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Motoring Q&A: Car wants to race instead of stop

Question: I bought a Mazda 6 with 25,000 miles from a leasing company. It has a smooth ride with excellent acceleration and gets over 30 mpg in highway driving. The problem: Occasionally, when braking to a stop, the engine accelerates. I have to hold tightly to the brake and press the start button to shut it. On restarting, it's back to normal.

Question: I bought a Mazda 6 with 25,000 miles from a leasing company. It has a smooth ride with excellent acceleration and gets over 30 mpg in highway driving. The problem: Occasionally, when braking to a stop, the engine accelerates. I have to hold tightly to the brake and press the start button to shut it. On restarting, it's back to normal.

I took it to my Mazda dealer and explained the problem, telling them it is a safety issue. They did something to the way the car determines the idle speed, and that seemed to help. However, it still happens occasionally, though not as often.

Answer: My automotive database pulled up a Mazda service bulletin (01-005/14) dated March 2014 addressing a high-idle-speed condition. It calls for reprogramming the power train control module to the latest calibration, which is likely what the dealer did.

But the bulletin also discusses the possibility that the driver is making contact with the throttle pedal when braking. This is called "gas and brake," and can be very dangerous because it is somewhat self-propagating. The pressure on both brake and throttle causes the car to surge against the brakes, which often causes the driver to push harder on the brake pedal, which opens the throttle even more, causing increased engine power against the brakes.

Check to see if this is a possible explanation.

Q: I have a 2015 GMC Sierra Denali pickup with 28,000 miles on it. A vibration has developed that you can feel in the seat and steering wheel. It is also noticeable to passengers and is similar to running over rumble strips. The vibration occurs at any speed between 35 and 75 mph, lasts about a second or two, and recurs every five to 10 seconds.

I brought the vehicle to the dealership twice. The first time, they balanced the tires but nothing changed. Not satisfied, I brought it in a second time, and they test-drove the vehicle. The diagnosis: "Inspected vehicle. Road tested on new road. Small rumble felt from fuel management system. Working as per design."

This is not a small rumble. What can be done to rectify it?

A: It's possible this vibration is caused by the active fuel management system, as it disables specific cylinders for increased fuel economy at cruise speeds. GM says, "Some customers may comment on changes in the exhaust tone when an Active Fuel Management (AFM) equipped 6-cylinder or 8-cylinder engine changes to 3-cylinder or 4-cylinder mode. Some drivers may also notice a very slight vibration in either the accelerator pedal, floor pan and/or the steering wheel. This is a normal condition for AFM equipped vehicles and no repairs should be attempted."

You may be able to determine if this is the cause by placing the transmission in manual fifth gear, which will keep the engine in V-8 mode. If the vibration is gone under the same conditions, you've confirmed the cause. However, since this vibration has developed relatively recently, you might try swapping the wheels and tires from another truck to see if they are the source.

paulbrand@startribune.com.