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8 speed fluid and filter change

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31 minutes ago, 2018GMC said:

My 2018 (Built 12/2017) just turned 21k miles and I haven't had any shuddering issues but decided to attempt at home driveway flush. 

I used the HF transfer pump Multi-Use Transfer Pump (harborfreight.com) to remove most of the fluid from the pan then dropped the pan. I was able to remove the pan without messing with exhaust or removing filter. The original fluid was dark in the pan but hardly any material on the magnets. Shined a light in the filter and it looked like new. I reinstalled the filter (Will prob change when I hit 50kmiles.)

I cleaned pan and I pumped in 7qts of new Mobil 1 Blue label fluid. https://www.amazon.com/Mobil-124715-Synthetic-Quart-Bottles/dp/B07Q3H159N I removed the trans cooler "pill" and inserted the hoses from the HF transfer pump (just the hoses disconnected from pump. But both into jugs and was suprised to see the hose connected to the rear port on trans was pumping clean fluid. I blocked that hose and ran the truck until the front port hose fluid started to look clean. 
Reinstalled pill and trans cooler metal lines.

Checking the level was a pain. Truck needs to be level. I used the offroad 4wd screen on the DIC so it read 0 for truck angle. I let the truck idle until it hit 180 degrees (this is the temp when the pill opens for my max tow optioned truck) I had to add another 1.2 quarts so there was a small stream running out of the 10mm check plug bolt. Reinstalled plug

I have not disconnected battery to perform trans shift relearn since I haven't had any issues. I believe my trans was low from the factory since I ended up using all 12quarts of new fluid and 1.2 quart of the fluid from the flush (it looked clean but would have preferred to use another sealed quart.)

Definitely miss the days of having a dip stick and not sure why both trans cooling line ports on 8L90 transmission were pumping fluid. I've at home flushed many older transmissions like TH350, TH400, 4L60, 4L65 and they all have a push fluid and intake fluid trans cooler line. 




I would consider double checking the level.  8L90 TBV is full open at 194F (max tow or not) and GM states the transmission must hit that before checking the fluid level.  


  • Quote

    If vehicle is equipped with a thermal bypass valve, the transmission fluid level should be checked only after the TFT has reached or exceeded an operating temperature of 90°C (194°F). Once the TFT has reached or exceeded 90°C (194°F), then turn OFF the vehicle and allow the TFT to cool back down to 35–45°C (95–113°F) before checking the fluid level as required.  Reaching or exceeding an operating temperature of 90°C (194°F) opens the bypass valve and allows the cooler to fill up with fluid, which will result in a more accurate fluid level check.


So 194F, cool it back to 95-113F and THEN check the level.  

Edited by newdude
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Will do. Forgot to mention I did blow air through the trans cooler lines when they were unhooked from the thermal bypass valve (pill) so when the TBV opens fully it should drop my fluid level a bit. When I checked it yesterday It was at 182 degrees, idling in park and the fluid was a light stream coming out of the check plug hole. I'll drive the truck until trans gets to 200F and then let it cool down to 95-113F then restart truck, and check idling on level surface in park.

Lots of info in this video


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Took all day to cool down to 110 but on a level surface, idileing in park, i drained half a quart out of the 10mm check plug. I put the plug back in when the stream slowed to a thin stream / spotty trickle. 

Should I leave battery disconnected overnight or a few hours since I put the new fluid in? Does this reset the transmission shift learning? I don't have shudder issues but I do have the occasional lurch in park when cold and sometimes harsh engagement when shifting into drive or reverse. Very cold winter temps there is the rubber band feeling when accelerating from a stop but once engine and trans warm up no issue. Summer driving and towing no issues.

Edited by 2018GMC
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  • 8 months later...

FYI - Best price for Mobil 1 Blue label Synthetic LV HP ATP 124715 $46.49

Make sure it is sold & shipped by amazon. I have ordered twice an both times received the blue label even though the product page pictures the old black label.

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