1989 Ford F150 standard trans 5.0 engine misses like bad plug or

P0320 means that an ignition pulse that's supposed to fire a spark plug was missed. It could be as simple as a chafed wire under the hood. Watch this video:


Ford has some information too:
P0320 2000 Ford F150 Ignition Distributor Engine Speed Input Circuit...

Blend doors not working properly due to vacuum leaks or bad actuators? Fan motor running too slow due to bad resistor or motor?

the O2 sensor works on one volt .5 is around the running voltage, i would refit the air hose as it will run the converter hotter at idle.

yes they both can do this. there is a coil pack for each cylinder, and at high speeds this should seem to go ayay and at low speeds and/or acceleration it will appear to get worse. think about a full tune up IE...plugs,wires,all coils, and if they all mount to a plate, then the control module as well, but I believe this truck has individual ones above each cyl.

idle air control . unplug it and see what happens.

You probably have one plug not firing. First, check all your plug wires to make sure the boot is pushed on all the way. If there is no improvement, you can pull one plug wire off at a time (with the engine running). If the engine doesn't die, or change when you pull a plug wire off, that will be your bad plug. It should change drastically when you pull a wire. Be sure to use a spark plug wire puller, or a heavy rubber glove. Check for spark in all your wires with a timing light.
If there aren't any dead plugs or wires, the next thing to check is the timing. Aim a timing light at the timing mark on the crank. If the mark jumps around a lot, the timing belt or chain has jumped, and will need to be replaced.
If that checks out okay, the next thing to do is a compression test. You may have bad valve.

when you changed the plugs did you notice any particular plug dirtier than the rest. If so, you probably have a bad ignition wire

Might want to change o2 sensor's, If they are not sending the right single its tell the computer to dump fuel. 1988 was using a fuel density system.
(1989 came out with Mass air flow system)
Check this
I think it might have a TFI module that's not working right. Located under distributor. Its gray and about 4" long and if equip, you need a 4 or 5.5 mm tool to unbolt it. This was a common thing back in the 80's. Its been so long I can't remember if the 4.9 straight 6 had it or not.
T- Thick
F- Film
I- Ignition module

The trans gearing is all the same, the rear end gear set is where final drive ratio is determined