Dear Tom and Ray:

I drive a 2002 Subaru Outback. During this past winter, we had a significant cold snap where I live, with temperatures below zero. After the first day of the cold snap, I noticed a puddle of coolant on the floor of my garage. So I visited a local mechanic who specializes in radiators. He attached some sort of pressure gadget and pumped it up. According to him, there was no indication of a leak. He looked around a bit -- without crawling underneath the vehicle -- and said he just wasn't sure there was a problem. So, he topped off the coolant and I returned home. Everything was fine, until we had our next cold snap. Once again, I noticed a puddle of gooey green stuff. Now that the weather is warmer, it's fine again. What's going on? -- Vicky

RAY: I'm guessing that whatever it is, Vicky, it's caused by shrinkage. We know things shrink when they're cold, right? (See also: George Costanza.)

TOM: If it's something serious, it will eventually start to leak regardless of the temperature.

RAY: My first guess would be a loose hose clamp. The rubber coolant hoses are clamped onto metal fittings under the hood, and there are lots of them. If one of those hose clamps is just barely tight enough, and that fitting is shrinking in the extreme cold, that could be enough to allow some coolant to leak out.

TOM: Then, when the temperature rises and the fitting expands back to its normal size, everything's fine again.

RAY: Of course, it could be something much more lugubrious, like a failing water pump, or even a bad head gasket.

TOM: But here's where we'd start. First, have your guy check it again with the engine cold. You'll need to leave it overnight so he can pressurize it first thing in the morning, before the engine has run. If a pressure test on a cold engine turns up nothing, then I'd have him check and tighten all of your hose clamps, including the ones that feed the heater.

RAY: And then keep an eye on it. See if there's any more coolant on the garage floor. If it starts leaking in more normal weather, then it should get easier to diagnose, and your mechanic can even add some dye to the coolant to help him find the source of the leak.

TOM: But once he realizes you're not expecting him to chill his shop down to minus 6 degrees to get the leak to happen, he may be more willing to spend a little time helping you figure this out. Good luck, Vicky.