Both posters should make sure the return line from their injection pumps are flowing fuel when the engine is running. It aids in keeping the internals of the injection pump cooler. If fuel supply pressure is low the relief valve will stay closed.

That may not be the problem both of you are experiencing is pretty certain. At the same time it should be returning fuel to the tank. Easy to check and rectify if not doing it. In both your cases I would want to know it is working.


Another function of the relief valve is to maintain a constant and adequate fuel feed pressure in the base of the injection pump.

I am not personally aware of any previous variable idle speed complaints from a return deficiency of any sort. The return system was designed to have function at the same time so it should be working. Many are not working properly. Either the relief valve is tired or blocked or the supply pressure is too low. Also have a look at your primary filter for air bubbles being in transit through it. Both when starting with a cold engine and after the engine is warmed up. There should be none present other than the normal stationary one.

If on alternate fuels the feed temperature of them can be a factor I suspect.