You are referring to the heat riser valve. It closes when cold & partially blocks the exhaust sending some of it through the passages in the intake manifold. This warms the incoming fuel/air mixture & gives better running & better gas mileage. As the engine warms up the thermostatic spring relaces & the valve opens. The butterfly is also mounted off center so it also opens under hard acceleration. I hope that explains it.
George
Last edited by Wrenchbender Ret.; 07/17/2017 1:05 PM.
