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. 

Idle speed misadjustedAdjust low idle screw on injector pump.
Misadjusted throttle/cruise control cableAdjust cables.
Throttle linkage rod bindingClean and lubricate ball connections.
Bent throttle rodReplace rod.
Broken return springsReplace springs.
Throttle/cruise cable damagedReplace cable.

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  1. Hello, I just discovered that the lock nut on my 1998 dodge 12v low idle screw has the edges rounded off. How do I adjust it now? Or do I need to get a new one? And if so, what is the part number and name? I ask because if I google search for the lock nut or anything to do with the low idle adjustment parts, only ‘how to adjust’ blogs and videos come up.

  2. After adjusting low idle speed she seemed to creep up to much now and then. I figured out the low idle speed and linkage rod easy enough but the spring tension escaped me.
    Thank You for the heads up!

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