Two problems related to cruise control have been reported for the 2009 Mercury Mariner. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2009 Mercury Mariner based on all problems reported for the 2009 Mariner.
Driving on highway with speed control on at 75 mph, all of the sudden vehicle rapidly sped up to 95 mph. The vehicle just took off raving engine up. Applied brakes and took off cruise control. If there had been traffic this could of been a dangerous or deadly incident. We have had the vehicle in before for the same problem, it would speed up while making a turn. Ford dealership said it was a transmission problem. All repairs have been done by a Ford dealership.
Traveling along with cruise control set at 65 when all of a sudden my vehicle decelerates. The "throttle body/throttle position switch" indicator light illuminated. Vehicle has had intermittently gone into "limp" since then. Lately it has happened at least once per day, often when trying to accelerate quickly or above 40mph. Check engine light illuminated yesterday (11/26/2013) and indicated a p2111 diagnostic code (throttle actuator stuck open). This is apparently a fairly common issue already under investigation. The solution seems to be have a dealer replace the throttle body (roughly $400 parts and labor).