4runner Transmission Fluid: Flush Or Drain And Fill?




4th gen 4runner gear lever

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Making sure your 4runner’s fluids are changed at regular intervals is an important part of it living up to its reputation of “lasting forever”. The transmission fluid is no exception – but there is a common debate among owners regarding whether you should have the fluid flushed or simply drain and fill it.

A drain and fill is a much safer way to change the transmission fluid on your 4runner. Flushing the fluid requires pressurizing the system, which can cause leaks in a high mileage transmission. It can also push sediment through the trans into places it shouldn’t be, causing performance issues.

Advantages of transmission flushing vs drain and fill

Unfortunately, this decision isn’t quite so simple. You see, there are pros and cons to both flushing the fluid and draining and filling it. Let’s dig deeper so you can choose which option really works the best for you.

Benefits of flushing your transmission

I know the transmission flush has gotten a bad rap over the years and it’s for good reason – these high-pressure machines have had many transmission failures traced back to them. “It worked fine before I had the fluid flushed” is a very common statement, unfortunately.

But there is a good side to flushing your trans. The truth is, it’s the most effective method of replacing the old, dirty fluid with new stuff. The pressure makes sure that all of the old fluid is pushed out of the transmission.

Draining and filling is much more gentle on the transmission. But that gentle procedure has a downside – it’s not as effective at pushing all of the old fluid out. As a matter of fact, some mechanics believe that you only remove 80% of the old fluid at best, while most others say that the number is much closer to 25%-30%.

That means you’ll be mixing a bit of new oil with a bunch of existing old oil. You’ll have to do the drain and fill procedure multiple times in order to get close to fully replacing the fluid with new stuff. Compare it to an engine oil change – you wouldn’t mix 2 quarts of new oil back in with 6 quarts of old oil, would you?

4th gen 4runner gear lever
Benefits of draining and filling your transmission

Draining and filling the fluid in your 4runner is much safer because it’s gentle on the transmission. The lack of high pressure eliminates the risk of any seals being damaged and sediment being sent into solenoids or other areas that can create clogs.

It might not be as effective and you might have to do it a few times to accomplish close to a full fluid change, but there’s no risk of harm to your transmission. For most 4runner owners, the safer way is the best way.

Tip: If you’re taking your 4runner to a shop to have the fluid changed, make sure you’re on the same page in terms of terminology. I’ve noticed when talking to local mechanics that they refer to it as a “trans flush” but they actually mean a drain and fill or a gentle exchange. It’s similar to someone calling a tissue a “Kleenex”. It’s just what they’re used to calling it.

If you’re against having a full flush done on your vehicle, make sure they understand what you’re asking for!

So what do you do if a drain and fill is too gentle and a flush is too aggressive? Well, you find something in the middle, of course. That is what some 4runner owners refer to as a “Fluid Exchange”.

Benefits of a fluid exchange

This is really a middle ground between the two options. It doesn’t add any extra pressure to the system like a traditional flush. But it does use the pressure that would normally be running through the transmission already.

This is done by unplugging the transmission cooler outlet tube and using the 4runner’s engine to pump the ATF fluid out. Once 2 quarts have been pumped out, you refill the trans with 2 quarts of fresh oil. This process is continued until the fluid coming out has more of a red color, indicating that’s it for the dirty stuff.

Here’s an awesome write-up on how to do this on the T4R.org forums:

4th gen 4runner gear lever
When should you service your 4runner transmission?

How often you replace the transmission fluid in your 4runner will vary based on how it’s used. A good rough estimate is every 100,000 miles or so. If you do a lot of towing or live in an especially dusty climate, you’ll want to change it more often.

Some people believe that the fluid should never be changed because it’s considered “lifetime fluid” in sealed transmissions. The issue here is that Toyota 4runners are known for lasting a very, very long time.

Would you want to keep fluid that has been used for 400,000 miles? It’s bound to get dirty and lose some of its lubrication properties. Clean oil is never a negative thing whether it’s in your engine, transmission, transfer case, differentials, etc. The cleaner, the better.

What fluid to run in a Toyota 4runner transmission

I’m a believer that when it comes to important things like an automatic transmission, it’s best to stick with Toyota fluid. With that said, there are many 4runner owners that swear by their favorite ATF fluid from companies like Mobil1, Redline, Amsoil, etc. The choice is yours.

Regarding the Toyota stuff, the recommended fluid is different based on whether you have a sealed transmission or one with a dipstick. The dipstick itself will usually tell you what fluid to run (more than likely “T-IV“). Sealed transmissions require this “WS” fluid.

4runner transmission reliability

As with the rest of the vehicle, the 4runner’s transmission is very reliable. As long as you take care of it, there’s no reason why the transmission won’t last the life of the vehicle. 4runner transmissions don’t have any known design flaws.

There is one common issue with 3rd gen 4runners but it isn’t a problem with the transmission itself. The transmission fluid runs through a chamber in the radiator and over time, this corrodes. That causes it to leak into the radiator with the rest of the coolant.

Mixing ATF and coolant can result in damage to the transmission and it can be an expensive fix. Again, this isn’t a problem with the actual transmission but it is very closely related.

This issue has been affectionately nicknamed the “Strawberry Milkshake” by 3rd gen 4runner owners. It can be prevented by replacing the radiator with an upgraded aftermarket one and adding an external transmission cooler. That ensures the fluids are kept separate no matter what.

New 4runners aren’t known for any transmission failures. As long as you treat them well, they’ll keep chugging along like the rest of the vehicle.


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