DEAR CAR TALK: For two years, my husband (a retired mechanic) had been driving my old 2003 Blazer two days a week. It has 135,000-ish miles on it.

It has a coolant leak, so he only drove it around town, no more than a 20-mile round trip. He says he smells antifreeze when he drives it, but he doesn’t see any puddles or leaks, and he doesn’t think it’s the heater core.

A month ago, my son (almost 40, so not a dumb kid) was visiting from out of state and drove the car about five miles to a friend’s house. Hubby explained the situation and warned him to keep an eye on the gauges.

Needless to say, a loud banging started in the engine while my son was driving. He immediately pulled over and called home. Hubby told him to cautiously drive home so he could look at it. Hubby then announced that it’s my kid’s fault that the engine threw a rod.

Then a week ago, Hubby tries to start the truck and … no banging, no puddles, oil level is good. Your best guess as to a diagnosis? Thank you. – Angie

RAY: My diagnosis is, in retirement, Hubby’s mechanical skills have clearly fallen behind his Mahjong skills.

If the truck started up and ran quietly last week, it never threw a rod. More likely, it severely overheated.

If Hubby didn’t keep the coolant topped up, or if the leak got suddenly worse while your son was driving (perhaps at 85 miles an hour), there may have been so little coolant in there that the engine overheated violently.

And the steam that’s created when the coolant boils is what makes that hammering sound. It’s like the knocking sound that steam radiators make when there’s air in your home heating system.

The question now is: How much damage was done to the engine when it overheated?

To find out, the first thing to do is to top up the cooling system. It’s probably empty or near empty. Once the cooling system is full, run the engine until it gets up to operating temperature. Then do an oil pressure test. Hubby probably still has an oil pressure gauge somewhere.

Often, when an engine overheats severely, it damages the engine bearings. And an engine with damaged bearings won’t be able to hold oil pressure. That’s the kiss of death. Or, as they call it in the junkyard business, “new inventory.”

So if the engine flunks its oil pressure test, then throw a jug of coolant onto the passenger seat, and hope the Blazer can make it to the nearest junkyard. If the oil pressure is OK, then you dodged a bullet, and you can probably salvage this heap, Angie.

And the next step would be to figure out where the coolant is leaking and fix it. It could be the water pump, a hose or even the heater core, despite Hubby’s earlier dismissal.

But whatever it is, he should fix it now. There’s a good reason you’re not supposed to drive around with a serious coolant leak. And you guys have just had a front row seat to it.

If it’s something expensive, like a head gasket, a cracked head or a cracked block, Hubby might decide it’s not worth fixing. And we’d probably agree with him.

But if it’s something relatively simple, he should fix it instead of risking the engine and jeopardizing the family peace at Christmas dinner.

Got a question about cars? Email Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.