B350 1 Ton Van V6-239 3.9L (1990)
Powertrain Control Module: Customer Interest
ECM - Idle Speed Dips Below Specified Rpm
NO.: 18-26-90
GROUP:
DRIVEABILITY
DATE:
Oct. 22, 1990
SUBJECT:
Closed Throttle Idle Undershoot During Deceleration 3.9L/ 5.2L/5.9L Engines
MODELS:
1990 (AB) RAM VAN/WAGON (RWD) (AD) RAM PICKUP/RAM CAB & CHASSIS/RAMCHARGER POWER RAM CAB &
CHASSIS (4WD) (AN) DAKOTA/DAKOTA SPORT/DAKOTA CONVERTIBLE DAKOTA CAB & CHASSIS
SYMPTOM/CONDITION
Some vehicles may exhibit an "idle undershoot" condition during closed throttle deceleration. "Idle undershoot" occurs when engine RPM dips below
specified idle RPM and results in a complaint of idle roughness. An idle speed increase may also occur due to system overcorrection. This condition will
most likely occur only after engine has reached normal operation temperature.
DIAGNOSIS
Using a DRB II tester and the 1990 Powertrain Diagnostic manual for 3.9/5.2/5.9L EFI (Publication #81-699-0025), verify that all vehicle systems are
functioning as designed. Be sure to check minimum idle air flow. If all systems are functioning correctly, proceed with the following diagnosis.
Condition may be diagnosed during a road test using low speed driving (highway speeds are not required). Release the throttle pedal completely. Engine
RPM will drop below the specified idle then back to the correct idle RPM. Test may need to be performed several times before condition can be
duplicated.
If the condition occurs, perform the following procedure.
PARTS REQUIRED
1
Single Module Engine Controller (SMEC)
AN Truck
AB Van/Wagon
