Had a check engine light P0305 & P0420, cylinder 5 misfire and catalyst efficiency below threshold bank 1 - swapped coils and spark plugs around to see if misfire moves, it didn't. Checked live data and can see only cylinder 5 is misfiring and only at idle. Once I increase RPM it goes away. Fuel injector maybe?

That would be the next logical thing to swap around and check, but the whole process is a major PITA

2007 3.7L with 150k miles

 

P0420 means that either 1) your cat is shot and needs replacing or 2) your downstream O2 sensor is bad and producing erroneous readings which is triggering a P0420 code. It is possible that due to the misfire, your cat ingested some excess air/fuel mixture, which caused it to overheat, and now it is operating less than ideally.

Take a Scan Tool, overlay the O2 Bank 1 Sensor 1 and Bank 1 Sensor 2 graphs; if the graphs more or less overlay each other (S2 rises when S1 rises, S2 drops when S1 drops, etc.) then you probably need a new Cat. Too much to explain here in one post, but there are plenty of great references online on how to interpret the B1S1 and the B1S2 readings.

Regarding your P0305, I would be willing to guess that it is due to some kind of valve leakage, whether due to bad/softened valve seats (most common on 2006 models, but can also affect later model engines as well), or simply due to some carbon build up on the valves causing them not to seal 100%.

I would recommend running either a compression test or a leak down test, measuring either the PSI or the Leakdown % on cylinder 5, then run some either Seafoam, Marvel Mystery Oil, or some type of PEA based fuel system cleaner through the engine, then re-do the compression or leak down test.

Here is a great thread from which shows Hummerz setup for running Seafoam through the engine. Just be careful not to add too much at once and risk hydro locking the engine.

https://www.hummerforums.com/forum/h...ling-h3-48822/

If you do this and your compression values get better, then the P0305 should clear itself out over time. If the compression values do not get better, then more than likely you have a failed valve seat.

Regarding the Fuel Injector consideration - what are your STFT and LTFT values when the engine is heated up and at idle? What about at 2000 RPM? Post them on here if you can.

 

Thanks for the reply. STFT and LTFT look normal. I went ahead and just replaced the injectors since I had the intake manifold out. no change. Still misfiring cylinder 5 once up to operational temp.

I did the compression test, cylinder 5 measured 155psi, the rest were about 180-185psi, added some oil to cylinder 5 and re-tested compression and it increased significantly to 295psi, this makes me believe something is up with the rings. Understand leak down test would be a sure way to tell, I just need to purchase the tool to do it.

About 2 years ago and 30k miles I did compression test while diagnosing engine running hotter than normal and every cylinder was about 180-185psi. I maintained the engine well since I took ownership, oil change every 5k miles 5w30 mobile 1 oil etc. I added external transmission cooler to keep temperatures at bay, even though the truck still ran hotter than I'm used to seeing with other vehicles 217-225F on very hot summer days (Doc said it's normal, Humz said it's not) so idk

So did something inherently cause cylinder 5 to fail prematurely? Are these engines known for cylinder 5 issues at this millage? I know 3.5L are known for head issues, what about 3.7L?

Anyway, looking into Marvel Mystery Oil or something I can add and see if it improves. Otherwise might have to sell this thing as I don't feel like rebuilding the engine 😔

 

CARBON BUILDUP on the valves will cause misfire - low cylinder compression!
LOOK HERE>>>>>>>>Service Bulletin

 

Oh yes I saw a few threads on this and to use seafoam through the intake to clean up. I guess without doing a leak down test I won't know for sure if it's valves or ring related. As mentioned I did add a little oil to the cylinder and it significantly improved compression, which made me think rings are a culprit rather than valves. If valves weren't sealing properly I imagine compression wouldn't improve with the added oil?

Is there a TSB for sticky rings due to carbon buildup?

 

NOPE!
Piston rings fail due to a combination of excessive wear, poor lubrication, overheating, and contamination. Chronic issues like not changing the oil or filter, engine overheating from a failing cooling system, or damage from dirt and debris lead to increased friction and premature wear. Ultimately, this causes the rings to lose their seal, leading to decreased compression and performance.
Monitor engine oil level & color:

 

This is basic stuff isn't it?

Anyway did a B12 cylinder soak along with some MMO improved compression a tad bit to 165psi - misfire improved slightly but was still there per live data. Sprayed Seafoam down the throttle body and that seemed to have helped reduce the misfire, only happens occasionally now but still present. Might do another can or just use Chevron only moving forward.

 

Smart move forward just remember patience is the key to success before throwing parts at it!
Decarbonization is a maintenance process that removes carbon deposits from internal engine components, improving performance, fuel efficiency, and longevity. This is achieved either mechanically by physical cleaning or chemically by using specialized additives or solutions that dissolve the buildup. Deposits can cause reduced power, poor mileage, and increased emissions, so decarbonization is a preventative measure that restores the engine to its optimal state.

 

Also forgot to mention, I do have a weird intermittent rough start, usually happens after the truck been warmed up and I leave it sitting in the parking lot or something while at the store. I get back in and it cranks a bit longer than normal, but does start. This doesn't happen all the time either, just randomly.

Battery is new, I just put new fuel injectors in, I will test fuel pressure probably this weekend. But wanted to get your take on what it could be. Just another symptom of lower compression in cylinder 5 or cam position sensor related? I saw some old azzed thread where you mentioned engine temperature sensor being the culprit.

 

Correct terminology. "Old azzed thread."

CPS might cause a slow/rough start. I would ride the misfire codes into the ground BEFORE changing anything else as making multiple changes simultaneously nearly always clouds the eventual solution.

Carry on and best of luck...