I have a 99 F250 7.3L just rolled over 500000 miles yesterday. It is hard starting and runs very rough until it warms up to temp. I have to turn it off for 30 seconds after it reaches temp and restart. It runs great after this. If I try to drive before warm up it will only reach around 1800 rpm and run very rough, then will suddenly climb rpm and smooth out. I bought this truck from a good friend and had a good maint record but a shop had cut the wiring to the reader port so I can’t pull codes. I have replaced the HPOP and multiple vacuum lines. Need to find which sensor does what to make sure I am not just throwing $$ at it. Worse on colder mornings and humid mornings. Any ideas?

 

Welcome to the site

Did they leave the DLC and/or enough wire to repair the connector?
At that mileage, and all the possibilities, I’d try to make a repair, if not for this instance, but something to come...

 

i Just repaired the reader port and reconnected the batteries. I am reading no stored DTC codes. Tried to start the truck and had to fight it. Finally started and had to feather the accelerator to keep it running. Couldn’t get it over 1000 rpm until it ran for 2 minutes. Finally got rpm to raise slowly to around 1800 and then it raised normally all at once. Temp gauge is reading below the first line. Once it gets to normal operating temp I can turn it off for 30 seconds and restart. It will run fine.

 

Getting any smoke on start up? If so, what color and what smell?
Has the IDM ever been replaced?
Are you getting decent system voltage when cranking and when running? I like to charge and individually load test batteries w/ cold start/run issues.
When was the last oil change?

 

7.3's and 6.0 liter engines have injectors that are prone to sticking. On Youtube you can see several 7.3 owners getting their fuel injectors working again by using ARCHOIL engine oil treatment. I use it in my 6.0, but my fuel injectors aren't that old. I suppose you could tell if Archoil would help by plugging in your engine block heater and see if it's better on a cold start. The fuel injectors don't stick as much when they are a little warmer.

 

Something to look at,
If the fuel is draining back to the tank or air is getting into the fuel supply, the engine will just feel like it has a "spongy" throttle pedal until all the air is purged out of the fuel supply.