I have a 1951 Chevy, straight-6, 3-on-the-tree, with 31,000 actual miles. Nice car.

If I coast downhill in gear (it doesn’t happen in neutral or if the clutch is disengaged), when accelerating after the downhill run, it will briefly (for perhaps 100 feet or so), put out a puff of smoke.

What gives?

— John

That’s the worst problem you have with a car that’s old enough to collect Social Security? You should be dancing a jig instead of writing to us, John.

On a car of this generation, blowing some blueish smoke after coasting downhill is not unusual. As a matter of fact, even some newer cars do it, but to such a small degree that it’s barely noticeable.

Your problem is that your piston rings are wearing out.

The piston rings are supposed to fit tightly against the cylinder walls and scrape off all the oil before combustion takes place, so gasoline and air get combusted and your oil doesn’t. Your piston rings aren’t doing a great job anymore. Not that they were stellar to begin with.

There’s a measure of industrial precision called “tolerance.” Tolerance is the space between parts. Back in 1951, manufacturing tolerances just weren’t that good. You could slip a hot air balloon between some of the parts in this car.

These days, our manufacturing processes are infinitely better, and tolerances are much smaller, resulting in better performance and much longer engine life. But back when your car was built, nobody expected you to go more than 75,000 miles without an engine rebuild.

You’re seeing that smoke because when you coast down a hill, the wheels are turning the engine rather than the engine turning the wheels. During that time, while there’s little combustion taking place, oil is getting pumped past those poorly made and worn-out rings, and it is pooling in the cylinders. Then, once you start to accelerate again, that oil is getting burned up along with the gasoline and sent out the tailpipe.

Unless it’s really driving you nuts, John, I’d just keep the oil clean and topped up and live with it for now. After all, given your annual mileage (we calculate 455 miles a year), you’re going to be due for an engine rebuild in the year 2115 anyway.

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