Oil control ring stuck on pistons 1, 2 and 3(long)
First photo is from yesterday. Positions from left to right are 654321. They were new 2500 miles ago.
Second photo is from four months ago before the valve job. Positions from left to right are 4321, and they had been used for six months.
Plugs from Cyl 7 to 12 look like those of plugs 2 and 4, so the driver side bank has no particular problem and thus the plugs are not shown here.
Compression (yesterday) 654321: 188, 190, 185, 195, 185, 195 psi. These numbers are encouraging as they suggest I haven't done anything too bad to the cylinders when I reinstalled the heads.
What's informative when comparing the two sets of plugs in the photos is that plugs from 1 and 3 were already more oily and wet back then. Now plug #3 has been plagued with oil deposits, presumably coming out past the oil control ring in piston 3. Plug #1 is also wetter than the rest.
Given that the intake manifold has been modified to not getting oil from the PCV valves, and new valve guides/valve stem seals everywhere(assuming good workmanship), and good compression numbers, I can only conclude that the oil control rings are failing fast in the past half year. It is also convincing that the front cylinders get more oil splashes from below because there is an oil baffle at the rear half of the M70 that minimizes splashes.
My particular case is probably due to the choise of motor oil that myself and the POs have used on this car. Reading some webpages about the GM Northstar engines, they seem to have similar problem to mine.
What I read is that the Northstar, being a high performance aluminum engine, is designed so that the oil control rings are not pressing tight against the cylinder walls. Thus it is easy to gum up by oil sludge and increase oil consumption, which even when normal goes as high as a quart every 3000 miles.
At this point I am looking for some chemical that I can pour through the spark plug openings to soak the cylinders, hopefully to free up the oil control rings. Cadillac has a heat soak treatment in a Oct 03 service bulletin (#02-06-01-009C) to specifically clean piston rings.
The Cadillac kit to go with the above bulletin costs $68. I am thinking if Seafoam or other chemical can be introduced directly to the cylinders and can similarly clean the rings. That approach is ten times cheaper and can be included as a regular maintenance (if it brings some improvements).
Patrick C 88 750 149K