RPM fluctuation

[crazycardfan1]

I have a new 17 2500 with the 6.4 and I'm wondering if anyone else has this issue or if it's normal. When truck is in gear at a stop with a/c on the RPM's fluctuates up and down and it's weird I can feel it in the brake pedal. I've had a bunch of new vehicles cars and trucks and I'm positive none of them did this. I have an appointment to take it to shop for some other minor issues but was just wondering if anyone had this problem.

 

[mowin]

Can't say if noticed that in my truck. Keep us informed on what the dealer says. If they say it's normal, test drive another truck to confirm.

 

[BlkZrx]

Doesn't do it on my 16..

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[gustheram]

Doesn't happen on my '15. Happened on my '95 after 18 years; I think this might be a sign of a vacuum leak. I'd take it in and get it checked out.

 

[Bigdaddy]

Take it back to the dealer.

 

[VicViper]

I'm also having this problem in my 14' 6.4....but I'd say it's worse and happens when I accelerate or am at a steady speed...engine randomly drops or raises RPM.


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[InvictusII]

Are you sure it's not the hill start assist kicking in?

 

[HvyDuty]

Mine does it at a stop with foot on brake, transmission in neutral. (like when waiting in a drive-thru) Never when in gear and only in neutral not in Park. Weird

 

[PatinAZ]

My '16 doesn't. My '98 5.9 & '99 5.2 did but the intake plenum gaskets were shot on those.

 

[whsk]

mine does not do it that i notice-looks a dealer visit is in the cards!

 

[SouthTexan]

Sounds like your A/C compressor kicking on and off putting a load on your engine. It is normal for the rpm to rise when the compressor kicks on, but should not be turning off and on that frequently to be noticeable. It should stay on for a while and then turn off for a while. Could be the A/C compressor is going bad, bad cycling switch, or low freon.

 

[HvyDuty]

In my case, its definitely not the A/C compressor. It does it a/c on or off. Idle will drop to as low as 300-400 and bounces back to 900 erratically. No other issues with the truck. I just don't shift to neutral at drive thrus anymore, go straight to park.

 

[crazycardfan1]

I ran into a guy at work that had the same truck as mine and we checked his out. There is definitely something up with mine his rpm's do not move at all it's as smooth as silk. Mine drives me insane lol. Taking it to the shop next Monday.

 

[crazycardfan1]

Mine only does it with a/c on.

 

[Ratket]

Could it be mds activating? I understand it only cuts 4 cylinders.. But at idle it maybe enough to drop the rpm and then kick it out of mds and bring the Rpms back up.

 

[crazycardfan1]

No, I checked another guys truck his as smooth as silk mine fluctuates. Like I said it's in drive with a/c on at a stop.

 

[HvyDuty]

I tried idling in neutral tonight at a drive thru. No more fluctuations in RPM.

 

[VicViper]

Mine is the MDS...with yours dropping low to the 300 range I was thinking low oil pressure.

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[HvyDuty]

Mine is the MDS...with yours dropping low to the 300 range I was thinking low oil pressure.

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I think its computer or sensor related. I keep my EVIC in Gauge Summary and all of readings have been in the normal range. Never had a Check Engine light either. Only did it in neutral with foot on brake.

It used to do it regardless of Engine temp (cold or warmed up) or A/C setting. I cycled the A/C on and off tonight and it had a negligible effect on rpm.

When I start the truck up in the morning I'm going to try it again and report back the results.

 

[HvyDuty]

Did the neutral idle at cold startup. Rpm fluctuations are gone at least for now.