‘90 4.0 oily spark plugs on one side

[RangerReviver1990]

I got a fairly early 4.0 in my ranger, built right at the end of ‘89, so it has the weak head casting right out of the gate. I came back from a week long vacation and went to start my truck up and it kind of stumbled on the initial start and then it seemed to be ok until I try to drive it. I’ve been used to having to let my truck run and idle for 5 or so minutes otherwise it’ll attempt to stall out when I hit the gas. But recently regardless of weather it was warm or not, anytime I’d bring it anywhere close to 2000 RPMS, it’ll start trying to bog its self down and start shaking. Eventually I got fed up and thought it was a spark issue so I replaced the wires and pulled the fairly new spark plugs out and thought I bought defective plug wires. But halfway through checking plugs and installing new plug wires, it had finally dawned on me that only my driver side bank of plugs were the only ones that were coated in oil. I sprayed them down with some brake-clean and cleaned them all up and put them back in and it ran like a champ, I asked around and they said it sounded like that the valve seals on one side had gone bad. But once I start thinking about it, it was borderline overheating really bad one day when it was raining and I kept hitting puddle after puddle and the truck’s temperature kept rising when I wasn’t moving, could it be a mixture of a cracked head and one side of valve seals going bad? Or could it just be one or the other?

 

[superj]

the truck's temp rising when not moving (idlingi nthe driveway or at a light?) means that your fan clutch is no good, no really that your head is cracked. a cracked head would give you issues the whole time its running.

bad valve stem seals usually allow oil to leak into the combustion chamber when the engine is not running so you get smoke at start up, but could also cause your running issue if the plugs are coated, like yours were.

here is an article on the seals

How to Check Valve Stem Seals & 6 Signs They're Leaking (globaleee.com)