Low compression

[trek686]

Noticed a shudder / stutter in mid rpms acceleration in my 2010 3.6r about a week ago. It got worse each day and sure enough the check engine light came on yesterday. Dropped it with the mechanic and guessed it was probably a coil. He called me and sure enough it was a misfire in cylinder 5. He did a compression test to make sure there weren't bigger problems. He ran it twice and unfortunately, the cylinder is coming back at 90 psi.

 

The car had been drinking about a quart every 1500 miles but I thought this was normal from reading other reports. Has anyone else had their engine go? The car has 138k. I thought I'd get more miles out it.

[MilesA]

Low compression does not mean your engine is gone. It probably needs some repair. Unfortunately, the compression number alone doesn't give you enough info to know the cause. Doing a vacuum gauge test and a leakdown test would help you know if the problem was in the valvetrain, or the pistons/rings.

[DrD123]

check all 6 cylinders, too (a single number - though that one is pretty low - is less helpful than knowing the spread across the engine)

[nads]

With variable cam/valve timing compression can change depending on where the cam sits. I don't know but there may be a procedure for doing this.

[YeuEmMaiMai]

if you got 90 psi, stick a boroscope down there and take a look around in there...

[FLlegacy]

With variable cam/valve timing compression can change depending on where the cam sits. I don't know but there may be a procedure for doing this.

 

Cam timing is only adjusted during engine operation, with no oil pressure the cam timing goes back to 0. And at TDC all valves will be closed no matter what otherwise no compression would build.

[trek686]

Thanks for the suggestions. I got a decent trade-in offer from the dealership so we traded it in for a loaded 2.5i outback. I was looking to get another 3.6 but it's for the wife and she only cares about the mpg.

[nads]

Cam timing is only adjusted during engine operation, with no oil pressure the cam timing goes back to 0. And at TDC all valves will be closed no matter what otherwise no compression would build.

 

Engines build oil pressure as you turn them over, even if they don't fire.

 

I've seen people build new engine with large cams and think they have a compression issue do to low numbers but its the overlap of the cam causing it, or they degreed the cam and crank incorrectly or not at all.

[FLlegacy]

Engines build oil pressure as you turn them over, even if they don't fire.

 

I've seen people build new engine with large cams and think they have a compression issue do to low numbers but its the overlap of the cam causing it, or they degreed the cam and crank incorrectly or not at all.

 

But that isn't even remotely the case here, the oil control valves only function with the engine running, and it would affect the entire bank, not a single cylinder. It would also set a code for the OCV and multiple cylinder misfires on the same side of the engine, if the OCV was stuck or not working properly.