Dear Tom and Ray:

I have a significant mystery on my hands. I installed a completely rebuilt
500-cubic-inch (1975) Cadillac engine in my 1981 GMC Suburban. I then towed a
travel trailer 11,500 miles and it used a quart of oil every 300 miles. I'm not
leaking any oil. The Suburban does not smoke on start-up or during normal
driving. A compression and leak-down test was performed and the results were
normal. The PVC valve was replaced.

I also tried switching from 10W-30 to 20W-50, and still no change in the oil
consumption. I then replaced the umbrella valve stem seals with official
Cadillac valve stem seals. Still no change. Any ideas? -- Charlie

RAY: Yeah. I think you need some official Cadillac rings, Charlie. I think the
rings on this engine never seated properly.

TOM: When engines are new, the metal oil-control rings take some number of
miles to mold precisely to the walls of the cylinders. That usually happens
during what we call the break-in period.

RAY: If the rings don't seat properly, oil will -- forevermore -- sneak around
the rings and get burned in the combustion chambers. And that's what happening
in your engine.

TOM: During the break-in period, you're supposed to drive the car especially
gently, in order to allow the rings to seat. But instead of driving gently, you
towed a house around behind you! And towing is probably the hardest thing you
can ask an engine to do.

RAY: So I think you ruined it, Charlie. Unless the engine was not rebuilt
properly, you cooked it by towing with it during break-in. The good news is
that you're driving a Suburban. And with all that extra room in the back, you
can probably just throw a 55-gallon oil drum in there and set up a drip feed!