My 1991 Ford F150 won't idle when cold,backfires when I accelerate,loses power when accelerating. I have replaced the fuel pump,egr,distributor,02 sensor,computer,all the sensors are brand new.I have replaced the lower intake,IAC controller,throttle position sensor.I checked for vacuum leaks.Could the problem be leaking vacuum at the throttle shaft in the throttle body. I don't know what else to try.I even took it to a mechanic and he could not find the problem.

SOURCE: 1993 Ford Van Econoline 150

Try changing the throttle body position sensor, or the ideling screw. , Service the fuel injection system (injectors and filter) have the gas tank washed as there could be rust paricles in it. and if that fails you will need to have an engine job done. Chances are the pistons are worn out or the rings are bad.

Other possible causes are the torque converter or the transmission itself, However if it coccurs with the ac, it means that it happens when extra stress is on the engine. This is caused by eighter the fuel system or the engine itself.

SOURCE: 1991 grand am 2.5 backfires wont start.

MAP sensor is probably bad ,located on the air filter cover

SOURCE: 1988 ford f150 running rough

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

SOURCE: 1989 GMC 2500 5.7L - engine surges and stalls at idle. Loses power at cruising speed.

Wow you have replaced a lot of stuff. Lets go back to the Fuel pump. When you replaced it did you replace the hose between the pump and the sending unit. These are common for splitting. Have you hooked up a fuel pressure gauge to check what pressure you have? It should be real close to 13 psi and should raise to at least 18 or 19 when you pinch off the return line.

SOURCE: My honda idles very high and low and erratic when iac disconnect

You need to have the throttle adjusted and since this will involve adjusting the air to fuel mixture you will most likely need a mechanic with a computer bench to do the work. Should cost around $40 to $50.