but the half of the block with the 1/3 cylinder bank is plugged, or otherwise affected.

As for your mechanic who has no clue as to what is wrong after 6 weeks, dump him. He is lost.
What bank are you getting the random codes on? If he can not answer that, you have a bigger issue.....

You can screw around, or take it to an GM dealer who used to sell Hummers.

 

Was it the driver side Cat that was bad?