Dear Car Talk:

I own a 2000 Jeep Cherokee. It needs fresh transmission fluid. What would your honest answer be about the benefits of "flushing" the transmission versus a transmission fluid "change"? Thanks! -- Francois



Well, I wouldn't flush the whole transmission yet, Francois. But you should flush the transmission fluid.

When you just remove the drain plug and rely on gravity to remove your transmission fluid, only a little more than half of it comes out. All of the dirty fluid that's sitting in the torque converter, and many of the passages, just stays there and contaminates the new fluid.

So a flush is a better alternative. We use a machine at the garage that connects to the transmission cooler lines. And, while the engine is running, it sends new fluid in and captures the old fluid coming out.

And because it pushes out the old fluid with the new fluid, you get a complete change of fluid. If your transmission holds, for example, 14 quarts of transmission fluid, 14 new quarts go in and 14 old, dirty quarts come out.

It costs about twice as much as just opening the drain plug, but it's at least twice as good. Plus, someone has to pay for that expensive machine we bought! But if you're going to go through the trouble of changing your transmission fluid, you might as well actually change all of it, right?

Unless your transmission has social anxiety, Francois. Then you might want to just change half of the fluid and give your transmission a chance to get to know the new fluid better first.