03 CTS success story with Trans Fluid & Filter replacement
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Cadillac CTS First Generation Forum - 2003 - 2007 Discussion, 03 CTS success story with Trans Fluid & Filter replacement in Cadillac CTS Coupe, Sport Sedan and Sport Wagon Forums; Hello everyone, I am new here & just wanted to share my experience with my '03 CTS I purchased about ...
  1. #1
    NoLeafClover is offline Cadillac Owners Member
    Automobile(s): 2003 CTS
    Join Date
    Nov 2013
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    03 CTS success story with Trans Fluid & Filter replacement

    Hello everyone,

    I am new here & just wanted to share my experience with my '03 CTS I purchased about 1 month ago.

    When I first purchased the car, I had some vibration coming from the rear and it would happen between 50-65 mph. It would come and go and it manly happened once the torque converter would lock.
    Car changed gears very smoothly but when coasting, it would shake, Torque converter would lock and unlock sometimes at random and was getting very annoying.

    I decided to replace the trans fluid and see if it made a difference.

    The 5L40E Trans calls for Dex 3 but GM changed to Dex 6 for later models.
    I DID NOT FLUSH the system, but only added what came out. I used Dex 6 full synthetic and added a small bottle of Trans fluid conditioner.
    I used about 5 quarts in total.

    Needles to say, I can not believe how much smoother the car is shifting (not that I could notice before). The best part is, well it is not shaking anymore when cruising.
    This car by the way has close to 200k miles hence why I was so scared to mess with the fluid period.

    I am typing this, because I have heard of all kinds of horrible stories when flushing or replacing the fluid & filter on high mileage cars.
    Perhaps I got lucky, but I would strongly suggest to stay away from doing a Full flush if you decide to do any maintenance on your cts (unless you have less than 70k miles on yours)


    Couple of notes if you choose to DIY:

    There is a kit that comes with the filter and the gasket @ Advance Auto Parts. The Original gasket, has a rubber and metal piece gasket. The aftermarket has a rubber gasket. Actually all non GM gasket for this car are rubber.
    **This is one of times you can tell exactly whether the filter was ever replaced, if the gasket is not metal then someone replaced it or removed the pan before.

    When adding fluid, only about 3 quarts go in before it starts to top and drip from the fill hole. You have to start the car and put the car in DRIVE as someone else is pumping more fluid into the trans.
    This will lower the fluid level for another 2 quarts. Please this is a 2 men job, do not attempt this by yourself trusting the emergency brake or a teenager. I can not tell you if the level drops if you use REVERSE instead (at least you won't risk getting run over.)


    1) Use Dex 6 not Dex 3 as it's better (for 5L40E ). Dex 6 are mostly full synthetic but they are okay to mix with Dex 3 (non synthetic).

    2) Differential fluid on mine was low. I would suggest removing old fluid and adding new one if you never done this before. It takes less than 2 quarts of 75w-90.

    3) Check the Center Support Drive shaft bearing. The rubber on mine was not fully attached. This can also cause vibration by taking the drive-shaft out of balance at random speeds.

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  3. #2
    kaitagun's Avatar
    kaitagun is online now Cadillac Owners Member
    Automobile(s): cadillac cts 3.6 2005
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    canada montreal
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    Re: 03 CTS success story with Trans Fluid & Filter replacement

    flushed my trans oil with super mobil dex VI . my car has 110k miles and no problem. My only probleme with this car is the shit trans cooler line alway pop out in the highway!

  4. #3
    briggy's Avatar
    briggy is offline Cadillac Owners Enthusiast
    Automobile(s): CTS 3.6L 2005 80,000 Miles
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    Re: 03 CTS success story with Trans Fluid & Filter replacement

    Quote Originally Posted by NoLeafClover View Post
    Hello everyone,

    I am new here & just wanted to share my experience with my '03 CTS I purchased about 1 month ago..................

    The 5L40E Trans calls for Dex 3 but GM changed to Dex 6 for later models.
    I DID NOT FLUSH the system, but only added what came out. I used Dex 6 full synthetic and added a small bottle of Trans fluid conditioner.
    I used about 5 quarts in total.


    I'm not too keen on mixing the fluid, the transmission holds 9 quarts total. This means you have 4 quarts of dirty non synthetic, and 5 quarts of Dex 6. I know everyone says it's ok to mix but many hose manufacturers actually specify non synthetic products. My point is is it really ok to specify sythetic in a transmission not used to it. I may be splitting hairs, but I do suggest doing a full drain and refill if switching to synthetic, you figure it costs a little more and is probably worth it.

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