Dear Car Talk:

A couple of nights ago, I was driving my 2000 Range Rover on the interstate when the heater suddenly started blowing cold air and the temperature gauge topped out. I pulled off to the side of the road and had the engine shut down within 15 seconds or so. I checked the overflow bottle, and it was empty, so I let the engine cool off and then put in more coolant. With the needle back on the "cold" side of the gauge, the engine would crank but not catch. By this time, the tow truck had arrived, and he gave me a ride home.

The next morning, I tried starting it again, and it started but sounded like only five out of eight cylinders were firing. Later that day after work, I started it again and it ran much better. I found and fixed the original leak, refilled the coolant and let it warm up. Now it's running like nothing was ever wrong. Why? Is it possible this car is so unreliable that a head gasket won't even fail properly?

-- Mary

Oh, I think the head gasket failed just fine, Mary. My guess is that you had a coolant leak that got catastrophically worse -- the hint is that you just happened to have a jug of coolant in the truck. How many people who don't have chronic coolant leaks drive around with a jug or two of Prestone?

Anyway, my guess is that when you lost enough coolant and overheated the engine, you damaged the head gasket -- hopefully just the head gasket, and not the head or the engine block. And when the head gasket was damaged, the coolant, which was under high pressure, seeped into one or more of the cylinders -- where it doesn't belong.

That's also why it sounded like it was running on fewer than eight cylinders the next day: because one or more of the cylinders still had coolant in it, and the coolant was dousing the flame. After running the engine, that coolant eventually burned off, and the truck began to run on all eight cylinders again.

But that's a temporary situation, Mary. What typically happens is that when a vehicle is driven hard, the pressure in the cooling system gets very high. And then when you shut off the engine, that coolant gets forced through and around the damaged head gasket, and gets into the cylinders again. So you should expect a repeat performance, especially when you start the car after driving it hard.

So, what to do? Well, if you were hoping to trade in this beast with a clear conscience, you should have stopped reading four paragraphs ago.

If you want to fix it, you'll first want to confirm our diagnosis. Your mechanic can do this by performing a head-gasket test, which looks for signs of exhaust gases in the coolant.

If the test is positive, you'll want to replace both head gaskets. You have two banks of four cylinders each on this truck, Mary. Both have a head gasket. And if one of them has failed, chances are the other one isn't far behind.

But that's going to cost you thousands of dollars. Before you plunk down that kind of coin on a 15-year-old truck, you'll want to get it checked out from stem to stern. Overheating can cause other damage to the engine, and you don't want to replace both head gaskets only to find out you're now burning a quart of oil every 200 miles because you damaged the bearings.

Or there may be other "home equity loan"-level repairs looming, and you'd want to know about those before putting a lot of money into this truck.

So if you don't need 100 percent reliable transportation, you can limp by with this truck for as long as you can stand it. But if you can't always wait for a couple of the cylinders to clear out the coolant while the truck shakes in your driveway, it might be time to look for a new car.

Bonus: You won't have to keep a jug of coolant in the back seat. Good luck, Mary.