The zemco counts the ball as it rides around a circular track. The fuel pushes the ball along horizontally. I agree there is definite need for a fuel cooler on a diesel if the fuel is recirculated and not returned to the tank. If not the fuel expands as it heats. New fuel is not flowing into the loop and mileage looks great. And then the float valve finally takes a big gulp of cool fuel and mileage looks poor. And the cycle starts over.
