-Replaced the headgaskets and had the cylinder heads rebuilt in early September, due to external oil leakage
-Car runs great, smooth, has plenty of power, typically gets 24-27 mpg in mixed city/highway driving
-All of the bores appeared smooth when I tore down the engine for the headgasket change
-After the headgasket change, the engine consumed no oil for about 2,000 miles, then rapidly burned through an entire pan til the oil light came on (while turning into my driveway) within a few hundred miles of highway driving
-I replaced the PCV at that point, refilled the oil (4 quarts), and the problem seemed solved. I could not blow through the old PCV valve (an aftermarket) so I think it was clogged.
-It happened again, about a monthl later, though not as quickly (refilled it before it got really low), so I put in a genuine Subaru PCV valve which seemed to fix it again, and changed the oil, also adding a can of Restore.
-That seemed to fix it until the last trip, when it burned about 2 quarts
-It seems that when it isn't burning oil, I get the P0420 code every couple of days and I have spark knock under light-throttle acceleration. When it starts burning oil, both of those problems go away.
-Oil is Valvoline 5W30, which I've used on the car for the past 3+ years with no consumption issues, only leakage before replacing the headgaskets
-The underside of the car and engine is bone dry. I do not believe I have any oil leaks.
-All four spark plugs look normal and none are oily or smell oily.
-Oil does not appear to be entering the coolant, the coolant appears clean and not oily.
-I did a compression test today, and here are the results (cylinder: wet (dry))
1: 182 (210)
3: 184 (230)
2: 182 (225)
4: 195 (230)
I'm stumped. I've also asked this question on Subarforester.org. A leakdown test was recommended next. Since my compression appeared good, I'm not sure if it would tell me anything but I may try one anyway. Could some other problem, possibly in my PCV system, cause this? The valve is clear right now, I pulled off the hose with the engine idling and it was pulling in vacuum. All of the hoses appear OK but are 13+ years old.
Edited by DirtRoadRunner, 29 December 2012 - 05:04 PM.