Car Talk: How did engine survive broken timing belt? Luck
By RAY MAGLIOZZI
I had a 1991 Civic. At 105,000 miles, the timing belt broke while we were driving. I pulled off to the side of the road — the cold, dark, middle-of-nowhere, Indiana road — and just to make sure to do the worst thing I possibly could do, I then cranked the engine. Everyone said that if the valves weren’t ruined right when the belt broke, I definitely ruined them by cranking the engine. I happened to find a mechanic who agreed to put on a new belt and just try and see if it would work. It worked fine, and lasted another 125,000 miles. But why did it work? Everything I’ve read and everyone I’ve talked to has said that this car had an interference engine. But it survived a broken timing belt and worked fine (until it was destroyed by a Jeep, but that’s another story). Can you explain this miracle? — David
Luck. You should have run right out and bought a lottery ticket after the Civic started, David.
I’d say in 95 percent of cases, when a timing belt breaks on a car with an interference engine (where the valves and pistons can collide if things go wrong), at least one of the valves gets crushed, and you end up having to rebuild the entire cylinder head.
But in order for that to happen, one of the valves has to be pretty much all the way open inside the cylinder when the belt breaks, so a rising piston can crush it.
And in about 5 percent of cases, the belt just happens to break during one of those few nanoseconds when none of the valves is fully opened.
So you just completely lucked out in terms of where the valves were positioned at the moment the belt broke.
And your mechanic did absolutely the right thing.
You might as well put on a new belt and try it.
Even though there’s only a small chance that the engine survived, a belt costs just a few bucks and takes an hour or so to install.
If the car starts, you’d be the happiest guy in middle-of-nowhere Indiana that day.
And if it doesn’t work, you already have your new belt ready for when you finish rebuilding the cylinder head.
As my late brother would have said: “You must have lived a good, clean life” up to that point, David.
As to what you were up to when the Jeep hit you, I guess that’ll have to remain a mystery.Visit the Car Talk website at www.cartalk.com.