Idle Speed - Effects of Incorrect Adjustment

NO: 18-07-98

GROUP: Vehicle Performance

DATE: Feb. 27, 1998

SUBJECT:
Effects Of Incorrect Idle Speed

MODELS:
1994 - 1998 (BR/BE) Ram Truck

NOTE:
THIS INFORMATION APPLIES TO THE 5.9L CUMMINS MECHANICALLY INJECTED 12 VALVE DIESEL.

DISCUSSION:

Incorrect idle adjustments (either too high or low) may cause many different customer concerns. Following is a list of items that explain the condition/symptoms associated with incorrect idle settings along with component checks and specifications to set it properly.

RPM BELOW SPECIFICATION

Hard Starting/Stalling during cold engine operation.
Engine roughness at idle.

Reduction in the amount of vacuum available during idle from vacuum pump.

Delayed power steering assist When cold.
Excess seat shake and/or mirror vibration.

RPM ABOVE SPECIFICATION

Excess creep at stoplights/signs.
Vehicle does not reduce speed with foot off throttle.





Typically low idle speed is caused by a misadjusted low idle speed screw on the injection pump but high idle speeds can be caused by many items. The list will help in diagnosing high idle concerns:

The idle speed specifications for 1994 to 1998 models are 750 to 800 RPM. All idle speed measurements must be taken as follows:

Automatic Transmission In drive with A/C switched on
Manual Transmission In neutral with A/C switched on

When making idle speed adjustments, always use a DRB III, or an optical tachometer such as Cummins 3377462, Snap On MT139 or equivalent to read engine idle RPM.

NOTE:
DO NOT USE THE TACHOMETER IN THE INSTRUMENT CLUSTER TO SET OR VERIFY IDLE SPEEDS. THIS TACHOMETER IS NOT ACCURATE FOR READING OR SETTING LOW RPM DURING IDLE CONDITIONS.

POLICY: Information Only