I'm not ruling them out, but nothing as far as a code is pointing me to cats and the shop that looked at it when it started the mis-fire code said they were fine. I recently found a exhaust shop that I talked to and thought the guy was decent trying to educate me, that I'm gonna have look at them if these other areas don't fix it. He is a Magna flow dealer but wasn't pushing for a sale. I got to look at live data on the O2 readings and fuel trims now that I'm learning more and see if those readings tell me anything. Thanks for the suggestion as they all help one way or the other. Hopefully Friday I'm done with other project and can get back on it.

 

my fuel trims where fine staying close to 0 and i had replaced all 4 of my O2 sensors with OEM ones. graphing the O2's i did notice passenger and drivers side rear O2's where a bit off from each other also i was not getting any O2 codes. my drivers side cat had partial clogging. i had replaced the rusted out exhaust manifolds and gaskets prior thinking maybe it was that as well. maybe a piece of rust scale found it's way in idk. i had blown out the front of the cat's out prior to fitting with an air gun. but my cats where the originals from 2003 so 20 years old.

 

I’ll start out with this. Definitely go pick up a VX Diag VCX NANO. It’s cheap, you’ll use it more than once, and it does everything. I’ve got mine to work on both windows 10 and 11. There are plenty of video guides on how to program modules and use tech2win on the nano specifically. It can be finicky at times and take a complete uninstall and reinstall every once in a while, but I still think it’s more reliable for programming than the tech 2 and it lets you see live data on a nice big screen.

another option is an hptuners mpvi2+. I don’t think it requires a subscription if you’re just viewing data but I could be wrong. Definitely verify before you buy. It is 100% the best tool for viewing live data with detailed graphs. It just can’t do programming or read codes.

After looking at everything you’ve done. The things I would do at this point are

-disassemble the fuel rail, have each injector flow tested and cleaned.

-inspect the fuel rail for crystals. This happened to me. Sometimes with bad gas or moisture, crystals form due to ethanol absorbing water and clog injectors and rails.

-check fuel pressure

-smoke test the intake to make sure there are no leaks

-inspect wiring harness for injectors, coils, crank sensor.

-check maf and map sensors. I might parts cannon a maf sensor if it was me. Oem only don’t go aftermarket

-ensure coils, plugs, and wires are acdelco. Might be worth pulling the wires and looking them over to make sure nothing funky has happened. Especially since I think you said you had headers on the car, sometimes things can get melty.

Did this start happening suddenly or gradually get worse?

First things first, get the scan tool and see if you see something there.

edit: I see now you’ve done a lot of these things I’m just a lazy reader.

 

I would of expected some type of cat related code. I'll definitely put them back on suspect list since one can't expect a code telling ya cat fault. With the recent downturn in performance it may be them finally bad enough to choke off. Thanks for the details on your readings.

 

Smoke test is on list to look into. Something I've never had to do before but when at NAPA getting scan that coded to throttle position sensor they also thought they heard a vac leak. With the open K&N filter I couldn't hear it but my hearing not best either. You brought up several good points to explore as well. Sorry to see about your brake fluid leak, that sucks too!

 

I started a new thread to continue this discussion in with a P0300 Engine mis-fire label to aid future searchers for help in their solving the issue on their Hummer.
Here is the link to it.
https://www.hummerforums.com/forum/h...13/#post404816
Thanks for your help an input, please move to the new thread to continue.
Roy

 

My VCX Nano arrived from Amazon. $129 with WIFI total. Strongly suggest upgrade to WIFI. I set it up on 64bit Windows 11 laptop without too many issues. (virus scanner kept deleting a necessary file, without sending notification) For H2's you won't use GDS2, only Tech2Win. Much, much easier to setup than my MDI clone from years back. That setup required a 24hr unlock each time I used it. This VCX Nano has a 90 day expiration and with their included VX Manager software there is a license renew button. I assume will keep me active. My instructions included links to Mega cloud server to get software, which I did NOT do. The usb drive had both GDS2 and Tech2Win. The GDS I followed the screenshots include folder included other files. 2 instruction video's and the PDF instructions, as well as VX Manager. The instructions varied slightly from reality but close enough to figure it out. I think the videos were more on point.

 

Thanks for this info, mine is supposed to be here Monday. I'm gonna put it on a old win7 that I only use for BRP/Evinrude diagnostic software and some old Garmin stuff. I bought off Amazon and didn't get the WIFI model and I didn't see option for USB drive with the files. I am a bit concerned about the download and malicious embedded **** from reading some reviews etc but I had already ordered it. By using that laptop I won't be risking any personal data. Will see how it goes.

 

AllScanner is the company that manufactures the VX nanao, use their site to download the Manager program
Also there is a wiki page for the nano that will help, need to translate to English

Neal

 

Thanks Neal for this info. It is supposed to arrive today and I will be on it all in the am I hope. Ready and excited to get the Nano running and to get this dam mis-fire issue resolved.