[Tommy13t]

2009 Buick Enclave CXL AWD with 98k. I have a noticeable misfire with codes reading misfire on cylinders 1,3 and 5. All coils have been replaced and all plugs are new. I also get a code reading catalyst system below effective threshold. I replaced the O2 sensor on the back side, pre catalytic converter, and still get the misfire codes. No vacuum leaks as far as I can see and detect. Do I need to also replace both O2 sensor in pairs on the back side? Assuming 1-3-5 are considered the back side.

 

[JayS]

Any evidence of coolant in the oil, or burning coolant in the exhaust? I’d probably do a compression test. That you replaced the cats and had misfires concerns me because if it was run with them plugged and misfiring long there’s a good chance of head gasket problems.

 

[electbc]

Did you get the misfires only after replacing the plugs and coils? If the misfires came after, I would suspect something that was disturbed in the process. If you moved the fuse block , that would be a good place to start checking as there are two fuses, one odd and one even cylinders.

 

[Tommy13t]

Any evidence of coolant in the oil, or burning coolant in the exhaust? I’d probably do a compression test. That you replaced the cats and had misfires concerns me because if it was run with them plugged and misfiring long there’s a good chance of head gasket problems.

The O2 sensors were white when I took them out but no other evidence of a blown head. No coolant loss over the last 6 months.

 

[Kingsfr]

Possibly carbon buildup on the valves. Its a common disadvantage of Direct Injection engines.

 

[RedSoxFan02116]

I've been in a cylinder misfire battle with my 2011 Enclave for a few years. Thought I would share some info. in case it helps anyone. I had misfires left and right a year or two ago. I replaced the spark plugs and all of the coils. When I did... I noticed some oil on top of 2 of the spark plugs (not good). Not a ton but enough. I still had misfires. Turns out- I used the expensive Bosch spark plugs and should have used the AC/Delco 41-109 OEM when I changed my plugs. Once I went back and changed them to the AC Delco, misfires went away and all seemed to be fine. In the midst of troubleshooting (the first time) - I also used Seafoam in the intake (which threw a Stabilitrak warning) but that reset itself. I used Techron Concentrate regularly since then.

Fast forward to this year. CEL came on again. Read the codes and its Cylinder 1 - 3 - 5 (pending) misfires. I checked the PCV valve that I corrected last year by drilling a larger hole. The MAF was clean and no oil. So I started researching again. I really didn't want to take my manifold apart again. I started wondering if I had bad coils. After tons of research - these misfires can be caused by a lot of different things:

1. Carbon buildup on the valves
2. Bad sparkplugs
3. Bad coil packs
4. Bad 02 sensor(s)
5. Bad fuel injectors
6. Stretched timing chain (uggh $$$$$)
7. Catylitic converter issues

So, this time I decided not to clear the codes. I used a 20 ounce of Techron Concentrate, filled her with 93 gas. Ran it a week. I was prepared to take it all apart and swap the Coil Packs from 2-4-6 to 1-3-5 and change the spark plugs in 1-3-5 when something happened . . . the CEL went away. There had been enough starts since the last misfire that the code cleared.

I remember reading somewhere on the forum that GM had a bulletin on a Firmware update to the On Board computer related to cylinder misfires. Perhaps the update causes the computer to "not be so sensitive" - Anyone have any info on this? For now, I think I'm going to keep monitoring it and do nothing else until the CEL comes back. Keep running 93 fuel and use Techron. It seems like you can throw a lot of money and time/labor at this issue.

 

[electbc]

Thanks for the update. I don't know of any Firmware update but GM does recommend using Top Tier fuel which has the Techron ingredients put in by the retailer. An ounce of prevention is worth a pound of cures. GM Techlink had a post on Misfires and Carbon Buildup, I can't seem to find it, it looks as if it was deleted.

Yes I believe this was it. https://gm-techlink.com/?p=6659

This is a similar procedure, not exactly the one on Techlink. https://static.nhtsa.gov/odi/tsbs/2017/MC-10137635-9999.pdf

I suspect that there are issues when carbon deposits fall off and get inside the cylinders wreaking havoc. I would suggest doing any procedure with the engine off, valve closed and clean out any residue with vacuum or compressed air before cranking and or starting engine. You want all debris out before opening any valves and getting in the cylinders.