where is this bad miss coming from

[FordFanatic12]

my 2003 ford explorer sport trac has a bad miss mostly when in gear sitting still at a light or drive thrue and really bad when giving it gas to go up a hill or passing I took it to a garage and they put it on a computer and cant figure out what to fix ..I have replaced the idle control . the fuel filter . plugs and wires and a couple sensors nothing the engine light isnt on this is making everyone crazy any ideas on what it could be ???

 

[david4451]

1. Could be the fuel pump on it's way out, working intermittently.
2. Leak in vacuum line, check PCV connections also.
3. Bad gasket on inlet manifold to head.

I've had 1. & 2. on my 2001 4wd ST

 

[swshawaii]

2. Leak in vacuum line, check PCV connections also.

Specifically the front PCV elbow. Very high failure rate and does NOT always throw po171/174 lean codes.
Also wouldn't hurt to clean the throttle body and MAF sensor with the specific CRC brand cleaners. Linked:

http://www.explorerforum.com/forums/showthread.php?t=314670

 

[TJ Sport]

Check this forum for issues with the timing chains. I had the exact same symptoms with my 2003 Sport-Trac. Finally I noticed a metallic clacking noise, and pulled the left valve cover, to find the timing chain guide had disintegrated. I was able to replace the left timing chain and jack shaft chain in vehicle.