Hey guys...first post.

I recently replaced motors in my 90 Ranger 4.0 2wd Auto. It was a 4.0 to 4.0 swap. The new motor has 80k miles on it. Truck fired right up after the swap but has a miss at idle and when just starting out (0-5mph). After about 5mph or 1000rpm I either can't notice it or it goes away. Until this point the ENTIRE car will shake and the engine looks like its loping, rocking back and forth, in the engine bay. If I take my foot off of the brake while in drive, the car I can feel the miss as is moves forward. I think this rules out mounts. After 5mph it drives great and has tons of power. No engine lights are on, and the car passed smog running this way.

I can take a plug wire off one by one and the miss will get worse each time. I cannot identify a lone cylinder.

Things I have checked:

MAF is clean and appears to be working fine.
Idle control valve is clean and working.
Changed fuel filter.
Fuel regulator is OK as far as I can tell.
All cylinders 160psi compression.
Replaced upper and lower intake gasket.
Replaced all vacuum lines.
Replaced spark plugs and wires.
Checked rocker arms, rods and valves. All look brand new.
Lifters appear to be fine.
Ran seafoam through intake lines, gas and oil.
Ran B-12 chemtool through gas.
EIS is firing fine.

I am out of things to do...Im clueless.

Could it be fuel even though it runs great above 5mph? No hesitation or loss of power through redline.

I bought the motor from a salvage yard. Could it be that the motor came from an explorer rather than a ranger? It is the SAME motor. If it was from an explorer could it be a computer issue? Shifts fine.

Pulled the plugs today. All but two look like they have little specks of pepper on them. Detonation? Why all of them? From the injector cleaners?

PLEASE HELP...

 

try disconnecting the maf and and see if that makes a difference, what does the fuel pressure look like running and sitting with the engine off? what year is the 4.0 from that you put in? You could have some injectors leaking down.