My idle in Park no AC is 500RPM. 5500 w/ AC. The engine sounds good and runs good, but the idle speed seems low. If memory serves me correct it should be about 700-800RPM. I know the earlier 12 valve engines had an idle screw adjustment. What about the 2006?

 

I'm assuming that you meant 550 with the AC... I need to get me an AC like that if you're posting right...

Sorry, the idle is adjusted by the ECM.

Do you have any codes? Changed your fuel filter lately?

 

You don't. Computer controlled. You have 1 of 2 problems. Computer problem, or tach problem. I'd photo-tach the engine to find out which.

 

550 w AC 500 w/o AC. Fuel filters changed 2 days ago. Engine runs smooth. I have had similar problems on gasoline engines that were computer controlled. It was usually fixed by either replacing an emissions sensor or repairing a vacuum line.

What is the correct idle speed w and w/o AC?

Could low fuel pressure at idle from one of the pumps cause this?

 

I have heard of 06's having dash issues, so thats probably your problem... But like bigwheels said use a photo tach to verify.

Idle speed should be 750 regardless.

 

There should be a sticker on the left side of the valve cover with all the info on the engine. On mine it says 750 RPM for Idle. Don't think the Cummins knows the A/C is on or if its hooked to anything when running.

 

The idle should be 750 unless you have had the ecm flashed and then it should be 800. There have been problems with the tach's on '06 trucks. Like has been said already confirm your actual idle speed before you go fixing anything.

 

After verifying if your idle is in fact low, you can change the idle speed with AutoEnginuity scan software. It does seem odd that your idle would change on it's own though. If it sounds OK, it probably is your tach going out on you.

 

Yup,
Mine idles at 500 also.
I think the tachs give out in the 06's, because I KNOW it isn't idling at 500 rpm's. That would be crazy.