I have a 1997 Ford F-150 with a 4.6L V8 with 178895 miles. Recently, when I start it up it idles fine for a few seconds and then drops down to about 300-400rpm. When the gas gets pushed it hesitates slightly but then revs up just fine. When I'm driving it works great but when I brake the rpms start to fall down to around 350rpm and then once I come to a complete stop it bounces between 350-600rpm. What could be causing it? I think it could be the fuel filter but I'm not 100% sure. Any suggestions on what could be wrong??

 

It's the time of year for IAC valves that are sticky to manifest.

Clean the IAC valve as a temporary fix to see if it helps, replacement is usually necessary, though, for a more permanent solution. Also, check for intake vacuum leaks as they will make the IAC's function ineffective even if the IAC is properly operating.

 

Ok I'll clean the IAC valve and check for vacuum leaks and see what happens. Thanks for the help

 

Ok so I changed the IAC valve and checked for vacuum leaks and it's still doing the same thing. What else could be wrong with it??

 

Did you THOROUGHLY check for vacuum leaks including the complete PCV system?

Is the MIL on and are there any DTCs?

 

Update: I cleaned the MAF and the throttle body and it's running smoother, just at a low rpm. It still bogs down and almost dies if I'm braking and climb over anything. I check for vacuum leaks often and haven't found any. The check engine light is off, and I'm taking it soon to scan for codes

 

If the MIL is off, there will be no codes.

 

Is it possible that the new IAC valve needs to be adjusted?