I’ve had my Subaru back together and tuned and stuff for a few weeks now. Since I’ve had my new motor it has spat a little puff of oil smoke out the tailpipe when you started it up, it was pretty minor so I’ve never worried about it, My parents Audi has done the same thing since like 60,000kmi, didn’t concern me. And i’d built this motor personally, its not like some redneck machine shop tossed it together and I know at least a decent amount about these engines so this is even more confusing to me.
About a week ago I took it on a few hour drive and about an hour into it it starts smoking like absolutely insanely bad. Not really bad when you were driving but if you stopped and sat at a stop sign or red light it would make a huge cloud when you went to take off, also did it when you got back on the gas after engine braking for a while. If I got into boost it would completely stop smoking and it still had plenty of power and started and ran great. When i got home I checked the oil, it burned a solid quart since i’d checked it that morning.
I got home and stripped it down thinking it was like a cracked ringland or something, fist thing I did was a compression test, first I should mention though that all the plugs were spotless except the #3 which was covered in oil. 1, 2 and 4 were right where they should be, #3 was about 20 psi high, I figured it was due to carbon buildup. Then I did a leakdown, #3 actually had the lowest leakdown, only like 4%.
Based on this I pulled the intake to look for oil seeping around the valve guide from a fucked up seal, there was like half a drop up against the back of the rear #3 intake didn’t seem like alot but hey, still shouldn’t be there. I went and replaced both of the cyl #3 intake valve seals, whats funny though is they looked perfectly fine when I took them out so I wasn’t too optimistic. Neither guide had excessive play either, at least as far as I could determine from just wiggling them around.
Anyways put it back together today and its exactly the same, no worse, no better. I went and threw a bottle of that stop leak stuff from the auto parts store in it and that actually made it like 90% better, it really only smokes now if you let it idle for more than 2-3 minutes but that isn’t a real permanent solution.
What else can I check. The only other thing I’d thought of was a crack in the intake port except i’m pretty sure that would make it burn coolant, not oil, I don’t think there are any oil passages adjacent to the intake ports. I also looked for a crack in there and didn’t see one but its hard to see and it could be under the valve stem I suppose.
I think i’m going to just buy a new set of heads and build them and hope it doesn’t get any worse in the meantime. The heads actually arent new, just the shortblock, I just stuck new valve seals and springs in them and lapped the valves. It isn’t too bad actually with the stop leak in it but I don’t want to keep it that way. Just wondering if there any possibility other than a cracked intake port.
TL-DR: Car burns oil on cyl 3 during extended idling, especially when taking off from said idling, #3 has high compression and perfect leakdown. Replaced #3 intake valve seals, didn’t work. What else can cause this?
Lets say the slugs is broken…Under boost the compression force will push the ring out and lock a broken ringland in place, sealing it up tighter than at idle with a vacuum on the bore. This is typically the case on a subaru because when a subaru breaks a ringland it usually snaps off in a large piece about 25-30% the pistons circumference VS small pieces that get dislodged under use.