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If you do double drain like what I did you'll get more than 90% of the old fluid out. Tranny flush is a tedious and expensive maintenance if you don't have the special tool.
If you look at this way… You stated you used a total of 8 quarts of new fluid. If you start with a full system at 9.5 quarts of old fluid, you then drain and replace 4 quarts with new fluid. You’ll have 5.5 quarts of old left over in the system. Once you run the trans, 4 qts of new and 5.5 qts of old gets thoroughly mixed together.

Then, you drain another 4 quarts of fluid. But this time it is mixed old and new. Assuming it gets evenly distributed and mixed. You’ll approximately remove 2 quarts each of old and new fluid. So you end up with 3.5 quarts of old fluid left over and 6 quarts of new fluid in the system. That is 37% old and 63% new with only using 8 quarts of new fluid.

Continuing with the drain and refill with only 4 qts replaced. You be at 1.5 qts old and 8 qts new. At 12 quarts of new fluid cycled. Your looking at 16% old and 84% new.

To finish up, you again drain and replace another 4 qts. That would be a total of 16 quarts of new fluid used.

If you don't believe me, You can test this with 9.5 quarts of water with food coloring and see how long it takes to totally remove the food coloring.


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Four ATF changes sounds like too much work for me, unless spread out over many years.

The photos show my setup, which I keep in a zip-lock bag (except for the ladder and funnel). Additional items are an oil-change pan, a clamp tool (needle nose vice-grip), jack stands, and computer to monitor the temperature. Most of the ATF flows into the transmission by gravity, but for topping-up, I drilled a hole in an ATF container's cap and inserted a length of Silicone hose. This allows for squeezing the ATF plastic container to add minor amounts of fluid. It's been mentioned many times, but leveling the car is fairly important, because of the fill port being at the extreme end of the transmission pan. Any tilt will tend to make the fill-level incorrect. Note that the filter is nearly centered in the pan, so it is kept submersed whether the car is going uphill or down.
 

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in dire need of an organic chemistry lesson
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You stated you used a total of 8 quarts of new fluid.
If I said that I misstated. I used 3 gallons of Valvoline MaxLife for double drain/refill and used almost all of the fluid.
Going back to the percentage exercise, I don't see how after two drainage there would still be more than 10% of the old fluid. All of these aside, a double drain is more than enough to have a clean fluid in the system. The rest is history.
 

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If I said that I misstated. I used 3 gallons of Valvoline MaxLife for double drain/refill and used almost all of the fluid.
Going back to the percentage exercise, I don't see how after two drainage there would still be more than 10% of the old fluid. All of these aside, a double drain is more than enough to have a clean fluid in the system. The rest is history.
Yes more fluid removed at a time the better. I calculate at 2 drainages at 5.75 quarts at a time would be right about 10% old fluid left over.
 

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The story about changing old ATF causing problems, is an urban myth.
Probably started because someone changed their ATF when the transmission was on its last legs.
And as should be expected, it failed soon after the change, but not related to the change of ATF.
 

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The myth started in late summer, 1958, when Ol’ John brought his sputtering Ford into Jim Bob’s Shadetree Service on a dusty corner of Tobacco Road in Eastern North Carolina. Jim Bob was no slouch, though, he had pored through every inch of


and had the soggy cigar butts to prove it. There wasn’t a damn spring, band or proportioning valve he didn’t know by memory. But Ol’ John was struggling, and wasn’t ready to pay the $6.95 inspection fee that Jim Bob’s expertise commanded. Instead, he opted for a simple fluid change for $3.25. He even helped pierce the ATF cans with the pointed opener to keep the costs down.

Needless to say, Ol’ John’s coughing heap broke down near Billy’s 5 & 10 4 days later, rolling to a stop while he revved the engine to no avail. Furious, he hitched a ride in Johnny’s immaculate 1937 Chevy GF pickup down to Jim Bob’s and demanded a full refund of his $3.25. Jim Bob wiped some greasy sweat from his brow, bent his head and looked authoritatively at Ol’ John over his reading glasses. He paused a moment, spat a sizable clump of cigar tobacco at the dirt floor and said, “I was afraid of that, John. When those old Fords don’t get they’s Fluid changed regular-like, the secondary pinion rides up on the primary pinion and a chunk of the forward band breaks off and clogs the passages of the rear servo accumulator piston. It was a gamble and we lost. Now I’ll split it with you and refund $1.75, but this is a business John.”

Of course Ol’ John’s memory wasn’t the greatest even when he was just John, but he got the gist of Jim Bob’s tale, and he would tell it to everyone who would listen. This included many a New Yorker heading south and stopping for lunch at Maybell’s Diner. And so, the myth was born.
 

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Personal experience:

Passat -- had the full pan drop and filter + fluid change at the 10-year mark, planning to do again later this year, at the 20-year mark. Very low mileage accumulation, but mostly urban "severe service." Exhibits very subtle second gear stumble when cold.

1996 A4 (R.I.P.) -- had full pan drop and filter + fluid change at 9-year, 66K mile mark, a month after I bought it

Four years ago, at about 100K miles, I noticed some slippage / engine rev. on the 4-to-5 shift, and a drain-and-fill, no pan drop. This resulted in severe slippage. My local shop did a full pan drop and filter change and followed the fill instructions by the book, and it worked superbly thereafter, until my power window short wiped out most of the wiring harness from the breaker/relay panel to the driver's door switches. When I donated the car to my local PBS station, the power train was still in superb condition, but the seats were shot, and I had given up on the electrical do-over, because routing and replacing wires got to be such a pain. (I am an electronics engineer and I worked my way through my undergrad years at UCLA as an electronics tech., so it pained my ego to admit that I just did not have the time, patience, and willingness to contort myself for extended periods to complete the repair on a 24-year-old car that had started to become untrustworthy. My wife really did not appreciate being stranded with acrid smoke pouring out of the driver's side vents.)
 
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