I had the engine pulled in my 2001 job one and the mechanic installed the complete cloyes kit. Had the front light chain rattle. Turns out the back one was broken. Engine still nice and quiet 20k miles later
My rear one broke like that on my job 1 2001. Had the engine pulled and a new complete cloyes timing kit put in it about 10k miles ago. Nice and quiet now. Now have about 155k on the truck.
My Sport Trac now idles in the same range as your Explorer. Before I fixed the gaskets it was worse during cold temperatures. I previously owned both a 1995 Ranger and 1999 Explorer with the 4.0 OHV engine and the upper gaskets started leaking on those engines at around the 120,000 - 140,000 mark.
"Brett, did you ever get a CEL as the gaskets were going bad? I had the upper gasket go bad on a 1997 Explorer and I did get a CEL, as well as bad idle."
Yes - as it got worse I started to get the lean code (I forget which one.) Mine is an early build job one.
The job 2s have an elbow...
As my intake gaskets aged my Sport Trac behaved like that. I replaced the upper and lower gaskets a few months back and the idle settled down after that.