I recently bought a 1986 Ford Bronco XLT.
5.0 Litre V8 with an automatic transmission with OD
There was very bad oil leak everytime I stopped the truck.
(Looked like there had been a Dodge in my driveway)
So I purchased a set of valve cover gaskets.
I went through the painstaking problem of changing them.
Removing all the top end of the engine.
I put it all together with new gaskets.
Then I had to take it on 400 mile trip. A few stops for gas at 13 miles to the gallon I noticed the engine loping well idling.
But seemed fine on the highway.
Can anyone tell me how to fix this problem ?
Im home now and stumped is it electrical or perhaps ignition

 

When you reassembled the upper intake manifold plenum, are you sure all the gaskets sealed properly? If so, are you certain that you reconnected all of the vacuum lines? It almost sounds as if you've got a vacuum leak of some sort...... You may double check all lines, particularly those hard plastic capillary tubes. They're at the age, now, where they're likely brittle and easily broken.

 

I also have an 86 303EFI with AOD with 215K miles. Great truck. I have learned a bit about the EEC-IV computer and ignition. I think restorit has it on the vacuum leak. That computer will chase it's tail trying to keep it idling at the right rpm. Did it lope before you changed the gaskets? Did you disassemble any of the throttle body components when you took off the upper intake manifold? Run a vacuum test on it. A good reading should be a steady 17 to 22 in/hg manifold vacuum. If you have a leak it will be lower. You can tell a lot about the condition of your engine with a vacuum gauge.