I am getting so fed up of my '90 F-250 7.3 IDI!

My engine had started to start up quickly recently, and now it is back to square one, but worse!

It takes about 20-25 seconds of cranking to get it to even fire, let alone start!

I replaced the batteries, glow plugs, air filter, and got the starter motor refurbished (thought it was maybe turning over a bit too slow), but it still takes ages to start!

It has half a tank of fresh diesel, so I can't blame the fuel...

What the hell is wrong!?

 

well you have air intrusion i would say.... the return lines leak air as the orings get stiff from the heat of the injectors over time.... bumping them can cause them to shift and will sometimes let air in but not let diesel out... bumping the air cleaner on the steel lines can cause this with old old orings.... the fuel will drain back to the tank due to the leak and the lines will be full of air. thus taking alot of cranking to push the fuel back to the IP since the pump is mechanical and not electric you have to be cranking to get fuel pumping.... also could be the selector valve that switches between the 2 tanks leaking air in.... though option #1 is more likely....

 

^ What he said. It can be a real pain to find these leaks. Replacing those O-rings and the other rubber parts of the fuel system should be considered a maintenance item, just like doing a timing belt on most modern cars.

 

are the glow plugs getting powered up ?

Ford diesel 6.9 7.3 IDI

where the return hose connects to the filter housing there is a orifice in there that causes air to get in the system when the orifice is worn out !
i blocked the one i have off and routed the hoes back to the return !

 

the ip should still be full of fuel though,so if it was air intrusion,she should still start right up and burn the fuel thats still left in the ip (which is quite a bit) and then all the sudden stall out.then it would take a while to prime the system back up (if still using the mechanical fuel pump.)
perhaps this depends on the status of your check ball in the filter head though,as to weather or not a drain back,could suck the ip dry too? im not too sure about that.
when i changed my filter the other day,due to my coolant filter,it was just as easy to just remove the whole unit.so i took off the filter head and made sure that check ball and spring was all clean and working up top there.all was great.

so that should be a clear sign of air intrusion problem anyway.a start then stall condition.
another way to check for that,is to pull the filter before the first start to see if she's full or empty.

testing for power at all the GP connectors when you turn the key on,is a good place to start like suggested.
also;how are the battery cables? a fresh starter isn't worth much if hooked up to old corroded cables.these suckers should crank real fast.
how about the condition of the injectors? changing these out on my truck was the final "instant" fire up and run like its been already running all day solution.

 

What's the mileage on the mechanical fuel pump, few years back mine was doing the same, cranking a while before even firing.

Untill the pump completely died in winter, after replacing it, started great!