low compression in one cylinder

[ForrestM]

Hi Guys! I'm a Subaru Noob with a question about my 83 GL Wagon. I got this car as a freebie, with the intention of fixing it. The front cylinder on the drivers side had severely low compression (60 as compared to 170 on the other 3, according to P/O). Wet test brought it up to about 90. P/O is a fairly competent mechanic and thought the rings on this cylinder were gone. My question is, how likely is his diagnosis to be correct, or should I look for other issues? Engine is in car with the head pulled, and access plug removed from behind water pump, so will I have to remove the engine no matter what the diagnosis, or can I R&R the rings in-frame? Thanks for any help.....

 

Forrest

[MilesFox]

The usual suspect is the head gasket. Rings never go bad. Head gaskets always go bad. In the world of subarus, anyway.

 

But if your mechanic is an old school GM kind of guy, the subaru is a whole different bag for him.

[ForrestM]

It did have a head gasket problem on that cylinder, also (I am not sure of the symptoms). I'll contact the P/O and pick his brain some more. Thanks for the info!!

[Deener]

I've had a bad ring before. It drove me crazy trying to diagnose it because it just didn't seem likely. A good way to tell if you have a bad ring is to do a wet compression test on the suspect cylinder. Just dump a tablespoon or so of oil into the cylinder and retest the compression. If the compression magically raises substantially versus the dry test number, then the oil you just poured in has sealed the cylinder where the ring couldn't and you will know the ring isn't holding the compression in.

[ForrestM]

Thanks, Deener. Wet test brought the compression from 60 to 90, I think it was (according to p/o), but I dont know if this indictes bad rings.

[Deener]

Well when I experienced this, mine was 70 at first then I tested it a few more times wet and the compression raised to around 110/120. Did you crank it for more than a couple seconds when trying? I would try it again very thoroughly just to be sure that you only get 90. If it doesn't raise much more than 90 with a wet test then you could have a bad/bent valve/stem/seal. Or maybe a combo of a little of both...ring and valve issues...(other board members opinions would be good here...)

 

If you can, pull that head off and take it into a shop for a valve job (or depending on your abilities - clean it yourself) and they will tell you if its sealing or not. I had a valve grind done on one of my heads for 40 bux up here and that was with me pulling the head and dropping it off for the machine shop. With the head off you will be able to spot a ring problem. Here is the thread where the ring was failing, there should be photos attached:

 

http://ultimatesubaru.org/forum/showthread.php?t=121266

 

You can see a definite 'charring' of the piston right near the edge and a big scar on the piston sleeve right near the 'brim/lip'.

 

Good luck and keep us posted!

[GeneralDisorder]

If the wet test only brought it up to 90 then it's got valve sealing problems. Possibly a burnt exhaust valve. The rings are sealing from the oil but the valve can't and continues to leak.

 

Unlikely to be a head gasket - those don't typically show up on compression tests. Also it would be having other issues related to the cylinder compression escaping to where it should not be.

 

Why wasn't the compression test followed up with a leak-down test? Wet/dry compression test is just a pass/fail for the rings. The rings passed that test - now you need a leak-down to check the intake/exhaust valves.

 

GD