TH350 flush lessons learned

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74flatbed

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Hello All,
I just thought I would share my pain so others could learn faster:

Initial plan: Flush transmission on my truck using a pulled line from the aux oil cooler. I figured I'd flush the cooler and clean out the pan, two birds kind of thing.

Actual:
Dropped pan and noticed 1/2" of sludge and a magnet that was so covered you could see the magnetic fields.
-I decided to replace the old dented pan with new Derale pan that would add two quarts and hopefully cool the fluid a bit. Ordered from Summit and waited a week.
-Installed new pan, refilled fluid, and pulled cooler line. When I started up the truck to start the flush, the oil came out of the wrong tube. After a lot of head scratching and cussing, I followed the oil lines back to the TH350 to discover they had been installed backwards. I tried to redneck the old lines to the right holes, but to no avail. Ordered new lines from Summit, along with inline transmission filter, and am waiting another week.

-Long story short: Suggest you don't assume anything about the PO and confirm everything before you loosen that first bolt or screw.

Next: Install new hard lines, install inline filter, install new aux oil cooler, complete flush, and pray my TH350 isn't going to die from the high temps it saw in a previous life (aka oil was routed through aux cooler to radiator then back to trans).

-Paul
 

74 Shortbed

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How was the fluid, nice and red or brown???, all you did was clean the pan and cooler, the torque converter holds as much or more than the pan, so if the fluid was bad when you put new fluid in it will be contaminated pretty much instantly when you fire it up, the converter needs to come out and flushed, another way is to run it with one cooler line in a bucket and run it and pour fluid in until it flows nice and red, but that expensive as hell, lol..
 

74flatbed

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Sorry for delay Art, forgot to follow my post. I actually did run the cooler line into the bucket and kept adding until it came out red. You are 100% correct, I did buy a ton of fluid and am not sure if I made the right choice economically. The original fluid was red, but very watery. I also had the great experience of touching one of the hard lines to the battery while I was underneath and watch sparks go everywhere. It arched a hole in the line so I had to wait another week for a new set to show up.
Now I have new hard lines from transmission, new filter in pan, new pan, new soft lines, and new inline filter from WEX. I'm hoping this is enough band aides to make the Th350 last while I save up money for a 4x4 700r4 conversion. Researching drive shafts, floor shifters, and tv cables. Know any good threads here (I know I'm not original). Thanks for the reply!
 

74flatbed

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I also missed tightening a couple of pan bolts and the guys at Valvoline were nice enough to catch that when they were flushing the radiator for me. Stopped a lot of leaks haha.
 

Iowan

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:driver: Aren't Sqaures such a fun thing to work on:disgust:. Just got my trans seals replaced and then find out the rear main is going out.
 

74flatbed

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Ha, indeed. They are fun to work on, but wow do they have a lot of surprises to give. What do you guys think about towing? I have to pull a '69 Biscayne 600 miles pretty soon. Do you think the 74 GMC can handle it? Just a standard 350/TH350 across pretty straight terrain (small hills along the way). I think the total weight will be 5700# when you add in the trailer.
 

Iowan

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Ha, indeed. They are fun to work on, but wow do they have a lot of surprises to give. What do you guys think about towing? I have to pull a '69 Biscayne 600 miles pretty soon. Do you think the 74 GMC can handle it? Just a standard 350/TH350 across pretty straight terrain (small hills along the way). I think the total weight will be 5700# when you add in the trailer.
Though a Biscayne is heavier, the Roadkill guys took a 73 Gmc macho grande with a 55 and hemi in the back.
 

Honky Kong jr

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Though a Biscayne is heavier, the Roadkill guys took a 73 Gmc macho grande with a 55 and hemi in the back.
Yup and it overheated multiple times lol.
 

jake wells

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it would be nice if theses trucks had a drain plug on the bottom of the pan.
 

Honky Kong jr

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Ain't nothin to it but to do it. If you pull the pan there is no sense in not installing one.
 

74 Shortbed

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I drill a 1/4" hole in the pan to drain it, then after I take the pan off I put a plug in it..
 

74flatbed

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https://www.summitracing.com/parts/der-14200

The transmission fluid shower I enjoyed motivated me to go with the Derale with the plug, then pinched the gasket by over-torquing the bolts ahaha (left a couple loose to balance it out). The whole "inch pounds" thing escaped me. I'm hoping the cooler veins are more then a gimmick. Do you have a favorite gasket type to use after drilling the hole Art?
 

jake wells

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unhooking one of the cooler lines and feeding it into a bucket works good also.
 

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