My truck is acting up the exhuast is sputering but when i accelerate its fine. I found one thing wrong i guess its a valve that has a air line going to it directly from the main vacuum port. when i remove the line the truck idles fine but when the line is put back on it idles real low and dies. i have picture of the valve below please help.The truck is a 1990 F-150 with the straight six 4.9l

 

If the thing is sitting on the fuel rail, then it's probably the fuel pressure regulator.