I just bought a 1990 4x4 Auto, 245k, and the previous owner says that when it is cold, the idle surges up and down until it warms up a bit. He says it has no trouble starting, but just the idle fluctuates for a few minutes.
Of course when I test drove it, it was 55*F outside, so I couldnt really verify his description. I will get the truck in a few days.
What should I check? Is this a common/normal problem?

This is my first cummins, so Im looking for any and all info I can get. Is there a website that outlines the common problems and fixes for the 1st gen trucks? Thanks.

 

If it's just lumpy that's probably injectors or the KSB system isn't working. If it really is running up and down, that sounds like a problem with the idle spring inside the pump.

 

What is the KSB?....remember Im new at this.

 

KSB is a system that advances pump timing on cold starts. It's just a thermal switch on the engine and a solenoid on the pump. The most common problem is no power or a bad switch. If the truck isn't white smoking while it's running funny, the problem is not the KSB.

 

Ok I started it this morning and it fires up IMMEDIATELY (like 1/2 turn of the starter), but the idle goes up and down, it goes so low that it sounds like its going to die, shakes a bit.
If I step on the accelerator just a little it stops surging and sounds normal.

There is no white smoke during the surging.

BTW, it was about 35*F this morning when I did this, and the grid heaters were cycling (volt gauge going up and down).

If I drive about 6 miles or so to get it warmed up, it will idle fine.

Is this a bad injector? what can I do to diagnose & fix this? I dont have a repair manual yet and Im new to dodge/cummins.

 

Pull off each of your wire connections clean and tighten them up. Change your fuel filter if you haven't yet. Run through a couple strong doses of injector cleaner. Get some preventive maintenance out of the way first. Pump look like it may be leaking anywhere? You might pull the shut down solenoid and check for any metal shavings, in case the pump is going bad.

 

I'm thinking that the idle is set too low to begin with and the grids are dragging it down. Mine loses 50 RPM when both grids are on, and it idles slower when first started. How rapid is the idle fluctuation? Is it a quick up and down, or is it in synch with the voltmeter? If it's not grid related, I think it may be a broken or weak idle spring in the pump.

 

I Think I found the problem!!!!!!!

Wire to the KSB cold start solenoid was was broken off right where it attaches. Hard to see until I actually touched it. I crimped on a new connector.

I will report back in the morning after it gets cold again.

 

Hmmm... Surprising that there was no white smoke? How cold are we talking here?

 

Well I went out this morning at 26*F, cycled the heaters, and it immediately started. It will hold a steady idle through the heaters cyling on and off....But here is the catch:

After it is started, if I touch the accelerator pedal while it is still cold, it will drop down to a low and very rough idle.

I can shut it off, fire it back up again, and the idle will be stable as long as I dont tap the gas again. It will do this until the temp gauge has moved about 1/4 of the way.

Its acting like there is a cold start/high idle position on the throttle, and when I tap the gas it falls off that position, and wont go back to the cold position unless I shut down and restart.
Does that make sense?

Reminds me of the old VW carburetors that had a stepped cam for different idle positions.

Any ideas?

 

Anyone?

 

Definitely the idle spring. The pump uses a small light spring to control the idle within a tight range (gives the governor more control). When you step on the pedal, the main governor spring bottoms out the idle spring, effectively removing it from the linkage (gives you more control). If the idle spring is weak/broken/sticking, the governor has less responsiveness/control and will let the truck surge at idle. This is something that can be fixed with the pump on the truck. Let us know where you are and perhaps somebody can chime in with the name of a good pump shop.

 

One more thing you can check.

Could it be that the idle is set low and that the throttle is hanging on the cruise cable or something. Bopping the pedal allows it to snap back and rest on the low idle screw?

Look and see if it is fully resting on the idle set screw before you start it. If not then remove the cruise cable and adjust the idle set screw. Then replace the cruise cable and adjust the cable so it's not hanging on it at idle.


Den

 

Can you tell me how to Identify the spring or describe exactly where it is? Is it visible in any of these pics:
bosch VE pump

is this an external spring? Why would I need to bring it to a shop?

 

It's internal, not in any of those pics. If you can get the part and aren't afraid to pull the top off the pump I guess you could do it at home.

It's listed as "top hat spring" in this pic. The problem could also just be the "top hat" sticking in the lever, impossible to know until the pump is apart.

Look in this gallery for more pics on pulling the top of the pump off (might as well do the 3200 RPM spring while the top is off ).

https://www.dieseltruckresource.com/...ssword//page/1

Directions are in the "tech facts" section, button is under the main banner.