Head Scratcher

[DaWgPoUnD]

2008 H3x I5 with 56k miles with a P0305 code on hand scanner.Intermittent P0305 code with an occasional dip in RPMs/miss fire on a warm start up.
PP TB, AirDo intake installed
All plugs changed, new coil pack on #1 and #5,TB is clean, MAFS is clean, clean air filter.
Swapped coil packs around trying to chase the issue...stayed on #5.
Took H3 to dealer...tech noted miss fires on cylinder 3, 4 and 5 (my hand scanner on picked up on #5). Tech moved coils around and checked plugs..all good and continued to get same miss fires.
Tech recommendation: remove cold air intake and PP TB and reinstall factory set up and see if problem continues.

Truck runs strong except for the occasional issue on a warm start up. I have tried top tier gas on several tanks with bottle cleaner (snake oil right...lol) and still have this code come and go.

Thoughts?
Thanks in advance.

 

[SuperBuickGuy]

did you get the tech to smoke check for a leak in the intake?

 

[atvspeed4]

Sounds like you may have the dreaded head issue. Tech needs to do a leak down test when the vehicle is warm and then keep it overnight. The valve seats may be worn. Unlike what many may think this is not just an early 2006 issue.

 

[DaWgPoUnD]

Negative..he did not do the smoke check....didn't think about that one. It is an odd issue...regarding the head issues is it common for the truck to run strong/smooth if it were to have a head problem?

 

[Happy Hummer]

I'm going to suggest IMHO that the following be looked at:* SBG suggested Intake agreed....* I'd replace all spark plugs and coils....* I'd replace all injectors and O rings on them...*I'd go back to all the oem parts before further testing...As always your mileage may very. Chris

 

[DaWgPoUnD]

Update: Replaced all spark plugs, all new coil packs...no more P0305 but now has P0300 and rough idle. Then swapped out PP TB with OEM and no change...same P0300 and rough idle. Going to swap the MAF sensor next and see what that does....after that it looks like either injectors or an internal issue...any other suggestions?

 

[DaWgPoUnD]

Additional update: dumped in 2 bottles of Techron additive and poof the rough idle went away. Granted...it was only a few miles driven from the time in until i arrived home so it may be too early to ​tell if this result holds.

 

[Joebot8484]

Any updates on this issue? I've had the same issue but no codes (2008 3.7L). It only happens on a warm startup when the truck has been sitting for more then an hour. I did have the dealer replace a broken head stud (that I found) and exhaust manifold under warranty. My Hummer has been to the dealer over and over again for this issue and the problem has yet to be found.

 

[DaWgPoUnD]

Idle issue did return but now it is running great. A few things i found during the process....BAD NEW PARTS...I found 1 new plug and 1 new coil pack faulty of which ultimately resolved my issues with the poor idle. It was quite a frustrating experience and didnt think a new part would be faulty...or I guess I could have damaged them??...It was not a constant poor idle and seemed to come and go based upon certain conditions but all is good now.