Bought my 1991 f150 a couple of months ago, ran fine. I was driving in a hurry 2 weeks ago and i drive her fast (80-90mph) and I did accelerate from 60 mph to 90mph flooring it! when getting close to my exit at the highway i noticed she drove differently she deccelerated. Once I entered the main road after comming off of the exit, on idel it was bucking like crazy! I changed the plugs, wires, temp sensor to the computer and to the coolant (just in case)..cleaned out the throtle body and nothing. the engine shakes steadily and when i floor it its fine but when i slow down and I'm ready to maintain speed bu giving it a lil gas , she bucks like a wild bronco!

Could I the highway trip flooring it have done somehting to make her run this way?

oh yeah #8 plug was a lil fouled and a bit oily when I pulled it out when replacing the old plugs with new ones.


any ideas?

 

Any check engine lights? Why not do the distributor cap too? Usually on any of our older engines, it was plugs, wires, cap and that's that. Some may ask what plugs and wires did you use, especially plugs. Be patient, a few guys on here are quite knowledgeable about the older engines.

 

I would finish the ignition tune up with a rotor and distributor cap, if you haven't replaced those. It doesn't really sound like an ignition problem though, if it runs good at full throttle. I have had or still have a lot of Fords of that vintage, mostly cars but same principle. I too would do an ignition tune up first when I had a weird problem. Although I never had a problem quite like yours, fuel injector cleaner has solved some strange problems for me. My suggestion, pour in a full container of a name brand fuel injector cleaner just before you fill your gas tank next time. If it don't work, you will only be out a few dollars.