My dad has a 99 F550, 7.3 obviously, and it acts fine except when its been sitting a long time and you start it up. It starts a little slow, but its cold right now so thats normal. Once it starts it seems to idle fine but if you leave it for a couple minutes to warm up it randomly starts speeding up and goes all the way to redline until you hit the throttle pedal, then it comes right back down to idle. We bought the truck in September and it never did this until December, so I'm not sure if it happened to the previous owner last year when it was cold. It may or may not have something to do with the cold, it may just be something wore out and it happened to be when it got cold. Any ideas what could cause this?

 

It goes all the way to redline or it makes a loud hissing noise, like a leaf blower on steroids and revs to about 1100rpm?

 

I haven't ever just let it do it and look at the tach so I'm not sure how high it revs. My memory is a little foggy because I haven't been there when it did it for a few weeks. I'm just wanting to figure it out because its happened to dad a few times in the last week. And I can't really ask him what it does because he wouldn't exactly provide informative information if you get what I mean haha he calls it a runaway even though I explained to him a real runaway is a lot scarier than that...

 

Well it did it today and you were right it only went to 1100 rpm but it only did it a few seconds this time, I didn't have to hit the throttle. I have the truck now because he wants it fixed so I just turned it on and let it idle and after a couple minutes it did it. So what is it then?

 

It's normal. It's part of the cold temperature programming in the PCM. If you touch any of the throttle, brake or clutch, it will return to curb idle then after a few more minutes it will idle up again.

 

Shows how much I know about diesels... Sorry for the idiot post. I would've thought that if that was the case it would idle up as soon as it started up, like a gas motor does... Thanks though.

 

It's not an idiot post. Everyone has to learn somehow, no?

 

Yep - I remember when my truck first did it. I think I shut it off because I was worried it might be broken or something..... Live n learn....

 

It's a function of the PCM, as Pikachu said. There is an exhaust back pressure valve, and the PCM commands it to close to heat up the engine faster. Then it will cycle on and off a little while and then it will stop once the engine oil temp gets up to about 150 degrees or you touch the break or throttle.

 

Ok, so the exhaust back pressure is used to put a small load on the engine and compress the exhaust gas which both heat up the exhaust gas and helps get everything warmed up. That's pretty fancy.

 

White Buffalo, Airborne and dope on a rope wings. I'm Airborne and crossed arrows. Never know who your talking to. Brothers in more than FTE.

 

Till the end! My hat is off to you, Special Forces have had quite the full plate mission...... Everything inside & outside the box.....