Hi everyone,

I have a 2007 H3X with 218k miles. Engine was supposedly rebuilt at 214k by previous owner. Since I bought it last December i always noticed a slight vibration at idle, just thought it was normal being a 5 cylinder. Anyways, a couple days ago I changed transmission fluid in the truck and let it get up to operating temp after. Just before the temp reached the halfway mark it started misfiring like crazy. Felt like it lost a cylinder. I shut the hummer off and restarted it, no more crazy misfire. So I got my scanner out to check misfire data and I have a cylinder 2 misfire. While the h3 was running the second time I did get a check engine light. It may have come on during the bad miss but I didn't pay attention. P0300 random/multiple cylinder misfire. But the misfire data showed only missing on Cyl. 2, but showed misfire history on Cyl 1, 2, and 4. I noticed that turning the AC on moved my cyl 2 misfire to cyl 3.

Another clue possibly is I checked my oil dipstick after the incident and found that there was no oil AT ALL. Not even a drip on the dipstick. I went to walmart and got a 5 quart jug and was able to put about 2 qts back in. So oil consumption is about a quart every 1250 miles. Someone mentioned a clogged PCV valve could cause this issue, I haven't checked it but I don't have any blowby (Saw that could be a symptom).

Where do I start? I'm thinking since it ran bad then didn't after restarting its a wiring issue somewhere but I don't know. I wish i had the scanner plugged in when that happened.
Thank yall!

yoh3

 

Mine vibrates too, I have another car with I6 twin turbo and it idles sooooo smooth..

​​If it were me, I'd first check all the ignition related stuff. Spark plugs could tell a story (check their condition), then coils. Swapping coils is a common troubleshooting method. For example if you're getting a misfire in cylinder 2 and swap the coils between cylinder 5 and all of the sudden you're no longer misfiring in cylinder 2 but now misfiring in cylinder 5 then you'll know.

Not sure about all the other stuff, I guess you could be misfiring and consuming oil if you got no compression, compression check can rule that out.

 

Back to the basics.
Suck
Bang
Blow
In order to Suck... you got to be able to get properly metered air. Start at air filter. Is it present, clean? Clean your MAF. Use maf cleaner. Verify connection.
in order to Bang...new plugs, coils, connections verified. Compression.
In order to Blow... exhaust must free flow.

There's your order. By the way if you were out of oil it would take 6 qts. Not 2.
I suggest IMHO to change oil and filter. Check for sparkles.

 

Ignition, compression. A common problem is a frayed cable on the ignition coil.

 

When was the last time the spark plugs were changed? Are you trying to us anything other than ACDelco OEM type replacement spark plugs? I5s are very high intensity discharge ignitions and will eat up off brand and fancy spark pugs.

 

Agree wholeheartedly with Doc.

 

I assume plugs were done when the motor was done but not sure. Started having issues with another car so having to drive the Hummer as-is right now. Happy Hummer one time I thought I smelled rotten eggs coming from the exhaust so that could be an issue. I have noticed lately on cold start it takes two tries now. Itll crank crank crank first time then second time it fires up immediately. Do these have fuel pressure regulators? I had a lincoln that would do the same thing so I was wondering if maybe that went bad on the hummer too. And also it runs smooth with the ac on so Im wondering if the added fuel is helping but maybe thats not the issue.

 

you got to be able to get properly metered air. Start at air filter. Is it present, clean? Clean your MAF. Use maf cleaner. Verify connection.
new plugs, coils, connections verified. Compression.
Try changing your spark plugs and coils with GM/ACDelco parts. Verify all your stuff is plugged in.
exhaust must free flow.

I would look at live data. Lng term, Shrt Term fuel trims. Also MAF and TB. Throttle Percentage and O2S.

 

You know what it means to assume something, right? Remove the air intake and pull a spark plug, LOOK at the brand & PN# as well as its condition. If they are not direct OEM replacements, get some ACDELCO41103 Iridium; Pre-Set Gap: 0.043" plugs. Maybe they are just used up as in like the second set ever in your truck? When somebody tries to use cheap off brand or fancy do it all super duper funky Ground Electrode plugs, they usually cough it up at about 5-10K miles. The OEM Iridium plugs will go 100K or more.

While you have the Intake resonator box off, inspect all the wiring to the 5 coil packs for compromise, clean all the connectors to each coil pack with CRC Electronic Cleaner.

Easy stuff first, and the above is pretty easy. Good luck.