My 2004 Ranger started missing. computer codes said misfire on cyl 3 I changed the plugs, wires, coil and fuel injector. When I put it back together it started and ran at normal speed with a miss then slowly the idle slowed down and it barely runs. Vacuum guage shows it fluxuating between 5 and 10" of vacuum. I then changed the head gaskets, intake gaskets and the issue remains. I didn't see any burnt valves. As I was troubleshoting I removed the vacuum line from the side of the throttle body and the engine sped up and started to smooth out but it popped through the port so I plugged it back in. Thinking maybe the air bypass valve? I removed it and the plunger is kinda floppy but I don't know what it's supposed to be like.

 

So I purchased an air bypass control valve and now the truck idles at about 2k. Codes show banks 1 and 2 are now lean and there is an issue with the air control circuit. The valve I purchased looked like it had been returned so it may be defective