why should you not change your transmission fluid if you’ve never changed it last 10,000 miles.

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For context: I have an older Toyota 05. 160k miles. Transmission fluid looks kinda brown and mechanic said I should do a flush and quoted me 300$. I’ve also heard that at some point you shouldn’t change the transmission fluid if it has t been changed in awhile. Why is this?

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16 Answers

Anonymous 0 Comments

Drain, filter change, and refill is fine and what is usually recommended in the manual. The “flush” a lot of shops will push is not necessary.

Anonymous 0 Comments

Your transmission can become damaged over time, and your transmission fluid can contain contaminating particles that help with grip, relatedly–changing the transmission fluid will get rid of these particles, and you can start experiencing slippage where previously this wasn’t present with the old fluid.

It is however, a case where the problem was already there in either case. Not changing your transmission fluid can lower the life expectancy of your transmission, but this old fluid will help “solve” the problem somewhat in the meantime, and the new fluid will make the damage more apparent.

Anonymous 0 Comments

From what I’ve heard, if you haven’t been doing regular “flushes”, then don’t start. It may dislodge some of the dirt or crud and it might find its way into the small nooks and crannies that you don’t want it in. It would be better to do a “drain and fill” instead of a flush. Drain what liquid will come out from the drain hole and then put new transmission fluid in to the proper level. That way you’re not dislodging any of the crud/dirt that has built up over time.

Anonymous 0 Comments

I’ve never heard any convincing evidence that this is the case at all. The common explanation of “oh the old metal particles in it are holding it together” or whatever is just totally made up and doesn’t make any sense at all.

It’s the *flush* that causes the problem. The flush can force bits of debris into places they normally don’t go. *That’s* what ultimately causes the problems you see and where the advice to not change really old fluid comes from.

The proper thing to do is not to flush it in the first place. I don’t think most manufacturers recommend flushes at all anyway. Follow the actual recommended service of draining and then filling the fluid along with a filter change.

Anonymous 0 Comments

While changing the fluid after it’s damaged can make it seem more damaged, maintenance too late is always better than maintenance not-at-all. A lot of professional and shadetree mechanics have seen transmissions fail shortly after a fluid flush, but that’s confirmation bias. Nobody thinks about the times they got another 100k out of the car. They think about a person who brought the truck in because the transmission was weird, found 100,000-overdue black transmission juice, recommended a swap, and the truck died a month or two later. The poorly maintained truck died. Did it die from the abuse? No! It died because I tried to DO something.

There are a few decades-old diesels that are held together and sealed by their ancient grime and adding fuel or lubricant with detergents would cause leaks and eventual failure, but the myth of the sealed transmission that will die if you swap out the sauce is just a myth, caused by confirmation bias. 

You’re not better off leaving the old stuff in unless you expect the car to die in a couple months and $100 worth of lubricants, filters and gaskets is just throwing good money after bad.

Anonymous 0 Comments

That’s what we call a hoax. The reason this hoax keeps getting perpetrated is that someone will notice their transmission having issues so they’ll bring it in for a flush. Not long after, the transmission will fail and the flush gets blamed even though the reason for the flush was because the transmission was having problems. Sometimes forcefully flushing can move clutch material that has built up in passages to locations causing binding in valves and pressure manifold switches, etc. If you just do a drain and refill, that should not occur.

I’ve been rebuilding transmissions for 30 years. There is nothing in old transmission fluid that helps a high mileage transmission to keep operating normally. Transmission have filters that remove particles from being able to flow throughout the internals so there are no particles to help with grip. If anything, old fluid breaks down and loses its ability to assist with grip.

Transmissions are not as complex as people think. High mileage transmissions typically will have worn down clutch plates which will cause slipping. Seals and o-rings stop sealing and can crack causing hydraulic fluid loss which causes things to not engage properly. It’s rare that were see catastrophic failure where things actually break inside. Also, clutch material can build up in passages before they get to the pan to be filtered which can cause issues with activating pressure switch manifolds and can cause issues with movement in the valve body.

Transmission rebuilds are expensive because of the labor. I can spend 14 hours removing a transmission from a vehicle, disassembling said transmission to get to a $15 part deep inside to replace it, reassembling and reinstalling said transmission. That $15 part may cost $2500 in labor. However, a refurb transmission may cost $2500 installed so there’s no cost benefit to the vehicle owner either way.

Anonymous 0 Comments

Yeah some point as in after 160,000 mi of never being changed, not some point as in 10,000mi down the road. I think routine maintenance is changing it every 30 or 60000 mi, vehicle depending. 

Something about how the transmission wears, then mixes with the fluid, and how if you replaced the fluid you’d lose whatever minor miracle was holding your transmission together

Anonymous 0 Comments

There’s a transmission flush and a transmission fluid change. The latter happens with a filter replacement as well just like an oil change. I’m going to compare this to prepping your body for a colonoscopy.

Think of a transmission flush like taking a bunch of laxatives and getting all the poop out of your intestines. It bad because it’s forceful and it will remove bacteria that is good and helps your digestive system.

A transmission fluid change is like going on a fast. You let all the poop come out naturally and then eventually you refill it with more food/transmission fluid when you’re done. The bacteria will still be there but you got the poop out so you could do your service.

Anonymous 0 Comments

Suggestion I’ve heard from my tranny guy is to do a pan drain and refill every 30-40k or less and do the filters every 100k if it has one.

They also recommended that if you have a 150k+ vehicle that’s never had a tranny service/unknown history to replace it in 1/4 or 1/3 of the pan amount over several thousand miles.

Anonymous 0 Comments

You must be missing a 0 because you never have to change your transmission fluid after only 10,000 miles.