JKofPA

So I have a 2012 JKU with about 113k miles. It runs smooth and never really stutters. However I get a P0304 misfire on cylinder 4 code randomly. It goes off after some time, but comes back. Well I had it in the shop for some other things (oil pressure switch/sensor and dual stage oil pump... that's a while other story...)

Anyway, while the mechanic was in there I had him do a compression test and cyl 4 is definitely low. See below:

Cyl 1 - 140 psi
Cyl 2 - 150 psi
Cyl 3 - 125 psi
Cyl 4 - 90 psi
Cyl 5 - 135 psi
Cyl 6 - 140 psi

So 4 is 90 PSI and according to his notes no cyl should be under 100 and they should also all be within 25 psi of each other. Well Cyl 2 is 150, which I suppose is high then.

Anyway, what could I look in to to fix cyl 4? Unfortunately I live in a state that requires years inspection/emaissions and a CEL will not pass. The guy at the shop wasn't too overly helpful, just suggesting staying under 5k miles in order to be exempt from emissions.

There has to be something that can be done, right? To be fair, I am not a mechanic, but I am able to do a little under the hood...

Thanks all!

Sv_dude

I hate to be the bearer of bad news but I would just assume you need a new head. If you have low compression, it’s either a piston that’s not sealing (not likely) or a valve that’s shot. Our engines have a bad history of valves getting destroyed. I just finished replacing the left head on my Jeep today. See my thread, it’s here on the first page “P0304 troubleshooting “.

JKofPA

@Sv_dude I did happen to see your thread. I read through most of it before posting. I didn't see anything about a compression test (but I was skimming pretty quick), so I was hopeful my outcome would be different than yours. Really dreading the need to do the head!

resharp001

Agree with sv_dude. unfortunately there is not some quick fix for any low compression issue.

Sv_dude

Well… I think if you do a leak down test you’ll find that one of your valves is shot. I doubt it’s a head gasket because I think you would have seen coolant in your oil or the white smoke coming from your exhaust.

either way, I think you’ll need a head. It’s gonna be about 3k for a shop to do it or you can do it yourself for about a grand or a bit more depending what you replace along the way.