notes about DIY trans fluid change/flush on 8sp

[hypntyz]

I have an 18 sierra denali with the 6.2 and 8sp trans, 18k miles, one of the very late build 18's (I had hoped that for this reason mine wouldn't be afflicted with any trans issues). After a recent long trip to go pick up and tow my c8 back from out of state, I noticed some vibration in the truck. It was light but apparent, it seemed like I was going across rough patches or rumble strips in the road every 2-3 seconds rhythmically, and I felt it in the steering wheel more than anything. Research told me that this was likely to be the torque converter shudder that is somewhat common on the 8sps. The "fix" for this is detailed in a GM document that I found elsewhere on this forum, which states that 20qt of blue label Mobil 1 LV synthetic HP ATF should be flushed through to remove all the factory fluid. The GM procedure uses a flush machine that connects directly to the transmission after disconnecting the stock trans fluid cooler piping.

 

Others here discuss dropping the transmission pan which will get about half the fluid out, but you still have half the fluid held in the torque converter, valve body, and trans cooler/lines. Plus you have to reseal the trans pan and hope it doesnt leak. I didnt like that solution so I wanted to do it close to the GM method of flushing, but without an actual flush machine.

 

I wanted to do this procedure myself at home and I was able to get the fluid from rock auto for about $185 shipped. I learned a couple things during the process that I thought I'd post here for others to consider.

 

I disconnected the transmission fluid pipe up front beside the radiator, I used the one on the drivers side, behind the headlight area, under the PCM/computer. There's a plastic retainer that you slide out of the way, exposing the quick disconnect C clip, once that's removed, you can separate the piping there. Use a couple towels to catch run-out. The pipe closer to the front of the truck will have fluid being pumped out of the trans and fluid cooler, and the pipe you pull back toward the engine would be the fluid return to the trans. My intention was to pump fluid out of the trans by idling the engine, catching and measuring that fluid in an old 5qt oil jug, then use the other pipe to pump an equal amount of fluid back into the trans. I scoured the shop and found an old small pipe fitting that I modified and it slid right into the open fitting beside the radiator...a piece of 1/4 or 5/16 tubing would probably have fit also.  I then clamped on an extension hose to my little adapter pipe...just a random piece of rubber hose that ran down into the oil collection jug. For the fresh fluid return pipe, another hose was going to my fresh fluid pump.

 

The GM flush procedure states to flush out 3-4 quarts of old fluid then shut the engine off and pump in an equal amount of fresh fluid, then repeat basically until you've done all 20qt. I was able to pump the first 3qt of old fluid out as planned by starting the engine and watching the amount measured in my collection jug, then shut it off. But then I found I was unable to get any fluid to go back into the return pipe. I tried a small electric fluid transfer pump, and a homemade pneumatic pressure vessel to push fluid, but the return pipe wouldn't accept any. Further research revealed that there may be a thermostat in the trans that prevents fluid from flowing (apparently via the return pipe side, which seems an odd place to put it instead of just stopping the trans from pumping out fluid to begin with) and thats why I couldnt get any to go back in.

The GM procedure has you remove a bolt from the front of the oil pan. This bolt leads to a pipe that is the fluid level check location...you're supposed to use an adapter fitting to fill fluid into this location,. The bolt/hole is tiny so a regular plumbing fitting won't fit there. The adapter that threads into that hole and has a barb for a fluid hose on the other end, even on the aftermarket, is $30, and you have to wait for it to be shipped in. I searched my garage and found a short length of copper tubing, about 5mm OD with a 2mm ID hole, and it fit pretty well in the hole in the trans pan so I put it up in there a couple inches. I then used some silicone vacuum hose and other tubing to connect that pipe to my electric fluid pump. I was able to, slowly, pump fresh fluid back into the trans that way. The pump pushed about 1 qt every 5 minutes but it did work. so I just put the pickup tube for my pump into one quart and then used other quarts to refill that one as needed. Every 3-4qt I pumped in, I would start the engine and run it a couple minutes to pump out 3-4qt of old fluid into my oil jug, using the marks and clear strip on the side to measure what I drew out. The first 8-12qt was really brown and dirty looking, even though the truck is rarely driven hard and has only 18k miles. The next few qt was cleaner, probably a partial mix of old and new, and the last 4qt I drew out was clean and bright, so that was probably mostly new. The idea of the process is that you have to waste some fresh fluid to ensure as much of the old stuff is out as possible.

