idle fluctuating 400-700RMP - Ford Powerstroke Diesel Forum
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Old 10-02-2013, 02:00 PM
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idle fluctuating 400-700RMP

Hello, I have noticed my truck when in park the ideling RPM is fluctuating between 400 - 700 RMP. I was hoping i could get some opinions on what might be causing this before i jump in and start trying things.

I am leaning toward a vacuum problem right now, becasue sometimes when i start my truck the park brake light comes on and the brake pedal is a little hard, but goes away in a few seconds.

Im not a pro so if vacuum pump has noting to do with RMP problems just ignore that.

Thanks for the Help
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Old 10-02-2013, 02:16 PM
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The vacuum pump is an external pump to the engine, it has nothing to do with engine function. Diesel engines do not produce vacuum in the combustion cycle; this is why they require an external pump, at least in the case of our trucks with their '50s-vintage vacuum-operated HVAC controls and brake boost. What you're probably experiencing with the brakes is that the RPM romp is causing the vacuum pump not to produce sufficient vacuum to provide brake boost. IOW, the vacuum issue is an _effect_ of the romp, not a cause.

Try starting it in Neutral the next several times you drive the truck. If that makes the romp go away, then there's probably a problem with the TRS - Trans Range Sensor switch (aka Neutral Safety switch NSS and MLPS Manual Lever Position Switch). It's not properly "telling" the PCM that the truck is in Park, which often manifests as an idle romp. Attached to the transmission. May be fixed with just an adjustment, but may require replacement.
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Old 10-02-2013, 06:51 PM
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Thank you for the reply, I will try putting it into netural the next few times and see what happens.

Also now that you have mentioned that, my 1-2nd shift is sometimes very hard while at other times very nice and smooth, I was thinking failing accumulator spring, but could this TRS sensor also be the culprite to the occasional very hard shifting?

Thanks
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