QDear Tom and Ray:

I drive a 1995 Ford Probe with roughly 138,000 miles on it. My mechanic says the rattle-like noise I hear as I drive is the result of valve tap. He indicated that it would be too expensive to repair. Can you please explain to me what valve tap is, and what causes it? -- Michelle

ARAY: If you're getting the noise only on acceleration, try putting premium gas in the car, and see if the noise goes away or improves a lot. If it does, you might ask your mechanic to check the stuff that can cause pinging -- like bad timing, a plugged EGR valve or carbon buildup on the pistons. All of those things are fairly easy to treat.

TOM: If the noise is there whenever the car is running, and not just during acceleration, then it probably is valve tap, more commonly known as "valve lash."

RAY: "Valve lash" is just another term for "slop," or "your engine is wearing out."

TOM: When your engine runs, there's something called a camshaft, which turns (some cars have more than one). The camshaft is what ultimately opens the engine's valves.

RAY: When you have lash, it means that there's slop somewhere between the camshaft and the valve train. And that rat-a-tat-tat sound you're hearing is all those worn-out parts slapping together.

TOM: If I had to take a wild guess, with almost 140,000 miles on the car I'd have to put my money on either a worn-out camshaft or worn-out hydraulic lifters.

RAY: Camshafts and lifters are big money, Michelle. Your mechanic is probably worried that if you spend a ton of money on that stuff, the transmission might fall out the next day. And he could be right. He certainly knows the car's condition better than we do.

TOM: So, if you want to just keep the car running as long as you can, we'd suggest two things. One is to drive gently. The more you do, the longer it's going to take for those parts to completely wear out. It could last another 10,000 or 20,000 miles like this.

RAY: And the second suggestion is to explore the wonderful world of "last ditch" engine additives. For generations, people have used stuff like STP, Motor Medic, Lucas Oil Treatment and Newman's Own Salad Dressing to prolong the agony of a worn-out engine. So take a little shopping excursion to your local auto-parts store, Michelle, and try a few of them.

Dear Tom and Ray:

Why do I keep hearing that you should never allow two vehicles to touch while jump-starting one of the vehicles? I recently e-mailed a local radio-show host who does an auto-repair show, but even he admitted that he had heard this but had never checked it out. Do you know? -- Brian

TOM: I would assume that's from the days when cars had exposed metal bumpers.

RAY: Cars still have metal bumpers, but they're now encased in rubber or plastic. But if you had two cars with exposed metal bumpers, and the cars were touching, and you hooked up a jumper cable incorrectly, attaching the positive of one car to the negative of the other, then you could possibly cause an explosion. You'd be, in effect, shorting out the battery in the car that was attached to the positive terminal, and it could blow up on you.

TOM: But it's largely irrelevant today, Brian. Unless you happen to have one of those few pickup trucks or SUVs that are still sold with chrome bumpers, and you're being jumped by another one, it's just not an issue.

Got a question about cars? Write to Click & Clack in care of The Post, or e-mail them by visiting the Car Talk Web site at www.cartalk.com.

(c)2005 by

Tom and Ray Magliozzi

and Doug Berman