Q: I bought a Mazda 6 with 25,000 miles on it from a leasing company. It has a smooth ride with excellent acceleration and gets over 30 miles per gallon in highway driving. The problem I am having is that occasionally when braking to a stop the engine accelerates, and I have to hold tightly to the brake and press the start button to shut the engine off. On restarting, it is back to normal. I took it to my Mazda dealer and explained the problem to them, telling them it is a safety issue. They did something to the way the car determines the idle speed that seemed to help. It still happens occasionally, but not as often.
A: My ALLDATA automotive database pulled up Mazda service bulletin 01-005/14 dated March 2014 addressing a high idle speed condition. It calls for reprogramming the PCM (powertrain 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. In racing parlance, 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 carefully to see if this is in any way a possible explanation for what you are experiencing.
Q: I have a 2015 GMC Sierra Denali pickup with 28,000 miles on it. In the past couple of months a vibration has developed that you can feel in the seat and steering wheel. The vibration is also noticeable to passengers and is similar to running over rumble strips. The vibration occurs at any speed between 35 and 75 miles per hour and lasts about a second or two and recurs every 5 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. Here is their 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 and is very aggravating. What is failing in the vehicle and what can be done to rectify it?
A: It is possible that this vibration is caused by the AFM — active fuel management — system as it disables specific cylinders for increased fuel economy at cruise speeds. Specifically, 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 V8 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.
Q: My 2002 Ford Explorer V6 runs fine until I turn it off and leave and it for 10 minutes, then it won't restart — it starts and stalls. I come back an hour later and it will run fine until next time.
A: Start and stall is a sign of loss of fuel pressure. Vapor lock from high underhood and/or cooling system temperatures could be the cause. Try cooling the fuel rails by spraying them with water before the next hot restart attempt.
Paul Brand is the author of "How to Repair Your Car" and "How to Repair Your Truck and SUV," published by Motorbooks.