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

 

That's the fuel pressure regulator. As long as there was no fuel coming from it when you removed the vacuum line then it's working fine.