runs great IF it starts!! HELP

[Jims-max]

I am having some serious trouble with my 99 7.3, It all started with a abrupt "dying" going down the road it would stall and restart all by itself, I got it home and it died in the driveway, I had it towed to my mechanic after I replaced the CPS, and tried a different IDM, with no change. the truck rolls great but doesnt fire, after cycling the key a dozen times it would start and run rough untill the codes where cleared and then it ran great untill you shut it off or it would just die at a idle. SO mr. mechanic says I need a new ICU or PCM. we plug the new one in and the truck fires right up, it runs great untill it sits for more than 24 hours then the same issue "rolls with no fire, no smoke at the pipe at all" plug the code reader in and it says there are IDM codes, no communication, clear the codes and it starts right up and runs perfect. untill the next time I let it sit for more than overnight! I have another PCM but I dont want to plug it in cause I dont know if I fryed the other two?

I am willing to try ALMOST anything

tested- glow plugs and relay, buzzed the injectors, disconected air heater, fuel pump/lift pump, batteries, alternator, inspected wires, fuel filter

[JJR]

How many miles? I might try checking the low pressure oil pump. It may not be supplying enough oil to the high pressure pump. Common symptom of this is a hard start or start and die after sitting overnight or a couple days.

[Jims-max]

I will check it out, any advise on how to test this pump, my oil pressure gauge is normal when running and it runs great if I can get it to start. my truck has 230,000 miles on it I have owned it for nine years and this is really the first time it has let me down. other than routine service.

[JJR]

Factory oil gauge is just a mechanical idiot light that will read normal at 5-7 psi. The mininum psi at idle is 10 psi and 40-70 psi at 3300 rpm. With your mileage it is possible it is out of spec. Next time it doesn't start check the high pressure oil pump reservoir level it should be an inch or less from the top. If it isn't fill it and see if that helps. If it does it can be likely the low pressure pump.

[Jims-max]

OK, that sounds very plausible, how do I go about checking this pump without the expence of replacing it ??

[JJR]

Hook oil pressure guage up and go by the specs I gave you. You can pull factory switch out of hpop reservoir and hook up guage there. Lpop is only about $80 and unbolts off front of engine after you pull the harmonic balancer.