I have the 3.6 DI engine, automatic transmission and all wheel drive.

I've noticed the transmission trys to upshift too soon, presumably to increase the gas mileage. However, when it upshifts, the revs are too low. They drop to anywhere from 1200 to 1500 RPM, which is too low for the engine to develop sufficient torque. What happens is I feel a vibration as though the engine were overloaded. When the revs are 1800 and above, the car runs smoothly.

I see this when traveling in the 40 to 55 MPH range, usually on a highway with a slight upgrade.

I've taken the car to the dealer, and there is nothing they can do with their shop computers, and there are no software updates available from GM.

Has anyone else had the same experience?

I'm going to call Cadillac customer service this week and see what they say.

what shifts are you referring to? 1st to 2nd or higher up in the gears? it doesn't sound like it's the engine's torque curve, even though the di engine does develop it's torque peak at a much higher rpm than the port injected engine. it sounds like a fuel starvation/delivery problem or maybe the mapping of the fuel curve.

jackg
06 sts6

I notice the vibration on the highway as well, in the shifter and the gas pedal. People mention that it goes away when putting it into Sport mode, and I noticed the same...I think that you're right, the engine is "lugging" in 6th gear, and when put into Sport it may be downshifting, resulting in no lug, and no vibration...

in top gear, how rpm's are you turning at 60mph?

jackg
06 sts6

DI, auto, AWD, turns about 1800 at 60-65mph.
is there a numerical rpm readout?
rg

I'm not sure, I'll look tonight

If you have a really light right foot and baby it when taking off from a stop, it will upshift accordingly (very gently and at relatively low rpm).
my response is don't do that. get going at a moderately brisk pace (don't have to floor it) and the engine is much happier.
rg

... even though the di engine does develop it's torque peak at a much higher rpm than the port injected engine. ...

I don't feel like searching for it right now but I remember reading that the DI engine has more torque than SFI at low (~1000) rpm, even tho the DI torque peak is at a higher rpm than the SFI peak.
rg

I don't feel like searching for it right now but I remember reading that the DI engine has more torque than SFI at low (~1000) rpm, even tho the DI torque peak is at a higher rpm than the SFI peak.
rg

I did this overlay of the power curves for both engines some time ago. It appears that the 3.6 DI engine has at least as much torque as the base 3.6 at all engine speeds. At around 3200 rpm or so they may have about equal torque.

http://www.cadillacforums.com/forums/attachments/cadillac-cts-forum-2008-beyond/38292d1198257780-just-got-her-back-3.6-overlay.jpg

I've noticed the same on my DI AWD. At about 40 to 50 in 6th gear the motor's rpm are in the 1000 to 1500 range and the motor lugs a bit and you do get some vibration. I do think this is based on getting better milage and the six speed transmission. The nice thing about this car is that if you don't like how that feels, you can shift it manually into 5th. I am guessing that you were driving a manual version of this car you would be cruising in 5th at this speed range, saving 6th mainly for highway driving.

There is a TCM update for this car, I'm not entirely sure if this is one of the issues it addresses though. The relase notes simply say "Adds robustness" LOL I can program these cars to do whatever you want but you might try getting the TCM update first to see if it helps :)

there's basically three parts to the puzzle: the gear ratios, the final drive ratio and the torque curve of the engine. does anyone know if the di engine has the same gear ratios as the portinjected engine? this could make a difference in the driveability of the di equiped cars if the ratios are the same, due in part to the torque part of the equation: the di engine makes more torque but at a higher rpm point.....this could be part of the lugging some of you are experiencing. also, i believe the awd cts's whether they're port or direct injected have the 3.23 rear end gear...the auto di and the manual pi have the 3.42 gears...this will add to the difference in the rpms turned at a particular speed.


jackg
06 sts6

Another factor, I haven't seen if this drivetrain includes a lockup torque converter. that would also bring revs down without the cushion of the TC.
rg

don't all automatics have tc's?


jackg
06 sts6

Lockup TC. eliminates slippage and power/efficiency losses at highway speeds.
if you have a 6 speed auto with lockup you should feel 6 transitions, not just the 5 of the tranny gear changes.
I haven't been able to detect that.
rg

yeah, like that...i didn't think tc would change the rpm speed.


