SWINGRRRR

iTrader: (12)

I dealt with this a little over a year ago. Took it to a buddy that knows these things and he hooked my truck up to his laptop.
Best guess was a clogged catalytic converter backing up exhaust gases, causing a misfire.
But mines a Hummer. The H2 are notorious for this.

Just throwing out ideas, maybe something to check.

Tim94gt

iTrader: (1)

I had this issue on my 2500HD. P0300, misfiring on both banks and the truck would fall on it's face over 2k rpms. Both cat's were clogged.

Also had a similar issue on my dad's '04 1500 truck. Chased this misfire for weeks. Finally decided to pull the injectors and inspect. They all had some deposits on them and the rails had some grit in them as well. Cleaned them all and put it back together, misfire has been gone for 3 months now.

Tydriver

iTrader: (2)

OK guys, sorry for the delay in responding, last weekend I didnt get a chance to work on the truck as house projects dictated much of the weekend.

So, fast forward to today..

I took the MAF off, inspected it, it looked brand new, but I sprayed it with cleaner anyway and reassembled (no change, but it was suggested so I can rule that out).

Hooked up a fuel pressure gauge and went for the cruise shown below in the video (Fuel Pressure holds steady at 65#), so I think this can be ruled out.

I logged a run to verify if the knock sensors were working, they are, the truck is pulling out timing based on detected knock so I think I can rule this one out as well.

I datalogged the run in the video below using EFILive (the datalog shows consistent misfires on cylinders 2 and 6).

This issue is why the truck sat, it was running well when I was driving it, I was getting off an on-ramp trying to hot dog (WOT, 4500+ RPMs) it and then poof !! It was like someone flipped a light switch, the truck started immediately running like *** at anything after part throttle, and you could hear the misfire, I came home and ordered new plugs and new wires which were installed. Once this didn't fix the problem I parked the truck as I had other things to work on. It's sat for about a year in a heated garage before I started to work on it again.

To me it feels like something broke, but the truck idles fine and runs fune at part throttle which makes me question if it could be a bent pushrod or ??

Anyhow, here's a video I shot today, conditions are 1st gear, WOT, idle to 4400 RPM.. I let out after a bit because its apparent the truck is struggling.



Does anyone have any additional suggestions?

What is involved with trouble shooting a plugged cat? Just replacing them or ?? I would think that if it was a plugged cat (or both cats), the misfire wouldnt be on just 2 cylinders..

Please advise, I'm all ears.

skolman91

iTrader: (28)

If you have a temp gun check before and after the cat, or hit them with a mallet and see if you hear anything rattling.. Ill be honest its been awhile since ive seen where the oem cats are, but dont both sides have one? Both those misfire cylinders are on the same side..

Tydriver

iTrader: (2)

I assume my exhaust is 'stock' to the cats. It has a typical hill-billy exhaust that the previous owner installed AFTER the cats, ( I've only put like 3000-4000 miles on this truck since I bought it a few years back) This is due for deletion/change all in due time. Anyhow, there are cats on both sides as part of the Y pipe setup that joins together right around the transmission crossmember..

I've also noticed some huge splits in the Inject Duty cycle and the long-term fueling is crazily split as well.. I'm starting to buy into the possibility of the cats being plugged more and more.. I'm also inclined to replace the O2 sensors as I have no idea what the history is on them but I'm not looking to throw $$ at this thing unless I can justify it, once my house sells if its not running correctly it may just have to get towed south.. Not ideal at all.

Since my last post, I pulled the valve covers off on the passenger side, I also pulled the rockers off and checked every pushrod on this side for straightness. I found no bent pushrods.

While I was at it, I swapped the coils around and the plug wires.. The problem still exists on the #2 and #6 cylinders.

Wolftrk99

iTrader: (5)

Pull ypipe, you should be able to see the substrate in the passenger side cat looking into the end of the pipe... Should look like clear squares.... Aside from that maybe do a cylinder leak down test may have a burnt valve or something

1FastBrick

iTrader: (17)

If you crack the exhaust loose were it attaches to the manifold you may be able to see if there is a difference in drive ability. If the pressure is backing up, it will have a place to escape.

Wolftrk99

iTrader: (5)

Or pull front o2 sensors out can work too...

Tydriver

iTrader: (2)

Thanks guys for all the suggestions.. I'm going to have to shelve any further investigation for this weekend.. I'll be back at it next weekend though..

I dunno, either way, its getting into the spendy part of the investigation now.. I find it hard to believe its ignition related since all the stuff has been moved to another cylinder..

I'm starting to think it may be burnt valves, I mean if it was a plugged catalytic most likely the misfire would appear on all of the cylinders on that bank.. Or at least I'd expect it to work that way...

Then again, if it was a burned valve or two, I don't see how it would make compression during the compression test I performed...

I guess we'll see..

Wolftrk99

iTrader: (5)

Pulling o2 or even ypipe to see if you can verify or get it to clear up is more time comsuming than money.... As far as valves being burnt and conpression test ive seen good compression with a burnt valve or even weak valve spring, its starting to sound like mechanical issue.... May even be carbon on the piston or valve causing detonation which is showing as a misfire.... What heads do you have??