 

I had 20qt of fresh fluid but I figure I lost 1/2qt in the whole process of trying to pump fluid in, failing, changing containers and tubing, etc. so I drew out 19.5qt total and put all the fresh fluid (about 19.5) I had left in. Then I reconnected the fluid pipes up front and reinstalled the c-clip and retainer, removed my pipe from the trans pan and replaced the bolt, sprayed the areas down with brake cleaner to remove spillage, and went for a drive. I shifted through the gears manually several times and the low gears were kind of jerky the first couple of times but after that everything seemed fine. So far, I have only driven a bit but the shudder seems gone. Of course, I will need to drive it more and get the trans heated up a lot longer to be sure it doesn't come back.

[pokismoki]

if you ever get a chance to pull the torque converter out, I always drill and tap two pipe thread holes in the converter for drainage, its best you take to a trans shop if you dont have confidance in doing this trick.

[hypntyz]

59 minutes ago, pokismoki said:

if you ever get a chance to pull the torque converter out, I always drill and tap two pipe thread holes in the converter for drainage, its best you take to a trans shop if you dont have confidance in doing this trick.

If I ever have to pull the converter for any reason, it's very likely that same one won't be going back in anyway. But, that's the first I have ever heard of that "trick". I'll try to keep it in mind for the future.

[pokismoki]

its a common mod on FOrds, factory does it on the Police cars

[Black02Silverado]

3 hours ago, pokismoki said:

if you ever get a chance to pull the torque converter out, I always drill and tap two pipe thread holes in the converter for drainage, its best you take to a trans shop if you dont have confidance in doing this trick.

I take it that the converters are not balanced?  I would think they are before any fluid is added.  Drilling and adding a drain plug would put it out of balance.  To me anything that rotates like that needs to be balanced, just like our tires and driveshaft.

[pokismoki]

12 hours ago, Black02Silverado said:

I take it that the converters are not balanced?  I would think they are before any fluid is added.  Drilling and adding a drain plug would put it out of balance.  To me anything that rotates like that needs to be balanced, just like our tires and driveshaft.

common sense would have you install two drain holes 180 degrees apart.  1/8"-1/4" brass pipe plug is not much to throw it out of whack. i bet you wouldnt even notice if you only installed one drain

[hypntyz]

Update, been a few months and no shuddering has returned after the fluid flush I did.

[YYC-SIERRA]

I installed a remote spin-on transmission filter a couple years ago. Whenever I want to change my trans fluid, I just remove the spin-on filter, start the truck, and drain a gallon of Dex 6 from the filter mount. Works great, super easy.

 

However, I was toying with the idea (had I not installed the remote filter setup) I would splice into the same location of the rubber transmission hose, below the harmonic balancer, and install a 3 way valve. Would make draining the fluid super quick and easy. Thoughts?

 

 

 

 

[YYC-SIERRA]

 

This is the remote filter setup where I drain my fluid, usually once a year when I change the filter. 

 

Coming up on almost 2 years and 22,000 miles since installation. On my third Wix filter. Everything has been flawless so far. 

 

 

 

 

 

[Z45]

On 11/15/2021 at 10:09 PM, Black02Silverado said:

I take it that the converters are not balanced?  I would think they are before any fluid is added.  Drilling and adding a drain plug would put it out of balance.  To me anything that rotates like that needs to be balanced, just like our tires and driveshaft.

I had this done years ago (circa 1980) on a 1979 Chevy van so its an old trans shop trick.  The mechanic who did it had done it many times before but mine was the first time in a vehicle.    The shop had a parts room full of cut open tc's he showed me.  You only need to drill one hole and you better understand how TC's are constructed so you drill between the interior fins.  Tapping and installing a plug will not cause imbalance as the plug compensates for the drilled out metal so it evens out. If you are ready to tap while the fluid is draining, no remnants of the drilling get inside the TC, they get flushed out because the fluid is under pressure.   I had this done at a Trans shop as I wanted to install this new 100% Synthetic ATF (AMSOIL) because the trans was very slow to engage in the cold upper Michigan winters.  We had had an extremely cold winter that year with about a month of temp's below zero and the van would not move until is was warmed up thoroughly. Once I installed the synthetic ATF and rear gear lube I never had a cold weather problem again for 6 yrs of ownership of that 1979 G20 Custom Conversion Van.

Edited January 30, 2022 by elcamino
typo