I've got a 94 3500hd with 6.5 and manual trans.

The truck idles high enough that you don't have to feather the fuel pedal while shifting gears, I am concerned about ruining the clutch.

I've read that the coolant temp sensor can effect idle speed but wasn't sure what else I could try to lower the rpm.

I don't have a gauge to see the exact RPM but I know that normally you have to apply a lil fuel when letting the clutch out.

Truck does drive fine power wise I've had it up to 100kmh just recently I do believe the LP on the frame is weak or the check ball is bad as I sometimes have to reprime the lines so that's getting done today.

Any ideas for why it is idling high? I don't see where a switch (high idle switch) is on which I will be adding once this issue gets sorted out the PMD is relocated on the inner fender and has been replaced once already.

 

Without a tach it might to the untrained ear sound like it mat be idling high and the coolant temp sensor mainly affects the glow plug cycle I have the NV4500 in mine and you don't really need to give it fuel to take off.

There is a high idle mod that can be done to the electronic 6.5's just google to find it

And there is no check valve or ball in these trucks what more than likely is happening is you are sucking air into the fuel system a couple likely spots are the fuel filter housing (there is a O ring in the bottom that with age go bad) and or the fuel lines especially where salt is used on the roads in winter time or near the ocean.

I have seen where people have drilled a hole in the fuel cap and put a tire valve stem to pump air into the fuel system to help with finding leaks

 

The new lift pump that I picked up has a check ball in it, as to where you can blow air only one way.

I'll see if I can find another cap or use a rag to stuff into the opening and use shop air to pressurize the tank and lines.