Q: I have a Turbo 400 transmission with a 4,000-rpm stall converter and a transmission cooler. My trouble starts when the transmission temperature gets to about 210˚ F—the transmission starts blowing fluid out of the top vent tube. Even this hot, the trans doesn’t slip and shifts just like it does when it’s cool, whether I’m cruising or cranking the engine up to 7,800 rpm. As far as I know, the pump is set up for stock line pressure. The transmission also has a flat filter and a deep pan.
A: Your converter will slip any time engine rpm is under 4,000—and a slipping converter generates heat. As the trans fluid heats up, it expands and fills the pan. A stock transmission pan holds approximately 4½ quarts of fluid, but your deep pan holds more. That means the fluid level when the transmission is cool is about the same as the level of hot fluid in the stock pan.
What all that means is when that extra fluid gets hot, it has to go somewhere—and that somewhere is into the transmission itself. All those moving parts whip that fluid into foam. Since foamy fluid takes up a lot of space, it is forced out of the transmission via your top vent.
The solution is one of two things—lower the fluid temperature (maybe by going to a larger cooler), or switch back to a stock capacity transmission pan.
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