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!

What's the best way to warm up your engine in the morning? Find out by ordering Tom and Ray's pamphlet "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1997 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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