1987 6.9 IDI Losing Prime Overnight

[Grumpy bruce]

Still have the water separator on it ? If it is motorcraft it might be bad.when I first got my 87 f 350 in 1990 I chased air intrusion for two weeks. Changed to after market one it's been fine since.
330,000 on her still running good. Banks sidewinder at 200000.

 

[Tod Rockstead]

The reason why the return cups are so touchy is that the O-rings inside the cups on the injectors get brittle over time, and if the cup gets bumped or twisted on the injector, the O-ring gets compromised and bingo, air intrusion