03 F350 6.0 diesel. Won't start. Sometimes starts when extremely cold. Was told needs oil pump. I assume it is the fuel oil pump. Quoted a $2000 price to change it. I am a mechanic but not a diesel mechanic. Where is the pump and how I diagnose it as bad? And how do I change it?

If it is a diesel then the cold start injector will be in the manifold . There will be a pipe connected to it and a wire as the injector normally squirts on a hot element which creates a fire that is drawn into the combustion chambers.. Check that the element is operational. If petrol then run the fault codes to find out the problem. I would be having a pressure test done on the fuel manifold to check the pressure regulator and pump operation

You are right that it could be either component, or something else electrical. The security system will kill the fuel system if the wrong key is used or the system is acting up.
A mechanic would check base fuel pressure with a gauge and also see if the pump is getting power from the relay when it fails to start.

If this is a diesel engine you probably have an air lock in the fuel system. It will need "bleeding" before it will start. If you have already tried bleeding the fuel system there may be an air lock in the injectors. If it still doesn't start I suggest you carry out a compression check because diesel engines need to be in good condition to start easily, especially in cold weather If it is a petrol engine, there could be problems with engine timing or ignition problems.

Here is some reasons and solutions for diesel generator fail to start:
1. There is air in the fuel systems
Check if the fuel line connection is loose and remove air from the fuel system. First unscrew the exhaust screws on the injection pump and the fuel filter and pump the oil with the hand pump until there are no bubbles in the spilled fuel. After, tighten the exhaust screw. Pump oil again, when there is return oil in the tubing, then tighten the hand pump. Loosen the nut of the high pressure tubing at one end of the injector, pry high injection pump plunger spring seat, when there is no bubble in the fuel out of the nozzle, spin the nut, then pry a few more times, so that each cylinder injector is filled with fuel.
2. The fuel line is blocked. Check if the pipeline is unimpeded.
3. Fuel filter block. Clean filter or replace filter.
4. Oil pump does not supply oil or intermittent oil supply. Check that the inlet pipe is leaking, and that the filter screen on the inlet joint is blocked. If no oil is supplied after removal, the intake pipe and oil pump shall be inspected.
More reasons and solutions, please see at:
https://www.dieselgeneratortech.com/generator-sets/Why-My-Diesel-Generator-Set-Fail-to-Start.html

There is a leaking oil seal in the injection pump thereby contaminating the fuel.Usually the front oil seal.

Check your fuel pump relay Alot of chevy systems back up the fuel pump power supply through the oil pressure sending unit . So even if the relay is NG the vehicle will still start and run because the oil pressure sending unit is feeding the pump . It also could be a combination of bolth a bad relay and the O P sending unit it needs to build up enough engine pressure to trip the switch inside the sender to send voltage to the fuel pump and it would do this much better if it was cold the oil is thicker

if you have the 6.0 then the filters are located on top on the motor right beside your oil filter and under the drivers side on the fuel pump, check to see if you have diesel in your bowl where the fuel filter is, no diesel in there your fuel pump might be dead, check fuses to fuel pump and check relay on the passengers side kicker panel, check your owners manual

Sometimes diesel engines will put out black smoke under extreme acceleration. Other than that, they shouldn't smoke. Black is excess fuel, White smoke is water and bluish is oil. White smoke on a cool morning is not necessarily a sign of a problem unless it happens every time you start it through a warm day. Hope this helps.

Have replaced your glow plugs?They dont cost alot to replace,try this

check the fuel press, a quick check of the pump can be made by turning the key on with the windows rolled up and all accessories off, , u should hear a faint hum for about 5 seconds, if u don't most likely the pump has stuck from the cold or wear or both.