5.9 12 Valve Cummins Diesel – Adjust Idle Speed

In this article, we discuss the 1994-98 Dodge Ram 12 valve Cummins Diesel Engine Idle Speed Adjustment. Towards the end of the article we include a helpful video detailing the steps to adjust the Cummins 5.9 12v idle speed.

Correct Idle speed is:

Automatic Transmission – in drive with AC on 750-800 RPM

Manual Tansmission – in neutral with AC on 750-800 RPM

NOTE: These speeds may cause stalling when the engine is cold. If this is occurs, adjust the idle speed to 900 RPM.

The dash tachometer is not accurate enough for proper adjustment. If you have adjusted the idle using the tachometer and have any of the problems listed below, you may use your tachometer to get the idle speed in the ballpark and then use your judgment to fine tune the idle adjustment.

Several things can happen if the 12 valve Cummins diesel engine idle speed is too high or too low. Incorrect idle adjustment may cause different customer concerns – low and high idle problems can and should be corrected.

5.9 Cummins RPM Below Specification

• Hard starting/stalling during cold engine operation.
• Engine roughness at idle.
• Reduction in vacuum available from vacuum pump during idle.
• Delayed power steering assist when cold.
• Excessive seat shake and/or mirror vibration.

5.9 12v RPM Above Specification 

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

 Typically, low idle speed is caused by misadjusted low idle speed screw on the injector pump, but high idle speed can be caused by many items.

The adjustment speed adjustment screw is located on the driver side of the injector pump near the rear of the pump. You will need a 10 mm wrench.

Idle Speed Adjustment

Loosen the 10 mm Locknut on the low idle adjusting screw at the rear of the injector pump.

Gently push the linkage rearward to take the pressure off and turn the idle screw counter clockwise (up) to increase idle speed, turn the idle screw clockwise (down) to decrease idle speed.

Check the RPM and adjust idle speed screw until the correct idle speed is achieved.

Vehicle should be in gear (if automatic) with the AC on. If the engine doesn’t try to stall then you are set.

Tighten the 10 mm locknut when idle speed is correct.

The following list is for diagnosing high idle concerns. 

Cause	Correction
Idle speed misadjusted	Adjust low idle screw on injector pump.
OK
Misadjusted throttle/cruise control cable	Adjust cables.
OK
Throttle linkage rod binding	Clean and lubricate ball connections.
OK
Bent throttle rod	Replace rod.
OK
Broken return springs	Replace springs.
OK
Throttle/cruise cable damaged	Replace cable.