The modulator on the side of the trans on gas engines and some diesels consists of a diaphragm with trans fluid on one side and manifold vacuum on the other side. During high manifold situations like idling or on decell a torn diaphragm allows tranny fluid to be sucked into the intake manifold. Just squirt a shot of oil into your manifold while the car is idling, and you'll get the picture. This is the first simple test that any professional mechanic worth his salt will run before yanking the heads off.

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