Dear Tom and Ray:

My wife went to a quickie oil-change place, and they told her that her radiator cap needed to be replaced because it wasn't holding pressure anymore. She called me and asked if she should let them replace it. I asked how much, and they quoted 15 dollars. I said no, and that I would take care of it. I went to the auto-parts store and replaced it for 4 dollars. The next time she needed an oil change, they told her the same thing, and she agreed to it and spent the 15. Needless to say, I was annoyed; her radiator cap was only three months old. Then I went to the same place with my car, and watched them change my oil. They took my radiator cap, and stuck what looked like the ball tap to a mini keg on it. They pumped the thing a few times, and told me that my radiator cap was not holding pressure and that I needed a new one. I said I would take care of it, but I am convinced that this pressure check was a scam. Was this a true test, or just a way to add 15 dollars to a customer's bill? -- Patrick

RAY: It's extremely unlikely that all three of those radiator caps were bad and needed to be replaced, Patrick. A leaky cap is not something we see very often in the garage, and to see it three times in the same family in a three-month period is pretty suspicious.

TOM: What happened was that these guys bought a new toy -- a new piece of diagnostic equipment. So they're using it on every car that comes in the door, in the hopes that their investment in the tool will pay off. And the test is real. But they either arenusing the equipment correctly, or, as you suggest, they're crooks.

RAY: The device they bought is a hand pressure pump, designed to pressure-test a cooling system. It's useful when you're trying to diagnose an overheating engine and the cause isn't obvious.

TOM: The hand pump is at one end of it, along with the pressure gauge, and you screw the other end onto the cooling-system recovery reservoir. Then you pump it up, and look for leaks. Or, if the leak is elusive, you leave it pressurized overnight and come back the next morning.

RAY: You can buy adapters for this pump that allow you to test the radiator caps, too. There's a boatload of different adapters, because there are so many different radiator caps in use.

TOM: So one possibility is that they're using the wrong adapters for your cars. Another possibility is that they're just not getting your cap to fit on there correctly, so air leaks out when they test it.

RAY: And a third possibility is that when they bought this piece of equipment, they also bought 4,000 radiator caps that they now have to get rid of.