I opened my radiator cap last night and found black sludge on top of the coolant, the oil is fine, could this be fuel leaking into coolant? This looks like thick oil, I'm kinda new to the diesel family. Oh by the way it's a 93 idi 7.3 Ford with 242000 on the clock.

 

There isn`t really any way for fuel to get in to the coolant. Maybe the oil cooler starting to leak.
My suggestion is to go down the main forum page to the Diesel Section, click on the Pre-Power Stroke page and post your problem there for the experts to give an opinion.
Good luck.

 

nice looking rig ya got there 63.
check out more info on lazy K's suggestion here:
https://www.ford-trucks.com/forums/9...lp-needed.html

 

Thanks, will do, also checked with local Ford diesel man who comes highly recommended and he also suggests the oil cooler. The truck comes from Tx area so NO MI cancer, and I am keeping all the places where rust starts WELL oiled with used motor oil rather than recycle it, had a 79 that looked like new in 94 when I sold it.