Is A Transmission Flush A Good Idea?

Here’s a question that can start a heated debate among auto repair experts: Could flushing a transmission cause harm, especially if the transmission is old and/or has not been maintained?

On one side, there's a group that says flushing transmissions - any transmission - can cause problems. (Click here for other common transmission problems.) On the other, many people argue that a transmission flush is nothing to worry about. Who's correct?

What Is a Transmission "Flush," Exactly?

Mazda auto trans

An automatic transmission is filled with fluid. In many cases (not all), the vehicle manufacturer recommends that the fluid be replaced after a certain amount of time or specific mileage. This fluid replacement can be done in one of two ways:

  1. The fluid is drained, then refilled
  2. The fluid is "flushed" using a machine

NOTE: Some transmissions are "sealed," which is to say that they use a lifetime fluid that does not need to be replaced.

If the fluid is drained and then refilled, it is not completely replaced. This is because about 1/5th of the fluid in any transmission is inside the valve body, and fluid inside the valve body needs to be pushed or pulled out. It will not drain on its' own. This is why transmission "flushes" were created: Rather than simply draining and refilling (and leaving a lot of old fluid in the transmission), a machine can remove almost all the old fluid by forcing new fluid into the transmission (and forcing it from the opposite direction of normal fluid flow).

The Problem With Flushing

While "flushing" a transmission removes most of the old fluid, it can also cause damage or even failure. This is because flushing tends to stir up any debris or particulate matter that's settled out inside the transmission.

Debris and particulates occur during normal automatic transmission use; parts inside the transmission wear, leading to particulates and "gunk." This gunk is accumulated over a period of years, and once it settles somewhere inside the transmission it's generally harmless. However, when the transmission is flushed, a lot of this debris is stirred up. It then circulates thru the transmission for a few thousand miles before settling out again.

If the debris is small enough - or if there is very little debris - it does little harm. In a modern transmission, the debris is usually small and harmless. However, if your transmission is an exception, a bigger piece of sludge will loosen and circulate after a power flush. That sludge can get stuck in an important valve body passage (for example), eventually causing the transmission to fail.

As a result of this risk, very few auto manufacturers recommend transmission "flushes." Automakers usually recommend drain and refill only.

Benefits Of Draining And Refilling Your Transmission Fluid

Provided you do not have a transmission with a "lifetime" fluid - also known as a "sealed" transmission - periodic draining and refilling of your transmission (as recommended in your maintenance guide) is beneficial in a few ways:

Flush benefits

The transmission will last longer. Transmission fluids (non-lifetime) wear just like any other hydraulic fluid. By replacing them, you increase their ability to lubricate and cool.

You may notice smoother shifts. If your fluid was in dire need of replacement, you may notice that the transmission shifts a little more smoothly after a replacement (the valves often perform a little better with new fluid).

You may notice better fuel economy. Since your transmission will run more efficiently, it will take less engine effort to make it run — meaning better fuel economy (and performance) as a result.

If you have an automatic transmission, and if that transmission is not a sealed unit with a lifetime fluid, draining and refilling fluid at the manufacturer's recommended interval is a smart idea. Not only does it ensure proper function of the transmission (and a long lifespan), it also saves you money at the gas pump and makes driving more enjoyable.