Never had anything good to say about the 300 out on the highway, always to the floor on hills. Never made much sense, they have good power. I think I've had 2 as well, last one was an 87 F-150 reg cab long bed 4x4 w/ the 4 speed manual, granny low. Engine ran at it's peak and never did like the highway. Piddlin around slow it would pull a house of it's foundation.

I recall 3,000 RPM being about all it wanted to go and that sounding like the engine was going to fly apart.

Yeah, check the codes anyways, don't need a reader, just a jumper wire and CEL. Haynes manual would come in handy for all that. Truck lifted? big tires? Any changes like that? Did it use to not do what you're describing then just start doing it?

The other truck was my dad's, it's about new back in 89ish. It was a reg cab long bed auto. It was exactly like you describe, to the floor always on just about any hill out on the road.

Oh, take out the AC spark plugs and get them as far away from your truck as possible, preferably at the bottom of a large body of water so they won't mistakenly be used again by you or anyone else. Use Motorcraft or Autolite. i personally would do the same with any Fram filter, but that's not an issue pertinent to your complaint. I always use the cap/rotors with the brass contacts.

 

Maybe a little off-topic but I have an Escape that was doing the same thing.
Hard shifts, deteriorating gas mileage, just no get up and go.

Turns out the right rear parking brake cable was seized. Whenever the handle was pulled, the shoes would expand and stay there. Not enough to smoke or smell, apparently, but enough to make it feel like it was pulling a trailer that wasn't there. New brakes and cables made it feel like a new car.

Might be something to add to your list.

 

if you listen and change the plugs to motorcraft, use copper plugs not platinums and especially NO bosch platinums.

 

maybe its just me but both my 300 trucks pull good on the highway. My 85 carb with an np435 4 speed will accelerate up steep hills from 65 at the bottom to over 80 at the top when most trucks slow down. Thats with 3.55 gears, 4th gear is 1:1 in a bronco. My 89 FI mazda 5 speed will accelerate up the same steep grade in 5th as long as im over 2500rpm.

the sweet spot starts at about 2500rpm and they really wake up over 3k. My 89 has pulled to 4500rpm once or twice but power falls off at about 4k. The carbbed 85 really likes 2500-3500rpm and with the small carb cant supply the fuel must over that.

I run the timing a bit hot in the carbbed truck but my 89 i run 10degree and 87 octane gas. comp is good. gets 15-18mpg in town and more on the highway.

Ive only driven one auto 300, it was a 92 2wd truck and it had some issues so its not a good comparisson.

Ps. ive run autolite, bosch, motorcraft and ngk plugs in my trucks. Never noticed a difference in running, but ngk and motorcraft seem to last the longest and look the best during inspections.

 

Put a timimg light on it and set the timimg to spec's. Also check the dizzy and make sure it is advanceing correctly. Also check fuel pressure and fuel filter. Good Luck Rick