I recently purchased an '01 F350 w/7.3L turbo. This is my first diesel engine. My truck starts easy, runs very well but when sitting in park (even after reaching operating temp) for a few minutes, the idle will go from about 700 rpm up to 11-1200 rpm. The speed change will take 15-30 seconds. If I touch either the throttle or the brake pedal, the idle speed drops back down instantly. 3-5 minutes later, the idle speed ramps back up. Any ideas what could be causing this? It is curious to me that the brake pedal will affect the idle speed...which leads me to think vacuum problem.

 

100% normal. When its cold..the truck will do that to warm up. It has a mind of its own And hitting the brake pedal will stop it. That is normal too, because it assumes that if you hit the brake pedal , you are about reay to drive, so it kicks the idle( truck) back to normal.

 

I seems to do this even after the engine temp stabilizes. Also, any idea how the brakes are tied to the engine idle speed?

BTW, Thanks for the quick reply!

 

100% normal. It is assumed that you need a higher idles speed when it is sitting around. When you touch the brakes, it is assumed that you are shifting it into gear hence the lowering of idle speed to what it was. I have a high idle for when I want to leave the truck run with the dogs in it during winter and need the heater and during summer when I want the AC.