I think that is MDS kicking in - In know they say it doesnt impact RPM but I think at those speeds it does. If you run CANalyzer and have it flag MDS activity you will see that happen. My truck does the same thing.

JD

 

Wow ! Finally someone that confirms that I am not crazy or the only one that this has been happening to. I also put a magnaflow on my 07 QC. I have gone to the dealer - first they said they didn't notice it. Then they acknowledge it a bit but said it's normal. I even test drove another new truck at the dealer and the same thing was happening but you don't notice it as much without the magnaflow. I reset the pcm myself on my truck but still it happens. Finally the owner of the dealership who has an 07QC has also noticed it after a few people mentioned the problem to him. However - still no resolution - only acknowledgement. If anyone finds any other info out on this - PLEASE post it.

 

Sounds like a combination of MDS and torque converter lockup to me. The better flow that I would assume you get from your muffler is possibly allowing the truck to kick into MDS more often. I notice an rpm drop everytime MDS activates on my truck. You'll also see some rpms drops when driving in tow/haul or OD off (MDS disabled) from torque converter lockup. MDS tends to work from 25-70 mph when a light load is applied most of the time, but with various factors involved it could kick in outside that range. That's probably why you did not notice the rpm drop at 75, but you did notice it at 65 mph. I drive around in tow/haul whenever my cutout is open, and I seem to notice a difference in acceleration/power when the torque converter seems to lock up.

 

Hey remarks,
Did you reset the pcm the way that Pappy had wrote or did you just disconnect the battery?

 

Me thinks minimum octane for our trucks is 89.

Unless you be running a tuner that lets you detune the engine to an 87 octane economy setting.

So you guys might wanna try 89 octane.

 

just went out and did the procedure that pappy described.mine was 10 amp red located in block 31.you will need a drop or flash light to see toread the fuse block lid.straight forward and only took about five minutes.i also noticed what i thought to be erratic shifting after i installed my exhaust which alot of people dont(wont) run.mine is a gibson super truck exhaust.it exits right in front of the right rear wheel.has two square stainless tips that look real bitchin!it does have some in cab noise but you can still hold a conversation at cruising speed without yelling.but when you drop the hammer differrent story.wot this thing sounds wicked.also installed a helix throttle body spacer and larger inlet tube along with an airmax cold air system.this is all mandrel bent 16ga steel and hand welded..these parts are made by Street and Performance Electronics in N.Little Rock Arkansas.i noticed a HUGE difference in throttle response and powerand get anywhere between 19-21 mpg on 87oct @ highway speeds(70mph).i highly reccomend these parts versus some abs plastic crap.plus the fact you can talk to a real person in customer service if you have a problem.i will repost and comment on the pcm and trans reset as soon as tonight or tomorrow morning.

 

Anyone know of a way to disable MDS on the highway? Around town I just turn OD off, but on the highway if I drive 65 I am fine, it is MDS all the time (1800 RPM or so) an only occasionally hops out of MDS (1950 RPM or so). But at 70 it bounces back and forth all the time and drives me a bit bananas. If I drive around at 80 then it is out of MDS all the time. Fuel econs at all three speeds are 20.2, 18.8 & 14.8 repsectivly so I dont like driving around at 80! Also, the truck still only has around 1400 miles on it so I imagine these will get better.

Anyone run a Borla? Those have a front resonator in the kit and I wonder if it does away with this issue.

JD

 

I reset the pcm the way pappy said. The hesitation I get does not happen all the time, but the main frustration for me is on a quick takeoff where there is a dropoff seems shifting into 2nd gear. Doesn't happen all the time though - can't pinpoint exactly how/when it does to recreate every time for the dealer. I also put a K&N on the same time as the magnaflow.

 

I feel that also. Feels like the *** end just fell out of the torque curve. I left my battery unhooked all day long at work. On the way home the rpm's stayed consistant through all speed ranges. If it goes back to jumpin I'll try Pappy's method. Anyone who tries Pappy's and gets results please post.

89 Octane-- Oh man I feel pocket book getting lighter. Unless it make a 4 mpg difference.

 

I reset the pcm like pappy stated and got really good result....the od doesnt kick in so soon now and all i here is mds....which still kicks in really quick....at around 10mph ifrmps drop....but the eratic shifting like before seems to be alot smoother and got my shift points back to normal. I really want to thank all you guys for the advise and wiil be on this forum from now on!!