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July 19, 2013

Car Talk

Very hot conditions can cause strange things to happen

Syndicated columnists

Dear Tom and Ray:
On a cross-country summer trip to Las Vegas, we stopped for gas in a town in New Mexico. After filling our '04 Ford Escape, it would crank but would not start. After a few attempts, I decided to stop so as not to kill the battery. Just down the road, there was a truck repair station. I gave them a call, and the owner towed my car into his shop. As he worked on it, it did start up. He hooked up a scan tool, but could find no codes to indicate that anything was amiss. As he was checking under the hood, he put a wrench on the battery clamps, and he said he found that the ground side was loose. He said that could have been what kept the car from starting. After paying the man, we drove to our destination and back to the East Coast with no problems. A friend of mine said that what the guy told me was impossible, and that a loose ground wire would not stop the car from starting. What is your opinion? — Charlie

Ray: A loose ground cable certainly can cause your car not to start, Charlie. But not in the way that your car didn't start.

Tom: If the ground cable was so loose that it was no longer making contact with the battery post, the car would do nothing when you turned the key. You said your car cranked but then failed to start. So we know battery power was getting to the starter motor. That means the ground was connected.

Ray: So something else caused your car not to start. The truck mechanic did the right thing by checking for trouble codes in the computer. It's often the case that if a problem occurs only once — or even just a few times — a code won't be stored.

Tom:
And unless there's a code, once the car is restarted, you really lose your diagnostic opportunity.

Ray: So if it happens again, you'll need someone to re-scan for codes. And if none are found, then he'll need to work on the car when it's actually in this "non-start" mode. He'll need to do some tests to figure out what's preventing the engine from getting either spark or fuel.

Tom: If I had to take a wild guess without the benefit of looking at the car, I'd suspect the crank angle sensor.

Ray:
We've seen the crank angle sensor misbehave under very hot conditions. And that's exactly what you were dealing with. You were driving across the country, so you presumably had been on the highway for hours — maybe all day. You were in the desert Southwest, and we know the desert can be, what? Hot! You stopped the car, and shut off its cooling functions, while you got gas. That's what we call a "hot soak" condition, where the engine temperature spikes after the car is turned off.

Tom: I'm guessing that the heat caused an open circuit in the crank angle sensor, which sits in the hottest part of the engine, near the flywheel. The sensor then sent an erroneous signal to the car's computer, and that's what prevented it from starting.

Ray:
After you had it towed, it cooled off enough to restart. And those exact circumstances weren't duplicated during your trip.

Tom:
It could be an open circuit somewhere else, but that'd be my first guess.

Ray:
And if you've really had no problems since, and the circumstances were like those I described (which would be unusual for you during your day-to-day East Coast life), you could simply continue to drive the car and assume that it won't be a problem again.

Tom:
But if it's causing you to live in fear, or you're planning another long, steamy trip, then you can consider replacing the crank angle sensor for a couple of hundred bucks.

Ray:
You're going to ignore it? That's what I thought. Good luck, Charlie.

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)

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