jackg
06 sts6

Found a document at media.gm, this auto (6L50) has a lockup tc with eccc which allows a bit of slippage even in "lockup".
under normal conditions that should not lead to lugging the engine like a hard lockup (especially when it is defective).
rg

back to kencurtis (anyone ever call you festus?) - the question is are you noticing something normal or unusual. to meet cafe standards all car mfr's have to do things to improve mileage of all vehicles under specific conditions. if you drive with a light foot then the tcm will respond with relatively low rpm upshifts and you are probably just feeling the engine strain a bit. I have a 96 s10 pickup with a 4.3v6 and od auto, it is *very* reluctant to downshift (or even release the lockup) going uphill at highway speed. it takes a deliberate stab on the throttle to get it to "kick down" and let the engine rev up to where it runs smoother.
rg

I cannot drive it more aggressively, as someone suggested, because I am driving in traffic. The guy ahead of me wouldn't appreciate my pushing him :)

The RPMs are < 300 times MPH in 6th gear. At 60 MPH, the RPMs are just under 1800.

I agree that if I had a manual transmission, or chose to use the manual shifting in the automatic, that I could keep the RPMs higher. But I bought the automatic so I would not have to shift.

This is definitely not a vibration that you stop by putting into sport mode or holding onto the shift knob.

Still haven't had a chance to call GM. Maybe Wednesday I'll get the time.

Yes, I've been called Festus before!

I just think that is the way it is. Always trying to eke out the max MPG.

However I think if it bothered me, I would try and use sport mode. I haven't even tried sport mode yet........
Supposedly Sport mode holds the car in each gear longer.
Of course you might find yourself in 4th at 2500......... But as long as you are happy, that is the important thing, right?

it would be nice to know how this turns out. some of it depends on what your expectations are - if you're used to silky smooth operation then normal vibes may upset you. I come from a long line of chevys and pontiacs (harleys too) including some pretty nasty stick-shift cars, so this CTS is like the most wonderful thing I've been in for years. OTOH there have been rumblings (sorry) of NVH (noise/vibration/harshness for you in rio linda) problems but nobody that I have seen has been able to pin this down.
best regards.
rg
p.s. you don't shift the auto in sport mode.

there is a website somewhere under gmpowertrain.comi that will list the power and torque curves for all the engines gm produces...somebody sometime ago posted the graphs for the pi and the di engines. if you can find those graphs you will clearly see that the di engine makes more power and torque but at a higher rpm...that coupled with the 3.23 rear end in the awd model could be the problem, especially with the six speed....that's why knowing the gear ratios for the 6 spd and the 5 spd is a critical part of the equation....especially if they simply took the 5 spd's and simply added on another gear that lowered the final drive ratio, without changing the 1-5 ratios. this could be part of the problem: the taller 6th gear ratio, the 3.23 rear end, the higher torque band of the di engine and finally the additional weight of awd.

it's something that taken individually might not be a smoking gun, but added together could be the culprit.

i'm not an engineer and could be way off base, but it's simply something to consider.

one suggestion, if your gate is like mine on the sts and has a slot for 4th gear (well it would be 5th on your 6 spd) simply use 5th in those traffic conditions that pose a problem for you. not very high tech, but this might also help you to analyze the problem better after drivng in 5th for a period of time.


jackg
06 sts6

refer back to post #9 in this thread, the graphs show the DI engine has more power and torque at low and high rpm. they are equal around 3200 but DI is higher everywhere else.
OP said automatic trans.
rg

The RPMs are < 300 times MPH in 6th gear. At 60 MPH, the RPMs are just under 1800.

I should have said RPMs are < 30 times MPH in 6th gear. Sorry.

I have always used mph/1000rpm. last nite I checked, in top gear with light load it was a shade over 70mph at 2000rpm.
that works out to 35mph/krpm. that's about as high as I've seen on any car or bike but it's nothing new.
rg

well that answers part of the equation. the engine makes more than enough power through the rpm range..it might something with the gear ratios, possibly.

jackg
06 sts6

kencurtis - did you get this checked out - normal or abnormal behavior?
anybody else complaining about engine lugging or vibration?
rg