Have a low mile 72k 03' H2.I was in a horrible car wreck ( not with the Hummer ) so my baby sat for almost 14 months..Jumped it fired right up but now it has a flashing engine light and an odd idle..I can figure that out did cars for almost 30 years..had to get my neck broken to stop lol...MY question is anyone ever replace that nutty expensive 79' group battery with a 78' group.? looks like it should work..love my Hummer..bought it in Chicago and drove it home rode great really enjoy the truck.

Thanks

 

You'd be better off with the larger battery due to capacity.

Get a code reader for the idiot light, chances are once you run it a bit it might clear up.

 

the difference in CCA is next to nothing between the 78 group which is a 80 dollar battery,while you can easily spend over 200 dollars on the 79 group.I have my own computer set up i had used while i was wrenching it is giving a very generic po300 multiple random misfires..on a truck that sat like this H2 did it could be an injector.I will p

 

flashing MIL usually means the computer thinks its rich, so rich it will cause damage. This is usually caused by a need to perform a crank relearn procedure. Why it would happen after being parked I cannot answer but I would try a crank relearn on it first (this will cause the P0300 code). The flashing MIL usually happens around 60 MPH on the ones I've worked on.

Not sure on the battery, can't hurt to try and see what happens. The reserve is where you will notice it, truck parked and how long it will go before the battery is drained low enough it will not crank the truck.

Neal

 

I would stay with the 79 group. Thats what it came with. Batteries dont seem to live a long life in these things. Especially if you dont daily drive it. Ive replaced 4 in the past 6 years.

Did you put fresh gas in the tank? 14 months is a long time for gas to sit. I could see this screwing with Maf sensor.

 

did the seafoam thing when it was parked and did add fresh gas..the light did not pop on instantly it ran smooth as glass for 5 mins..i have a SNap On solus and it is kicking' the dreaded P0300 already trying to find a dealer to relearn the cranksensor and to fix the AC as well as it had forgot how to work just pull the relay and start it shut it off plug it back in and viola working ac...i really did wrench for years..then ran stores then ran districts i have done it all from being a GS at Western Auto to an Area Supervisor for Midas here in St.louis

 

I had the po300 code.It can be many things as i found out and was very frustrating to troubleshoot. Initially i could feel a slight misfire at idle. However, it took a month before it latched a code.i brought it to my mechanic who insisted it was a dirty MAF. But after cleaning it. Was within tolerance. Had the truck back 3times to no avail. I had been researching the hell out of it and was leaning towards intake gaskets. Of course, he didnt want to listen to me. After the third visit, i told him to smoke and water test it. He again cleaned the MAF and said he didnt find any leaks.

So i take the truck back and head over to neighbouring city. No code but i swear its still misfiring. Then a new problem arises. The truck turns into a slug and now i have a code thrown. Knock sensor. During the water test the knock sensors had become submerged in their bowls. To replace them the intake would have to be removed. Seeing as it was warmer now i did the repair myself. Two knock sensors, harness and steel fel pro gaskets. It was a pretty easy repair and guess what. No more misfire or codes and she has ran mint since.

Not saying this is your issue as a po300 can be a long list of suspects but is a common issue in lq motors