Jeep’s ‘Big Bang’ problem is exhausting all options

Dear Car Talk:

I have a 1997 Jeep Grand Cherokee with a six-cylinder engine and four-wheel drive. I never had any problems with it until recently.

My wife was driving down the road at about 50 mph, and it started missing then blew the muffler off. I took it to a mechanic, and he did a diagnostic test that showed nothing wrong. He put another muffler on it, and about two weeks later, it did the same thing.

I took it to the Chrysler dealer and they said it wasn’t showing any codes, except about every few minutes, an oxygen sensor code would show. They put two oxygen sensors on it and charged me $300. Two weeks later, it blew another muffler. We even tried it without the catalytic converter, and it still blew off the muffler.

This has happened four times so far (four mufflers). Do you have any suggestions? I can’t give up — I love this car.

— John

John, what brand of mufflers are you buying? I want to buy stock in the company.

It sounds like you’ve got a bad backfire. When the spark gets interrupted, some of the raw gasoline that doesn’t get burned will end up in the exhaust system. And then, when the spark returns, combustion can take place in the exhaust system in addition to the cylinders.

That creates what astronomers call the “Big Bang.” Except it’s not a theory. That explosion of unburned gasoline inside your exhaust system is what’s blowing off your mufflers, John. So your mechanic needs to figure out what’s interrupting your spark.

My first guess would be something called the crank angle sensor. The crank angle sensor does a lot of things, including directing the spark to the right cylinder at the right time. And we’ve seen bad crank angle sensors in a lot of older Jeeps. They tend to act up when the engine is hot.

It could also be a wiring problem, a bad coil, bad rotor or a bad distributor pickup. But I’d start with a crank angle sensor. That’ll cost you a hundred bucks or so, including installation. But not including your next muffler. Good luck, John.

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