Idle rpm funny while coasting in neutral - JeepForum.com

 
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post #1 of 4 Old 01-24-2014, 08:09 PM Thread Starter
kb9mfd
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1995 ZJ 
 
Join Date: Dec 2012
Location: Janesville
Posts: 91
Idle rpm funny while coasting in neutral

I have not noticed this before on anything else I drive, but if I am driving along and shift into N (transmission, not TC), at 15mph and higher the rpm jumps to 1.2k, then every two seconds it drops to 600, then back to 1.2k (like if I would hold down the accelerator a little ways and let go of it every two seconds), it does that until I get below 15 mph then it drops to 600 and stays there. I'll do that when it is very icy out so I am not fighting against the drive train and engine, (also when I have my big snow boots sometimes I will catch the accelerator with the brake by mistake), or when I am shifting the TC. Is that normal? What would cause that?

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post #2 of 4 Old 01-24-2014, 11:11 PM
KJK
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1998 ZJ 
 
Join Date: Jan 2007
Location: Dallas, Texas
Posts: 1,014
Mine will do the same thing. I first noticed it a few years ago. Every time I bring it up all I get is a bunch of posts stating it's illegal to coast in neutral.

5.9 ZJ SCT tuned PCM/ KJK CAI/ Thorley headers with 3" Magnaflow cat and muffler/ Modded kegger/ TransGo shift kit/ Vanco brake kit/ Braided steel brake lines/ Mopar cupholder insert!
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post #3 of 4 Old 01-24-2014, 11:50 PM
Oldfrog
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2007 WK 
 
Join Date: Feb 2008
Location: Louisiana/Texas
Posts: 5,092
My GUESS would be that , even in neutral, the torque convertor has some fluid resistance and therefore the engine rpms are affected. I base this on the fact that the tranny fluid is to be checked with the tranny in neutral and warm, indicating that the fluid pump is operational. But I know NOTHING about auto trannys...so it's only a GUESS. I'm sure a tranny guy will set me straight if I'm way off base.

2007 5.7 Hemi, Ltd. QD II -
1997 ZJ, 4,0 select trac, Up country, track lok.
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post #4 of 4 Old 01-25-2014, 07:41 AM
newfieZJ
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1995 ZJ 
 
Join Date: May 2008
Posts: 6,557
Quote:
Originally Posted by kb9mfd View Post
I have not noticed this before on anything else I drive, but if I am driving along and shift into N (transmission, not TC), at 15mph and higher the rpm jumps to 1.2k, then every two seconds it drops to 600, then back to 1.2k (like if I would hold down the accelerator a little ways and let go of it every two seconds), it does that until I get below 15 mph then it drops to 600 and stays there. I'll do that when it is very icy out so I am not fighting against the drive train and engine, (also when I have my big snow boots sometimes I will catch the accelerator with the brake by mistake), or when I am shifting the TC. Is that normal? What would cause that?
Normal . Not a good practice regardless . Not concerned if there's a legality involved , I don't think it's heathy for the trans/engine period .

Old frog , my hypothesis is the the pump works all the time , neutral allows the oil to flow through the trans w/o restriction and be safely checked by one person . The process also accounts for any mods or issues that could effect the oil level in , let's say , Park . (ie: check/drainback valve removal) The gauge then set on the dipstick accordingly .
Torque converters work based on engine RPM . wheels are spinning , it's transfered via drive shaft , through the t-case , to the trans and eventually to the engine . The torque converter will stay engaged till approx 1500rpms , therefore it will increase the engine rpm until the vehicle speed is reduced enough to unlock the converter . It's not the engine revving , it's being pushed by the energy transfer of the rotating assembly which everything is connected .

That's my stab at it
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