2017-10-25 / Columns

What’s the cause of acceleration before you stop?

By RAY MAGLIOZZI

Dear Car Talk:

I just bought a used, 2010 Lexus GX 460 with 110,000 miles. It has an unusual problem: As I brake to approach stop signs and stoplights, the engine accelerates slightly. The engine speed will increase from 750 rpm to 1,100 rpm for a brief second or two as I come to a stop. After spending almost $200 to clean the throttle body (to no avail), I thought I should check with you and see what else to dig into. — Terry

The next thing to dig into is your wallet, Terry. Because you’re a Lexus owner now.

Actually, I have three ideas for you. I’ll give them to you in order of likelihood — least likely to most likely: Idea No. 1 is that your feet are swollen, and you’re accidentally scraping the gas pedal when you hit the brake. Look, I said I was going to start with the least likely idea.

Idea No. 2 is that you have a failing power-brake booster. If there’s a vacuum leak in the booster, when you step on the brake, the drop in engine vacuum could fool the car’s computer into thinking that you’re stepping — very slightly — on the gas pedal. In other words, if the engine experiences a reduction in vacuum, that’s like adding more air. And when the airflow sensor reports that more air is coming in, the computer responds by sending in more gasoline.

Idea No. 3, which I think is the most likely, is that all you’re experiencing is a downshift. As you slow down, the automatic transmission downshifts into its lower gears. And just before you come to a stop, it shifts from second gear into first. That sends the engine speed up a few hundred rpm, which is exactly what you’re reporting.

Car Talk

I don’t remember that last downshift being noticeable on any of the Lexuses I’ve driven lately, so it could be that you need to go to the dealer and see if you can get a software update for the transmission.

You can do a little bit of diagnosis on this yourself, Terry. If it’s the power-brake booster (Idea No. 2), you should be able to reproduce the problem while idling in park. So try that.

If you can make the problem occur only when the car is actually moving and slowing down, then it’s more likely to be the transmission downshifting (Idea No. 3).

And if it’s neither of those, cut down on the salt, try a diuretic and see if you can fit back into your size 11s (Idea No. 1). Good luck, Terry.

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