Ok my 90 diesel starts up instantly at any temperature, and will idle great too.
However, when it is cold out (say less than 20*F) it will still start instantly and idle great....until I tap the accellerator pedal. If I hit that pedal before she's warmed up, it goes into this rough idle/hunting for speed mode. I can hold my foot on the pedal to keep it from running rough, but it wont idle smoothly again until I drive a few miles.

Any ideas what this might be and how to fix it? thanks

 

I'm baffled. I can't think of anything that would make it idle OK until you tap the pedal and then idle poorly until warmed up.

 

wanna i might be on a limb but i thought that i had read someone say that some of the 89-90's had something similar to the setup on my chainsaw. there's a catch (on my chainsaw) that holds it off idle till i tap the thottle.... i really thought i had ready that somewhere.... if only i could remember where... and when.

 

Sounds like the KSB may not be working, mine had a similar problem and I found the switch was bad. The engine was rough running below 50 F until the truck warmed up unless the block heater was plugged in and if it got cold enough the symptoms came back.

 

Thanks for the replies. Today it was really cold when I got up (1*F) and it was idling rough right from the start....so you can ignore what I said about tapping the accelerator pedal. I guess it was just a coincidence.

Should I be looking at the KSB system? How do I diagnose this?

 

I would look at the KSB if the engine is crapping all over itself at idle and some reasonably heavy white smoke is coming from the exhaust. Also when you ease into the accelerator the engine will run rougher. As the engine runs at idle and builds heat in the cylinders it will eventually smooth itself out.

The KSB on your 1990 is different than my 1992. Yours is the wax motor style KSB set up. The temperature switch that runs the "motor" (outside of pump with the little steel pipe running into it) is located on the drivers side of the engine block slightly ahead of the fuel filter, there should be 2 wires into the connector of the switch.

The switches 2 USED TERMINALS are the ones to test with an ohmmeter, be aware that one terminal of the switch is unused. If the engine is overnight cold and/or below 160 F and the ignition is in the "run" position, the wire going to KSB motor on the side of the pump should be dead with a test light it should be hot when the engine temp is over 160 F. The switch is directly wired to the KSB solenoid, so if the KSB wire is hot or dead when it shouldn't be look to the switch, test for power at one wire of the connector with the ignition in the "run" position.

If all seems ok to the KSB wax motor after testing remove the solenoid/unscrew from the pump. Inspect the bore for signs of the plunger sticking. Ground the motors body and supply power to the male terminal. If the plunger doesn't extend after 1 minute check connections and retest, if nothing happens again the motor has seen better days.

I all still seems ok it may be necessary to look at the possibility of a stuck advance piston in the pump, this is not very likely.

Also, at 1 F or lower I highly recommend increasing the dosage of diesel antigel in your tank. We commonly see problems with gelling when power service white bottle is mixed as directed and the ambient temp is about -7 F or less for 2 days or more, we usually mix double dosages at these temps.

Cummins Parts Information

KSB switch 3915945 (number may be superceeded)
KSB wax motor ?

Best luck