I have this 1991 F150 4 x 4 , Some may remember me a couple of weeks ago talking about it being a new toy. With the lift kit and big tires. My wife thinks I need to grow up.... I told her it was my mid life crisis....
So to get to the point. I have a slight problem with the motor when I'm driving down the highway, or around town. It will be running along perfectly, and then all of a sudden it cough's, a couple of times. back fire's out the throttle body, and then carries on.... It may do this one or two times, then run normal again for awhile. At first I thought is was water in the front fuel tank, (truck was sitting for 2 years.) Then when I switched tanks. It seamed to go away, so I was convinced that was the problem.... Nope. Now it does it on either tank. I changed the plugs, Cap, rotor. ( was surprized it even ran with the amount of rust on cap and rotor) New Fuel filter, cleaned the throttle body, added gasline antifreeze and some injector cleaner to both gas tanks.
Still sputters and spits on occasion. One time the truck actually seamed to die, and then just as I stepped on the clutch, and was about to restart it. It came back to life without turning the starter over. ????? The tach actually dropped to zero, for a split second.

Anybody got any ideas on what I could check into next? I changed the Ignition moudule. on the side of the Distributer, but that was replaced with another used one, they want $130.00 for a new one. So I would like to test it to see if it's really the problem, before buying a new one.

The above info was written yesterday. Last night and this morning, driving to work. Nothing happened. It ran perfect.

 

Old truck.. rust can be a problem for electrical connections as well as the body. Have a look at the fuel pump and computer relays on the drivers side inner fender, if you find corrosion on the connectors clean it up. The power wire corroded clean off the fuel pump relay on my truck and left me stranded in the middle of the road last year. Before it let go completely it killed the motor a couple times while driving. Also check the ignition switch in the steering column, these wear out and can momentarially kill the motor if it opens when you hit a bump for example.

 

Thanks for the reply. It's been running great ever since I sent this out.

I changed the fuel filter, and while I was at it. I noticed an excess amount of wires bunched up, under the truck.

You could be right about the wiring. Next chance I get I'm gonna clean all that up.

I also had the old module tested.... It did not pass the diagnostic's test. Three stages on the tester out of the four were ok. But I don't think that was the problem.... it just added to it.
Thanks for the tips. I'll be spending some more time tinkering with it, cleaning up the wireing.

One other question for you. Does the truck have one fuel pump, for both tanks. Or one in each tank?

 

The '91 will have a pump in each tank.