Dear Tom and Ray:

I recently purchased a new GMC Suburban -- diesel. This is my third Suburban, and having logged 200,000 miles on my other two, you could correctly assume that I like these vehicles and I am very serious about maintenance. On my last service, I noticed on inspection that the oil level was overfilled by at least a quart. I brought this to the attention of the dealer, who dismissed it as a bonus for me, since I hadn't apparently been charged for it. Perturbed by his lame explanation, I decided to consult you two gurus (guri?). What are your thoughts? -- C.W.

Tom: I think you should go back to the dealer, tell him to drain out the extra quart, and as a bonus, tell him that you're not going to insist on store credit for the returned oil.

Ray: Actually, yours is a borderline case, C.W. Normally, overfilling the crankcase by a quart IS bad for the engine. If the oil level inside the engine is too high -- if it touches the bottom of the crankshaft -- the crankshaft can do a "Betty Crocker" on the oil, and whip it up into a nice, bubbly foam.

Tom: That's great if you're making chiffon pie. But the problem is, the oil pump can't pump foam. And since it can't pump foam, some parts of the engine won't get properly lubricated.

Ray: In an engine that holds four quarts of oil, an extra quart could definitely cause this problem. But since your Suburban holds seven quarts of oil, it's more of a borderline case.

Tom: In general, I'd use the 20 percent rule. If your crankcase is overfilled by 20 percent or more, it's worth draining out and correcting right away. But since you plan on keeping this beast for 200,000 miles, I'd play it safe and insist they take care of it, C.W.

It's amazing how many bad driving habits and wacko theories have been passed down from generation to generation! Tom and Ray set the record straight in their booklet "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.

© 1996 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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