Often times when the A/C is turned on, the engine computer will boost the idle speed, because the A/C pump is about a 20 hp load. When the A/C is off, then the idle speed will not boosted. Have the throttle body inspected and cleaned if need, and have the base idle re-set. Do you ever get any check engine light on? If so, have those codes traced, that will also be a big help.
THIS IS A FUEL INJECTION, COMMERCIAL ENGINE. WHAT ABOUT A LOOSE FITING OIL FILLER TUBE TO THE VALVE COVER, THAT HAS ALWAYS BEEN LOOSE IN THE HOLE.I BOUGHT THE TRUCK USED A YEAR AGO LIKE THAT. NO ONE EVER QUESTIONED IT. THE CHECK ENGINE LIGHT HAS COME ON SOME TIMES. THE LOCAL SHOP RE SETS IT. THE COMPUTER CHECKS EVERY THING OK UNTIL THE NEXT TIME. THANKS.