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Old 02-21-2006, 02:49 PM
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Question Cold idle RPM cycling

When the engine is cold and running at idle, the RPM revs up slightly for a second or two then goes back down. It cycles through this untill the engine is warm. My 300TD does this, I assume everyone elses diesels do it too. I had a friend that had a diesel F150 that I believe did it too. Why do they do it?

Thanks
Andrew

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Old 02-21-2006, 02:51 PM
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Have you checked to see if your Rack damper pin has a weak spring or not....mine was doing that till I replaced it. THere may be other things that can cause that however.
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Old 02-21-2006, 03:14 PM
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Nope, but I will.
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Old 02-21-2006, 08:11 PM
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It's called "hunting".

The SD does it for about 1 minute when it's very cold. Probably changes rpm's by no more than 100 rpm.

I believe the engine warms slightly, and increases speed, then the governor notices the increase and pulls the rack back. This is repeated as the engine warms. At a certain point, it stops, presumably because the engine no longer increases speed fast enough for you to notice.
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Old 02-21-2006, 08:17 PM
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Mine does that when its real cold out, and my rack damper pin is only a year old, and of the newer style. I can watch the oil pressure gauge go up and down from 1.9 to 2.1 bar in a wave motion, its weird. It does not do it when in drive though, only in park/neutral, and only when its reeealy cold out.....
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Old 02-21-2006, 08:30 PM
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Quote:
Originally Posted by pawoSD
Mine does that when its real cold out, and my rack damper pin is only a year old, and of the newer style. I can watch the oil pressure gauge go up and down from 1.9 to 2.1 bar in a wave motion, its weird. It does not do it when in drive though, only in park/neutral, and only when its reeealy cold out.....
Mine is similar, and I too have the new pin. I've got some doubts that the pin adjustment is the cure for the hunting. It seems perfectly normal to me for the engine to warm slightly and increase speed, and then have the governor pull the speed back down.

Some of the 12V71TI's that are in the ferry boats will hunt so badly that they almost stall.......the rpm gets down to almost zero......before the governor finally brings it back up.

Remember, most folks won't notice the hunting. If you just back it out of the driveway and don't let it idle for five minutes..........like some people are fond of doing .......you'll never see it.

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