idle fluctuating 400-700RMP - Ford Powerstroke Diesel Forum
LinkBack Thread Tools Display Modes
post #1 of 3 Old 10-02-2013, 02:00 PM Thread Starter Rookie
Join Date: Sep 2013
Posts: 3
Thanks: 0
Thanked 0 Times in 0 Posts
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
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
Rat250 is offline  
Sponsored Links
post #2 of 3 Old 10-02-2013, 02:16 PM
Compression Ignition Addict
Join Date: Aug 2008
Location: Madison, WI
Posts: 4,075
Thanks: 4
Thanked 148 Times in 144 Posts
Mentioned: 3 Post(s)
Quoted: 58 Post(s)
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.
madpogue is offline  
post #3 of 3 Old 10-02-2013, 06:51 PM Thread Starter Rookie
Join Date: Sep 2013
Posts: 3
Thanks: 0
Thanked 0 Times in 0 Posts
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
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?

Rat250 is offline  
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:


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
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

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

For the best viewing experience please update your browser to Google Chrome