I have an 89 F150 302, today I pulled the upper intake plenum to do some cleaning and noticed there was oil all over inside the runners of the intake. I dont have the tube hooked up off the valve cover to intake because I am running aftermarket valve covers and it has a breather on the oil fill side and a PCV in the other cover in the factory location. I noticed after running seafoam through the truck that it developed a rough/mis-fire at idle which it had before but it seemed to make it worse. It would stumble while idling at 800rpm and shoot back up to 1000rpm.

What would be the cause of the oil getting into the plenum? A bad PCV? Would that also affect the idle to make it very rough? I checked all vacuum lines prior and they were fine

I have already done a new set of motorcraft plugs and which the old ones looked fine for their age, new cap and rotor and new motorcraft wires. It still had the rough idle prior to all the new parts

 

Pull the idle air control valve off and give it a good cleaning. I would change the pcv valve it don't cost but like a few dollars anyway.