Dear Tom and Ray:
Is it safe to add a few capfuls of denatured alcohol to the crankcase of my two older vehicles to remove oil sludge before I change the oil? — Lorilee
Ray: Is it safe? Well, I don't think it's ever a good idea to dilute your oil. But you can put a couple of capfuls of almost anything in five quarts of oil without doing a whole lot of damage. I mean, you can add a couple of capfuls of cat urine to your crankcase and still drive the car for a long time. The question is, Why would you want to?
Tom: The same question should be asked of denatured alcohol. I don't see a good reason to do it.
Ray: First of all, while denatured alcohol is a solvent, we have no idea whether it does anything to remove engine sludge.
Tom: Second, we don't even know if you have excessive engine sludge. Did your mechanic tell you that your oil is not draining down quickly enough from the top of the engine? Or do you just have S.A.D.: sludge anxiety disorder?
Ray: Unfortunately, even if you're sure your engine is sludged up, it's unlikely that any oil additive is going to fix it.
Tom: We work on Volvos that develop sludge problems sometimes. What happens is that the crankcase ventilation system gets plugged up and doesn't allow oil fumes to be purged from the engine. So those fumes get trapped and solidify, forming sludge and gunking up the top of the engine.
Ray: At that point, the only way to remove the sludge is to remove the valve cover and actually scrape off the stuff.
Tom: If you're wanting to add the denatured alcohol simply as a preventative measure — if you're worried about getting sludge in the future — then the single best thing you can do is change the oil regularly instead, and make sure your crankcase ventilation system is working properly.
Ray: Save the denatured alcohol for cleaning your windows. And on those nights when you toss and turn, restlessly worrying that sludge is somehow building up inside your valve train, get up and change the oil, then go back to bed.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)