Mercury Preliminary Evaluation PE11018: Idle instability

2005 Mercury Montego

Idle instability

Vehicle Speed Control

On November 29, 2012, Ford Motor Company (Ford) initiated Customer Satisfaction Program 12N03 to extend warranty coverage terms for repair of engine idle RPM surge caused by throttle body deposits in all model year (MY) 2005 through 2007 Ford Freestyle, Five Hundred and Mercury Montego vehicles.The program extends the coverage for up to 10 years of service or 150,000 miles from the warranty start date of the vehicle (all vehicles are eligible for the program through May 31, 2013, regardless of mileage).Dealers will repair vehicles exhibiting the condition by removing and cleaning the throttle body and reprogramming the PCM with software containing an updated idle speed control strategy.According to Ford, some of the affected vehicles may experience momentary, intermittent engine idle RPM surge (idle flare) when stopped or during low speed driving maneuvers such as in a parking lot or driveway.Unstable idle speed control (dips and flares) around the target engine speed (700 rpm) can occur in vehicles with deposit build-up in the throttle body when the system adjusts engine speed in response to changes in engine load, typically from power steering application or air-conditioner compressor cycling.Idle speed control logic is only active at vehicle speeds below 3.5 mph and when the accelerator pedal is not applied.Ford testing of a vehicle with a "worst case" throttle body measured a maximum idle flare of 1360 RPM lasting approximately 1 second, which Ford indicated was consistent with system design to limit vehicle speed and acceleration in idle control mode.Ford indicated that the test vehicle was held in place with normal brake effort and that vehicle speed remained under 4 mph without the brake pedal applied.In addition to throttle body deposit accumulation, symptoms associated with the condition may include Check Engine lamp illumination and diagnostic trouble codes (DTCs) P0505, P0506 or P061B.Ford attributed the surge condition to changes in idle air flow resulting from the progressive buildup of deposits in the throttle body from normal Positive Crankcase Ventilation (PCV) system operation.This condition was not adequately compensated for in the original Powertrain Control Module (PCM) calibration.Ford issued a technical service bulletin on August 15, 2011 (TSB 11-8-5), releasing an updated powertrain control calibration for vehicles with CVT transmissions.On October 19, 2011, Ford issued a second bulletin (TSB 11-10-21) superseding TSB 11-8-5 and including updated powertrain control calibration for vehicles with 6-speed transmissions.Prior to Ford issuing the TSB-s, owner-s experiencing the alleged defect either received no repairs, if dealers could not duplicate the condition, or had the throttle body cleaned or replaced, which only provided temporary correction until the throttle deposit build-up returned.Ford-s revised powertrain calibrations appear to be effective remedies for the idle instability caused by build-up of throttle body deposits.However, even after the TSB-s were released, many owners experiencing the condition continued to have difficulty with proper diagnosis and repair as dealers were often either unable to duplicate the condition or were unfamiliar with the new repair procedures.Ford's new program addresses those concerns.NHTSA's testing and complaint analysis indicate that the idle speed control issue in the subject vehicles can only occur at very low speeds, are very brief events, are easily controlled by moderate brake pedal forces and result in minor increases in vehicle speed even with no braking.This investigation is closed.The closing of this investigation does not constitute a finding that a safety-related defect does not exist.For additional information regarding this investigation, see complete closing resume in the document file for PE11-018.
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Date Opened
MAY 11, 2011
Date Closed
FEB 07, 2013
NHTSA Recall #
No recall issued
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