How did engine survive broken timing belt? Just plain luck

Dear Car Talk:

I had a 1991 Honda 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

RAY: 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.

A few last-ditch things to try before replacing heater core

Dear Car Talk:

My brother has a 2006 Toyota RAV4 with 125,000 miles. We think he has a clogged heater core. The Toyota maintenance schedule calls for changing the coolant every 100,000 miles; he takes his RAV4 to the local Toyota dealer and has followed the schedule. He still has a clogged heater core. They have tried to flush it, to no avail. Now they are telling him he needs the heater core replaced. It's a nine-hour job and could cost up to $2,000. What should he do? – Paul

RAY: Buy some blankets.

Yeah, that’s a terrible job. It requires taking out the entire dashboard. And then when you’re all done, 15 hours later, if you haven’t stabbed yourself with a screwdriver, you end up with a pile of nuts and bolts left over. So you want to avoid that job if at all possible.

Your brother might want to try to find an independent mechanic who is a little more motivated to work with him.

If I were working on this car, I’d try everything I could think of before I started pulling out the dashboard. First, I’d double-check to see if the heater core is actually the cause of the problem; it’s possible there’s something else preventing the passenger compartment from getting heat. I’ve seen a bad thermostat or a failing water pump cause a lack of heat. I’ve also seen a bad blend door in the ducting system keep warm air from flowing into the passenger compartment, even when the heater control on the dashboard is set to “hot.”

So first check to see if the heater core is working. Once the car is warmed up, feel both of the hoses that attach to the heater core (one goes in, one comes out). If only the hose going in is warm, then the heater core isn’t working. But if they’re both warm, then hot coolant is flowing through the heater core and your problem is somewhere else.

If the heater core fails that test, I’d make sure the heater core isn’t air-bound. If the dealership flushed it, it’s possible there’s still air trapped in the core.

These heater cores are particularly hard to bleed, because they’re up high – higher than the rest of the cooling system. So they tend to trap air. We’ve resorted to removing a heater hose and filling the heater core with coolant through a funnel to get all the air out.

If your brother is a good guy, and he didn’t give you too many noogies when you were a kid, maybe the flush did work, and there’s just air blocking it now?

If you’ve successfully bled out the air, another thing a mechanic can try is doing a more intensive flush. For instance, you can remove the hoses and hook up a small, low-pressure submersible pump to one of them. Drop both hoses and the pump in a bucket of chemical radiator flush, and just let it pump slowly through the heater core for hours. If you see some rust and gunk collecting in the bucket, then keep going. And then reverse the direction and pump it the other way. Do that a few times if it seems to be helping.

Maybe you’ll unclog it enough that, when combined with a pair of Bronko Nagursky long underwear, it’ll be enough to get him through the next few winters. And if not, my next suggestion would be to go back to the dealer and trade it in in the summer. Good luck.

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