idle fluctuating 400-700RMP - Ford Powerstroke Diesel Forum
Please Visit our Site Sponsors is the premier Diesel Truck Forum on the internet. Registered Users do not see the above ads.
LinkBack Thread Tools Display Modes
Old 10-02-2013, 02:00 PM Rookie

Join Date: Sep 2013
Posts: 3
Thanks: 0
Thanked 0 Times in 0 Posts
Feedback Score: 0 reviews
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
Reply With Quote Quick reply to this message
Sponsored Links
Old 10-02-2013, 02:16 PM
Compression Ignition Addict

Join Date: Aug 2008
Location: Madison, WI
Posts: 3,515
Thanks: 4
Thanked 95 Times in 91 Posts
Feedback Score: 0 reviews
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.
Reply With Quote Quick reply to this message
Old 10-02-2013, 06:51 PM Rookie

Join Date: Sep 2013
Posts: 3
Thanks: 0
Thanked 0 Times in 0 Posts
Feedback Score: 0 reviews
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?

Reply With Quote Quick reply to this message
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Ford Powerstroke Diesel Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
What state or province do you live in?
Security Question
*Required, this field is not shown to others. Enter the last two characters of the word "Powerstroke"
Security Question #2
*Required, this field is not shown to others. "While balancing on a piece of wood, two inches by four inches known as a 2x4, john and his friend sally both spotted a dalmatian inside a truck with sirens, headed to put out a fire. State the animal mentioned in the sentence above."
Please select your insurance company (Optional)


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Thread Tools
Display Modes

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -8. The time now is 05:53 AM.

Powered by vBulletin® Copyright ©2000 - 2015, Jelsoft Enterprises Ltd.
Search Engine Optimization by vBSEO 3.6.1
Garage Plus, Vendor Tools vBulletin Plugins by Drive Thru Online, Inc.