Missread post #8,if you have the key in the run position and the LP is working it may already have a dual relay mod in place,otherwise the LP prime relay in the underhood fuse center is likely stuck shut-it should only work with the key in the start position.

If the key is off and the LP runs it could be either the LP relay or a stuck OPS.

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An easy way to test is put the key in run,remove the fuel pump relay from the underhood fusebox and see if the lift pump quits.
If it keepos pumping,rput the relay back and disconnect the Oil Pressure Switch harness plug and check the LP again.
Whatever makes the LP stop is the source of the problem.

 

Truck is running wow how could I let something as simple as checking block heater sorry for all the confusion now for real problems . Heat gauge will run at normal then shoot to max temp ad check gauge light comes on then it will drop and be fine will also blow cold air from heater at idle . Coolant is disappearing I don't see any leaks from pressure reservoir or rad... Help could it go into motor sounds like a thermostat? ?? Whatd y'all think

 

Start the engine from stone cold and squeeze the upper rad hose,does it get firm/hard very quickly long before it gets warm?
If so you have a blown head gasket,cracked head or cracked block.

If the rad hose stays soft until hot then a stuck thermostat is a good candidate,rplace with AC Delco only.

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How cold is it where you live?
The truck should start just fine on the glow plugs alone down to around 10F - 15F,or -10C.