I have been working on my wife's 1990 F-150 4.9L i6 5 speed and I do not know if it could be the, fuel pump, EGR, or TPS? If i give it a little gas like 1/4 throttle it's fine but as soon as I get to 1/2 or 3/4 throttle it sputters and will not increase in rpm like 1/4 is the most I can get.

The truck is an old farm truck so most of the instruments and lights do not work but it was running fine a couple of months ago and not it is acting up.

So far the truck will idle but i have to remove the A/C and Cruise Control vacuum lines from the manifold for it to stay on. It idles around 1100-1300 RPM and will run but it is odd, i will let off the clutch and it takes off but as soon as i press the gas it bogs and wants to die but if i do not touch the gas pedal it runs and will even run through all 5 gears, i just do not control the throttle at all.

I would also like to add that the front fuel pump is dead and the rear i can hear go on when i turn the key but the fuel gauge is not working.

Does anyone have an idea of what it could be? I already replaced the spark plugs, wires, cap, rotor, and i just bought a new fuel pump for the front tank but i have not installed it.

The truck is at her families ranch so i will not be able to get the codes for another week but any guess would help right now.

 

If you have to take vacuum lines off for it to idle, you're introducing a vacuum leak. If it won't idle with vacuum lines connected, it's most likely the idle air control, which is mounted on the throttle body. Try cleaning and lubing it.

The induced vacuum leak may or may not be causing your other issues.