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Toyota Repair/2002 Corolla Oil Consumption

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Question
I recently purchased a 2002 Corolla automatic with 120,000 miles for my son to drive.  The car drives well, has good power, great fuel economy and starts right up.  I thought these were great cars according to Consumer Reports, etc.  However, it is burning a quart of oil every 150 miles or so.  The back of the car has soot on it.  I have done a lot of internet searching and found this is frequently an oil control ring sticking problem with this 1.8 liter engine, or a PCV issue. To try to reduce the oil consumption and keep it running properly I have done the following:
-replaced the PCV valve and verified approx 22 in Hg vacuum is being supplied to the PCV valve at idle in neutral.
-replaced the leaking valve cover gasket
-replaced the spark plugs, all of them had ash deposits consistent with oil burning
-checked cold compression which was within 10 psi across all cylinders at 140 psi average (don't trust the absolute psi value as it's a Harbor Freight tester, but do trust that they are pretty equal across all 4 cylinders)
-completed a 24 hour piston soak with Kreen (checked on Bob is the Oil Guy website to try to find the best chemical to use and that was one of the best).
-I have a good quality manometer and checked the crankcase vacuum at idle and with the engine revved up a bit (2000 rpm? - no tach) in gear - the vacuum level at the oil dipstick tube is about 3/4" water column at idle and at approx 2000 rpm in drive.  No change when engine is revved up.
I have a few questions that I hope can help me avoid tearing down this engine:
Is the crankcase vacuum level not changing with rpm indicative of a crankcase ventilation problem?  I thought it should get to about 4-6" water column when the engine was revved up.  If the vacuum level should be higher, what could be causing this to be low?
Is there a better piston soak chemical and procedure that I can use to try to free up the oil control rings?  Is there any kind of commercial pressure type cleaning process that would loosen up the frozen oil control rings?
Should I just use heavy weight oil (20w50) and STP to try to reduce the burning by thickening the oil?  Currently using 5w30 Shaefer (Sam's Club) oil at $30/doz quarts.
Thanks for any advice you can give.
Pete

Answer
As you already know by now this is a not uncommon problem on some of these engines, the oil rings don't just stick they collapse and freeing them up or trying chemicals to do so is a waste of time and money, by now you probably also realize this is why the car was sold by the previous owner. Toyota has updated the piston rings, there should be a copy of the service bulletin out there somewhere online, they recommend replacing the pistons and the rings with updated parts, there are no oversize pistons in case you are wondering whether to have it bored oversize, this is not possible on this engine due to it's design. This is what it takes to really fix the problem, you can try heavier oil etc. but I can't give you any recommendations on that Like I said, the rings collapse because the tensioner spring behind the oil control ring is too weak, after they collapse there is a lot of carbon that builds up, the rings loose their tension, the result is the engine burns oil.

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