About to lose my mind – Hummer H2!

[Rebel Richey]

I been chasing this issue so long….. idk what to do but share the full story….

I have a 2003 hummer H2 with 479k. Good running rig but got a few issues to deal with…
This is for you lq4 6.0 Chevy guys.( I was towing a race trailer when this happened )….. Iv been struggling with a hesitation on take off – only when it’s up to full operating temp – when I first got the truck someone had been messing with the cats – they had been cut and rewelded ( not sure why )…… They did a crap job welding – so I sent the Hummer to a shop and had them fixed…… Anyway about 2 weeks of having a beautiful sounding exhaust – the left side cat started rattling – to the point it sounded like a rod knock at idle…… Iv been driving like that for about a mth……. Now this whole issue with the stumble and lazy take off has been present the whole time – since last year when I replaced the transmission – and fixed some vacuum leaks ( I found the evap purge line busted and pulling a vacuum – ever since fixing it – the issues has been present….. ( That was a yearish ago ) some days the sumble is there and other days it’s not even noticeable.. even with the lines fixed – I still get a code for the evap….
As far as the hesitation Its most noticable when up to temp – isn’t present when cold….. Sometimes you turn the key off and back on and it’s gone…. Totally random….. Iv looked at the mass air flow – iv changed parts around with known donor vehicles – can’t get rid of it………. That’s a lil back story….
So like I said – had the exhaust welded up and few mths ago – 2 weeks after having it fix (it started rattling inside the cat)……… The other night (Sunday) we wear driving home from Muskogee to OKC 136 miles……. When leaving Muskogee I first noticed the Hummer seemed to be burning gas a lil bit more then normal….. Then I noticed it seemed down in power – like it wasnt pulling on the top end as hard….. I brushed it off – thinking maybe it was the extra wind….. Finally on the other side of Shawnee – the check engine light started flashing – but seemed to drive okay…… Few mile late I lose the ability to accelerate – it’ll rev but won’t pull….. Rpms will climb to 6kish and then the engine stops pulling…. Pulled over at a truck stop – checked codes ( found cylinder 1 miss fire detected )……. I could personally tell the engine didn’t sound like it’s normal self but anyone else probably wouldn’t notice…… I pulled the coil wire for cylinder 1 and it was very clear – it was working….. When reving engine – no hard misfire could be heard….. But below a 1000 rpms you could here it sputtering and surging…. Once you reved it past 1000 it sounded clean…….. I decided to limp the truck home – when I took off It would not pull past 40 mph… it would get up to high 5k rpm and just set there reving – like it was on the red line….. Wouldn’t shift or anything – the moment you let off the gas it would shift and only then you could apply part throttle and start to accelerate again….. Once I got it up to speed 60 – anytime you got the rpms above 3k it started having sputtering issues – it acts like the cats are clogged and it can’t get the exhaust out…… It wants to stumble and die when coming to a stop – will take off and shift fine under normal conditions but you can here the engine isn’t hitting right…… The cylinder 1 misfire – changed to random multiple misfire……. Something else I noticed – if you shut the truck off and let it cool down – when I restart it – it sounds better instantly…… It now has a very noticeable exhaust leak – almost worse then before…. One more thing I noticed…… The blend door for the front driver side heater has been stuck for sometime – I feel heat from time to time……me and the wife both noticed there was alot more heat coming from the vents then normal/hotter heat – more then iv ever noticed……. Temp gauge reported normal or at least close to normal……. I’m think the cats are clogged and this is causing the engine to run a lil warmer then normal but not enough to show on the gauge… I had a few vehicles do similar stuff like this in the past and it was the cats….. Thoughts……… 479k miles…..

IV been driving the truck for work – I noticed the exhaust leak is way worse – but it was stopped all it’s goofyness……. I live about 10 miles from work….. I can’t get the Hummer to repeat the issues…… And even the surging and fluttering idle has cleared up…… Yesterday I noticed I didn’t even has a hesitation!!!!!?
​​could someone give some advice on this!

Idk if this is related but I Also I got a issue that happens randomly……… Sometime the engine does a long hard crank – may take two turns of the key to get it to fire…… Sometimes it will smoke black out the exhaust like it’s rich…. But instantly clears up… Sometimes it won’t do it at all……. It’s seem do it most in the evening when I get off wor- after setting a bit…. Altho it does happen sometimes – after only setting a few mins…. Sometime it will go a week without a issue….
This event tends to happen once a day and never repeats in the same day…… Does not do it in mornings after setting all night……

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