Sounds about right. I'll just have to get used to it I guess.

 

I have the same exact issue with my '08 hemi QC. I have about 3500 miles on it and the rpms constantly move up and down with cruise set at 65. I also feel a slight surge each time the rpms change. If I engage the tow mode, then the rpms stay constant. I don't know if this is the MDS, or TC Lockup, or some combination of both. I do know...this is extremely annoying and should not be something Dodge would pass off as "normal". I'll be having this looked at by the dealer on my next visit. Maybe they will have a flash to fix this?

 

Notice that this goes away if the highway changes to slightly down hill?

You are trying to drive about at the speed where the MDS can't create enough hp to operate in 4 cylinder mode, except when their is either wind at your back or downhill pavement, or both. Recently Dodge engineers told reporters at the 2009 Ram demo day that even the new Ram can't operate in MDS much above 60 mph.

Here's three easy 'one time' tricks to try:

increase your tire air pressure to the max and go back out on the highway to see at what increased speed this now happens with the MDS

fold back both your mirrors (improves aerodynamics slightly) and go back on the highway to see at what mph MDS can go up to

turn off your air conditioning, then test

You can 'permanently' increase the speed at which MDS can continue to work by
improving the aero with modifications like:
lowered suspension,
tonneau,
partially blocked off one third to half grille opening,
vortex finders or 'Airtabs' on sides just before rear tail lights and just before rear window,
extended front air dam using plastic lawn edging,
tape over body joints,
etc

As the tread depth on your tires wears down the rolling resistance will decrease and the MDS will be able to work up to a slightly higher speed.

As the heat of engine operation 'relaxes' the tension of your piston rings over several tens of thousand miles, internal engine friction will decrease, MPG will go up, and MDS will be able to still work up to a higher speed. This is the primary 'engine break in' effect.

 

i'm as confused as lx is. I would think that rpm would stay constant as long as the TC is locked, whether its in 4cyl or 8. maybe it is the TC locking and unlocking so that the engine can stay in 4 cyl mode?

 

The bottom line is it shouldn't do it, the "engineers should've done something so that it doesn't do it. It's annoying and sometimes aggravating. If it only does it at a certain speed, then we're gonna have to drive at this certain speed to realize any (if there is any) benefits of the MDS. Mine will do it at any speed. When it senses it's not under a load, it starts dropping cylinders and the surging starts. Don't get me wrong, I liked my '07 and I like my '08. I just don't like this rpm change. I'd rather see it lock up and stay locked up.
Oh well.............

 

On most of the late model Chryslers
the PCM and TCM are programed to
first unlock the torque converter clutch
if the cruise control can't hold set speed,
then second, after a few seconds,
do a gear downshift
if the speed continues to fall
or the TPS throttle movement is too rapid.

This is the old problem called 'shift hunting'
where customers don't like trans shifting on what they consider slight hills,
but they also demand that their vehicles get better MPG on flat and downhill.

MDS is one solution.

Carlos Ghosn of Nissan alternatively believes that the 'best' answer is
CVT transmissions,
like his Jatco division sells to Chrysler & Jeep for the small cars.

Big, high torque CVT transmissions have been tested in Cummins Rams and Ford Expeditions:

http://web.archive.org/web/200503070...p?article=5029

overview of CVT's
http://www.roadandtrack.com/article....ber=1&preview=

 

Some more details on my setup...

I have all tires at 38 pounds pressure, I have a Truxedo bed cover, however I do have the towing mirrors which may cause slightly more aero drag. When I experienced the problem I was going on a long straight, flat stretch of highway (Houston area); My cruise was set to 65 exactly and I saw no speed fluctuations. I could see my RPMs jump up to about 1900 and then it seemed the RPMs would "bleed" down to about 1650, the "jump" back up to 1900. This cycled continuously about every 5-10 seconds. Very annoying while travelling a long stretch. I set the cruise to 60 and also 70, and I had similar results. Above or below those speeds seemed better.

 

I have been having the same issue since I used the Superchips programmer to drop it to 87, and my mpg dropped noticeably.
been considering reinstalling OEM tune

 

Oh...also I was using the "recommended" 89 octane and my truck has about 3700 miles on it. I wonder if any non-MDS 4.7 guys have this problem because I think the transmissions are the same. If they are, then at least this would eliminate MDS as the culprit. I'm getting the feeling my '03 Ram was a much better truck than this '08.

 

It's the MDS...mine does it too. Learn how to keep it in MDS mode on the freeway and the rpms won't fluctuate...it'll stay steady at around 200rpms higher than when firing all 8. Tow/Haul mode disables MDS...that's why your rpm's remain constant.

I said this in another post but cruise control uses too much throttle too fast...that's why it keeps jumping out of MDS. I don't use cruise unless the road is completely flat...even then it'll jump out of MDS once in awhile.