I have an issue with my F250 and it is that it will not idle properly. About a month or so ago the truck started to idle strangely. At start up it would idle at around 1200 rpm. After the truck warmed up the idle started fluctuating as sometime it would idle around 2000 rpm and sometime it would idle around 1100 rpm. If driving on the freeway in traffic and the traffic stopped it would idle high and slowly drop down to the 1000 -1100 rpm area but never to the normal idle speed.

To try and resolve the issue I tried process of elimination.

1st I took the throttle body off and cleaned it thoroughly and checked the shaft and bore for clearance. Replaced it on the truck with no change.

2nd I replaced the MAF and installed a new air filter. Again no change.

3rd I replaced the TPS. No change

4th I replaced the IAC. No change.

5th I replaced both of the 02 sensors. No change

6th I replaced the PCV hose at the intake and installed a new PCV valve. No change

7th I tried my best to check for vacuum leaks, there are not that many hoses so it did not seem to difficult, and did not find any.



One observation I did make is that I clamped off the IAC hose and at startup it almost idled normally and when driving it the idle did not seem to go as high, maybe 1400 – 1500 rpm, and it seemed to settle down to an idle around 1000 rpm a lot quicker. Once I took the clamp off it went back to the varying idle !!! So here I am posting in hopes that someone has run into this issue and found the solution. I need to get the truck smogged as soon as I can and really am hoping someone out there can help !! As always thanks for any information you can provide and hopefully it will help.

 

Everything I reading is basically saying you may have an air intake manifold leak. Maybe someone with real knowledge of the V10 can answer your questions better.