I've got a 99 Forester with 77K. It ran well enough when I bought it, but it blew blue smoke out the exhaust. Compression was right around 150 in all cylinders but went up to 180 when I did it wet. So I figured it needed rings. When I got the engine out, I did a leak-down test and all cylinders were under 5% leakage. I expected to find more leakage past the rings.

 

I've got one piston out, hoping to see something definitve, but I don't really know what I'm looking at. The cylinder walls look fine and the pistons look fine with a bit of carbon build-up on top. The rings look healthy. Ring-to-groove clearance is well within specs.

 

Can the oil control rings be weak and not show on a leak-down? My plan was just to slide new rings on without cracking the block.

 

One other thing: When I rotated the crank with the heads off, I could see a film of oil on the cylinder walls above the pistons. I haven't noticed this on an engine before.

 

Thanks for your input. Tom

 

So my question is:

I would have checked the PCV and breather system before tearing the engine down.

Did you happen to notice any oil on the valves? That might indicate the valve stem seals are leaking.

 

The oil control rings don't really affect compression as far as I know. So it could be that they're allowing some extra oil past.

Do the rings move freely in the grooves? They're not all clogged up with carbon?

 

5% leak seems fairly high to me for an engine in good working order. Not sure what Subaru specs are for cylinder leak down, but most that I've done the 2-3% range is acceptible, and anything higher is usually worn rings or cylinder wall damage.

 

How does the rest of the crankcase look inside? Is it clean? Is there any goo/tar like substance in the corners?

How does the hone in the cylinders look?

FairTax,

 

The PCV valve was working. No oil on the valves. Rings slide freely and look unworn. Crosshatching is very visible on the walls. No gum or goo anywhere. Some carbon on the tops of the pistons. Some buildup on the underside of the exhaust valves in cylinder 4, about 1/16". Mostly a very clean looking engine.

 

I didn't take real careful measurements on the leak-down but it was clearly under 5%. I couldn't hear any air blowing past the rings. I was under the impression that under 5% was quite acceptable. I was also under the impression that a rise of 30 PSI on a wet compression test indicated worn rings.

 

Thanks for your help. I don't want to pull all the pistons unless I need to, though, at this point, I may as well. I wondered whether anyone had seen this kind of thing before.

 

Thanks, Tom

Valve stem seals - probably not rings. Stem seals won't affect a leakdown test.

 

GD

I'm no expert, but I'd be inclined to say valve stem seals are your culprit. It sounds like the internals are in good shape, and with no other obvious signs of wear the chances that the rings are worn seem pretty slim. Unless you can find some pretty big score marks in the cylinder walls. But I think cylinder wall damage would have been evident in the leakdown test.

 

If the PCV valve is clean and in working order, did you check the breather hoses for obstruction? (i'm going to guess they're clear)

I'm inclined to stay out of this except the rings would influence that "oil film" you mentioned. The Haynes is good about this area for it shows two particular things. The 2nd ring is tapered and if installed upside down can cause this. Also the oil ring has a certain way. Page 2B-21

I'm inclined to stay out of this except the rings would influence that "oil film" you mentioned. The Haynes is good about this area for it shows two particular things. The 2nd ring is tapered and if installed upside down can cause this. Also the oil ring has a certain way. Page 2B-21

 

Good info Fuzpile.

I dug into the FSM after reading your post.

Here's what Subaru has to say about that:

4. PISTON RING

1) If piston ring is broken, damaged, or worn, or if

its tension is insufficient, or when the piston is

replaced, replace piston ring with a new one of the

same size as the piston.

CAUTION:

I “R” is marked on the end of the top and second

rings. When installing the rings to the

piston, face this mark upward.

Thanks for all your thoughts. I don't think it's the seals for the following reasons:

 

The wet/dry compression test

The blue smoke wasn't worse at start-up.

The oil film that the pistons seemed to drag up the cylinder when I hand-rotated the crank.

There is nothing in the throat of the head around the intake valves to indicate that there was oil dripping through. (The exhaust valves point uphill, so I don't see how oil could leak past the seals there.)

 

I set the heads upright last night and squirted oil around the intake valve seals and none had dripped through by this morning. Not proof, just more evidence.

 

One explanation I found for the good leak down test was that the oil seeping by the rings helps seal them up.

 

I'm still thinking it's the rings. I'd like to know why. Was it run low on oil once and the oil rings are the first to go?

 

What Haynes do you have Fuzpile? 2B-21 is a glossary of terms in my book.

 

Tom

Did you spray the exhaust valves as well? Those would not leak into the cylinders, but small amounts of oil dripping into hot exhaust would still burn off and cause it to smoke.

due to a piston slap issue sometimes the pistons are replaced. i would try and see if possibly this piston (or more than one) have been replaced on this engine. maybe it looks different or has a part number or markings on it somewhere? you could search on here or google the piston slap issue and the parts used to fix it and maybe hit something helpful there to look into that.

 

in general piston rings are not a concern with subaru's. i realize it's possible, just not very probable (understatement).

 

but on the off chance that the pistons were replaced, which i've heard of people/Subaru doing due to the piston slap issue then maybe something was installed wrong or not lined up, etc. maybe they were done under warranty or even later?

 

i believe the rings need to be offset too - the gaps in the rings can't be lined up or that will encourage oil passage if these are gapped rings which they should be i think.

 

but i'm with these jokers, valve stem seals are the most likely culprit i'd guess. how long did you drive the car like this? i would have driven it for awhile and put some miles on it to get a gauge for what's going on.

Ring failure on a Subaru engine with 77k on it is unheard of. I would be looking for a neighbor kid that squirted oil in the plug holes every morning before I went looking for ring failure. It's that uncommon.

 

If you think it was rings, then hone the cylinders and replace them. I don't know what else to tell you since none of us are there to look at it.

 

GD