To flush....... or not to flush! THAT is the question!

Arth · September 4, 2017

158k on my 2006 Avalanche.

Dropped the pan, changed the filter, and filled it back up with new fluid. Guess we will see what happens.

Did this on a 2003 Tahoe with 170k and didn't have any issues.

Just my experience.

Supreme Pizza · September 4, 2017

I wonder of there is any real benefit. Like.... It either does nothing, or it causes problems type of thing. If I had to guess, you guys wont have any problems. From my understanding the problems develop over the next year after service. Maybe a coincidence.

Colossus · September 5, 2017

its an old wives tale that properly done transmission flushes (using the tranny pump itself) will cause a high millage transmission to fail. Generally speaking, if problems do occur after a properly completed flush, (keyword/phrase there, "properly completed") the transmission was already on it's way out. Now, what I've heard and come to believe from experts in the transmission repair/service industry is that for transmissions where there is no service history, some may prefer to drop the pan first to inspect for debris in the pan as well as filter and/or screen. (some have a filter, some have a screen) If very little or no debris, most feel ok then flushing the system to get the rest of the old crap out of there and freshening things up. If there is significant crap in there, then fill it back up with new stuff minus the flush and see how far it gets you. All transmissions will bennefit to some extend with fresh proper fluid. I hear good things about longevity of the 2007+ 6-speeds and I will assume 8 speeds as well, but for the old school 4-speeds that are notorious for going out between 150,000-250,000 miles.. well.. keep your fingers crossed. Transmission services is the second most important service on the vehicle behind good oil changes. Good luck.

Supreme Pizza · September 8, 2017

Supreme Pizza · September 8, 2017

My fluid looks like the center one. Still cant decide if I should change the fluid. I agree that it might be too late, and it may just cause problems. I agree that the fluid change might not cause any new problems and if the trans fails, that it was going to fail anyway. Both possibilities are theoretically possible and reasonable. There is debate on the subject, because its a grey area. Or brown area in this situation...

What oh what shall I do.....

What would you do?

(the picture above is from Google)

Colossus · September 8, 2017

change it.

waumo1 · September 8, 2017

Over the year I've come to the conclusion that a lot of "my transmission failed after a fluid/filter change" were really "I was noticing some transmission issues, changed the fluid/filter to fix it and then it failed".

KMGZ400 · September 8, 2017

I don't know what the real answer to this question, but I have had 4 vehicles with 4L60s. 2 of them I never changed the fluid and they worked fine until I sold the trucks at 160k plus miles. the other one I "flushed" the trans at 110k miles and it completely grenades at 115k. again not sure what the answer is, but the vehicles was babied its whole like.

MaverickZ71 · September 8, 2017

About 12 years ago, I managed a fleet of transit vehicles. Fleet contained GM/Ford/Dodge vehicles--everything from front-wheel-drive midsize sedans and minivans to Ford F-550s. A bunch of dealers then bought fancy flush machines. At their insistance, we quit doing transmission/transaxle pan drops/filter changes/new fluid like the manuals called for, and did their "much better" faster/cheaper flushes. Had more trans failures than usual after that. Then we went back to doing it the old-fashioned way (had to argue with some dealers) and had better luck and actually lower long-term expenses.

mookdoc6 · September 8, 2017

A passive flush is completely harmless and is the best way to freshen fluid in a trans. Forced flushes, such as the ones done in car shops are the ones that cause all the problems. Passive flushes are done using the 2 cooling lines and the trans pump. New fluid will not mess with the valve body.

Correct! That is the only way to fly.......it's like saying you spun the bearing because of an oil change? Nope, wrong answer!

Supreme Pizza · September 9, 2017

For what reason do the "forced flushes" cause problems? Specifically, what is the cause of the failure? Forcing debris into the wrong area? Damaging seals? A chemical reaction of some sort?

Nanotech Environmental · September 9, 2017

For what reason do the "forced flushes" cause problems? Specifically, what is the cause of the failure? Forcing debris into the wrong area? Damaging seals? A chemical reaction of some sort?

A forced flush uses an external pumping apparatus to change the fluid. It can force fluid to go thru the tranny in a way it wouldn't normally go. The thought is that this action can stir up shit, which causes problems. That is one of the theories anyways.

A passive flush uses the tranny's own pump by starting up the vehicle. The fluid gets pulled and pushed thru the entire transmission in exactly the same way it would during normal service, at normal service pressures. It's also a lot cheaper to do (if done by the owner at home etc.).

Short of avoiding severe overloading and outright abuse, fluid health is the most important key to automatic transmission life.

Supreme Pizza · September 10, 2017

The thought is that this action can stir up shit, which causes problems.

I wish there was a study done where 100 cars were ran until trans failure with no fluid changes, and and another 100 cars got fluid changes every 125k miles. I wonder which batch would last longer on average. You might be right about the flushes. I'm just not sure if the new fluid's fresh detergent will loosen up crud over time and cause problems. If I do anything at all, I would remove the drain plug and refill. Drive, repeat, repeat.

Nanotech Environmental · September 10, 2017

I wish there was a study done where 100 cars were ran until trans failure with no fluid changes, and and another 100 cars got fluid changes every 125k miles. I wonder which batch would last longer on average. You might be right about the flushes. I'm just not sure if the new fluid's fresh detergent will loosen up crud over time and cause problems. If I do anything at all, I would remove the drain plug and refill. Drive, repeat, repeat.

It appears to me that you're worrying about the wrong things and not worrying about the things that are important. Doing a passive flush with fresh fluid won't cause any issues that weren't already going to happen anyways.

Good fluid is critical to proper operation of the clutches, bands, valves, servos etc. Worn out, dirty fluid causes issues like excessive slippage & heat buildup, as well as impaired operation of the other critical parts of the transmission. Those are the things that cause auto trannys to fail. (other than outright overloading & abuse)

swathdiver · September 11, 2017

General Motors in the service manual uses the Transflow J 45096 machine to back flush and exchange the fluid for at least the 6L80/90 transmissions. That's what was done to mine recently. It was contrary to my owner's manual but the service manual has it done this way and the GMT900s design makes this procedure more practical.

In the 1990s we started using a similar machine called the T-Tech. We exchanged the fluid on thousands of cars of all kinds and only one, an old RV, failed to make it on the road, the transmission was toast but the owner wanted us to try anyway despite our protests. Sure was easier than getting fluid all over myself and the driveway when doing it the old way. However, the Fleet Manager makes a compelling case with real world data against doing such.