Re: 97 F150 4.2 about to give up
the 4.2 is a notorious motor for blowing a head or piston so my guess is its a serious problem that needs to have every cylinder checked one at a time to determin what is really goin on.
first thing to look for is run it for a few min which you probably did and then see if antifreeze is in the oil and crankcase after it cools some and seperates outta the oil.
its really bad if the oil isnt clean and looks more like brown mud.
if the guy changed to oil to prevent you from seeing this problem before the sale i bet oil residue will still be inside the rad overflow tank.
i would rather push my Ford than drive your Chevy !