Dear Tom and Ray:

Automobile engines get dirty as they age. I would like to clean my engine, but I am concerned that the water and strong chemicals might damage electronic components, as well as rubber and plastic components, and that lubricants in the front end might be washed out. How do you feel about do-it-yourself engine cleaning? Is it better to take the car to a shop that has steam-cleaning equipment? Is it best to just let the dirt accumulate? -- Bill

RAY: We're generally in favor of letting the dirt accumulate. That's the same approach we take to my brother.

TOM: Here's why. First of all, steam is the best thing to use, and pressurized steam can damage expensive electronic equipment. And there are components under the hoods of lots of cars that are vulnerable, like computers, air-mass meters and the like.

RAY: Second, all that stuff that comes off of your engine is an environmental hazard. So if you wash it off in your driveway and it goes down the sewer, we'll all be drinking it eventually.

TOM: The reason engines get dirty is because they leak stuff. Sure, there's a certain amount of plain old dirt that just kicks up from the road. But when you see an engine that's really filthy, it's because it has been leaking, and dirt adheres to that leaked oil or grease and forms a disgusting cake of crud.

RAY: But the only good reason to get rid of it is because it makes it easier to find the source of the leak. If the whole engine is covered in black gunk, it's difficult to see where newly leaking oil is coming from. Whereas if you clean the engine, you get a lot more contrast, and it's easier to see where the leak is originating.

TOM: So, we generally recommend engine cleaning as a diagnostic tool rather than a regular maintenance practice. Of course, having a clean engine all the time will make it easier to spot leaks if you're fastidious about checking. But in our opinion, it's not worth the risks.

RAY: If you do want to clean the engine yourself, you have to find a car wash with a grease trap. Make sure you know exactly which electronics are vulnerable to water and steam, and be sure you cover them tightly with plastic before cleaning.

TOM: If you don't know which pieces to cover, take that as a sign that you should have it done by a professional. Not only will a professional properly dispose of the hazardous crud that comes off your engine, but he'll probably have ruined -- and paid for -- enough computers in his day to know how to protect them correctly.