Cylinder 4 misfire - 07 pontiac g6

I’ve already done all the typical stuff to try and fix this problem - new spark plugs, ignition coils, fuel injectors. I know for sure there is a bad valve cover gasket, the old spark plugs had some oil on them, but I was told It’s probably not related. Any help would be appreciated, the problem is getting annoying and expensive.

Oh, also, this is some chattering in the engine while accelerating, coming probably from the side cylinder 4 is on. And in addition to the misfire code, I also get a lean fuel/air mixue code (forgot the number.)

I can’t tell you much if you don’t tell me the engine in your G6 and how many miles on it.

That said, #4 has a mechanical problem. Bad rings, valves, leaking head gasket, bad piston… Needs a compression test on all the cylinders so you can determine the problem.


Thanks, I will end up doing a compression test whenever I get the tools. It has 131k miles.

Compression results are:

Sorry, forgot to mention the engine is the 2.4l.

Cylinders 3 and 4 are toast. If you have been losing coolant, the head gasket may have failed between 3 and 4.

If you squirt a little oil in and do the test again, if there is no change the valves are leaking or the head gasket has failed. Remove the head so you can see inside. If it is only the gasket or valves, replace the gasket and re-grind the valves. A machine shop can do this and check the head for flatness.

If the compression comes up significantly - 20-50 psi, the rings are shot and a new or used engine is required. This seems likely given the rattly noise you hear.

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Well thanks for the info. I don’t have the compression gauge to check again right now, but I can try again tomorrow. Do you think it would be safe to drive until I can get a new engine put in? The mpg is literally at 17.5 right now :unamused:

To find out why compression is low on cylinders 3 & 4. do a leak-down test on cylinders 3 & 4.


As long as it runs you can drive it. It won’t have much power. It might leave you stranded at some point. But not going won’t hurt you.

Let’s be brutally honest . . .

The engine is barely running, as per the compression results

I suspect #3 has plenty of misfire activity, probably barely under the threshold for generating P0303

A smart guy with a pro-level scanner could confirm this

My gut feeling is top end . . . as in valve train

And I somewhat disagree with the op’s notion that oil on the spark plugs is not an issue . . . we’ve all seen cases where plugs that were drenched in oil couldn’t fire a spark through that stuff

I agree about the 3rd cylinder, but I’ve checked the spark plugs today, and the oil wasnt bad enough to cause a short. Just cleaned off the blackness - which they all had, and put them back in. The worst offender with the oil is cyl #3, if i remember correctly. Two of them didn’t really have any at all.

The cylinder head might be due for a valve job

Misfires can send raw fuel into the cat, which will eventually damage it. Since that part is expensive to replace, don’t defer on this repair too much longer.

Top end or low end; it really doesn’t matter. The engine needs major work. Drive it to the bitter end if possible without spending more money.

For what it’s worth in the future, always verify compression issues before spending any money on what is likely unneeded parts.
One simple way of doing this is to connect a vacuum gauge to check engine condition. It’s quick and easy to do this and if the vac. gauge shows a mechanical problem then it can be verified with a compression test to determine which cylinder(s) are at fault.

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