I have a 2000 Ranger with 2.5l SOHC, this engine has 280+ on it. The problem is that after about 45 to 100 miles it starts to run rough, I first tested the compression and found that it had none on cylinder 3. I opened the valve cover to find that one of the rocket arms has fallin off. I compress the valve spring and reinstall it, after about the same amount of miles it happens again. I reinstall it and look for any wear that is noticeable to no avail, what should I be looking for or is it time to replace the rocker arms and possibly the cam?

 

Welcome to the forum

Just for some background FYI
You have the "Pinto engine", introduced in the Ford Pinto in 1976
Built in Ford's Lima, Ohio engine plant, so more common and popular name is the "Lima engine"
Made and installed in Fords until mid 2001, Rangers got the last ones
There were 3 versions of this engine
2.0l Lima, not used much
2.3l Lima, most used version, even came turbo charged in some Ford cars
2.5l Lima, a stroked 2.3l, only made from 1998 to mid 2001

Very reliable engines, built for 25 years and many are still running well , 500k + miles

Good read here on the Limas: The Ford Ranger 4-Cylinder Motors - The Ranger Station
And here: Ford Lima 2.3 SOHC Specs - myPOWERBLOCK

So you can widen your search for possible cause of the problem to 2.3l Lima engine, as outside of the longer stroke, they are the same engine and used the same heads and valve trains

As far as the rocker coming off, I know the obvious answer would be that rocker is losing tension, so broken valve spring or bent/sticking valve stem, that's all that comes to mind.
But I would think this would be proceeded by "ticking noise" because of the lack of tension on that rocker, which you don't mention

The intake valves on these engines can "tulip" if a cylinder is overheated, like any intake valve can, but that shouldn't cause rockers to slip off, just severe drop in compression.

And could just be the rocker is damaged in some way