Today: What's Causing Engine to Rev?

Dear Car Talk

Dear Car Talk | Aug 20, 2015

Dear Car Talk:

I have a 2007 Honda Civic. For several years, I have been experiencing an intermittent problem. When I am stopped at a stoplight or stop sign, my car's engine unexpectedly revs up. If I put it in neutral or park, it subsides. It happens only occasionally, sometimes months apart. I have reported this to Honda on two occasions, and have brought it in for a testing procedure. Both test sequences came up with nothing. I don't know what to do. I am nervous about driving the car long distances. I know that what I am experiencing is real. What should I do? What is wrong with my car? I cannot in good conscience sell it privately.

-- Howard

No, you can't, Howard. So the time-honored solution, then, is to trade it in! But before you go to that extreme, I'll give you a few things to check.

Start by making sure it's not that size 14 Buster Brown of yours. Assuming that's not it, one possibility is that your throttle is sticking. The throttle is electronically controlled in this car, but the actual throttle plate itself sometimes can hang up if there's dirt on it.

So if you've been driving along and then you come to a stop, the throttle might hang up and not close completely, causing the engine to stay at, say, 2,000 rpm, rather than returning to idle speed right away.

The other likely scenario is that the problem is emissions-related and you have a bad sensor of some kind.

For instance, if an oxygen sensor incorrectly senses that your fuel-air mixture is too rich (too much fuel), it will signal the computer to send in more air, which would cause the engine to run faster.

Similarly, if a coolant temperature sensor senses that the engine is cold, it will signal for more fuel, thinking the engine is in warm-up mode and needs to idle faster.

Most of the time, an abnormal reading from a sensor will trigger your Check Engine light. You don't say whether the light has ever come on. If it has, and you didn't tell us that, we're going to penalize you 15 yards for withholding information, Howard.

If the Check Engine light does come on, it will "store" a code for perhaps as long as a few weeks so that your mechanic can scan the engine with his scan tool and see which sensor is reporting a reading that's out of range.

You say you went to your dealer twice, and they "tested" the car. I'm assuming they scanned it and found nothing. But it may be that you waited too long and the Check Engine code had cleared itself. Or that the problem occurs so infrequently that the computer doesn't have enough data to show a pending code.

So, my suggestion is, next time this happens -- especially if the Check Engine light comes on -- go right to the dealer within a day or two, and have him scan it. If nothing turns up on the scan, have him clean the throttle.

And if you have 150,000 miles on this car, I probably would replace the oxygen sensor, too, since you likely need one anyway. That could very well be the source of the problem.

Good luck, Howard.

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