Engine Control Module - Hesitation on Acceleration
NUMBER: 11-53-98DATE: 1-05-99
APPLICABILITY:
97 - 98 Legacy, Impreza, and Forester Manual Transmission vehicles with 2.5L & 2.2L engines
Subject:
Hesitation On Acceleration
In the event you encounter a customer complaint of a slight engine hesitation between 1500 - 2500 rpm's when the engine is cold or hot, perform the following:
Eliminate all external influences, such as a incorrect or dirty air filter, loose or cracked intake duct, dirty fuel filter, low fuel pressure, PVC system, or low engine vacuum that would indicate an external leak or an internal engine component. Check for trouble codes and repair as necessary. Verify the fuel quality.
If all external components are confirmed to be operating within vehicle specifications, the hesitation may be caused by the ignition control logic in the BCM. Under certain low rpm driving patterns, the ignition control system can pick up engine vibrations through the knock sensor and may retard the ignition timing. This ignition timing is learned by the ECM and placed in memory. Note: This area of the memory can not be viewed by using the Select Monitor. When the vehicle is driven under these conditions, the timing may be retarded and could cause the engine to hesitate on acceleration. To confirm this condition, road test the vehicle while viewing the Knock Sensor Signal on the Select Monitor. If you duplicate the hesitation, and the reading on the monitor is around -10 Degrees, you will need to change the ECM to correct the concern.
Note:
Some vehicles (shown on the following two charts) were produced with two different emission systems during the same model year. To identify which system you have, look at the Vehicle Identification Number. Refer to the chart to identify which system you have on the 98MY Forester and 97MY Impreza vehicles and choose the correct part number from the chart in this bulletin.
98 MY Forester
97 MY Impreza
Use the chart to locate and order the correct part number for your application.
Refer tot he Warranty Labor Time Guide for claim information.