I have a 1992 Dodge grand Caravan with a 3.3L engine. I would like to measure the back pressure on the catalytic converter to see if it is clogged. There are many YouTube video’s showing how to use a vacuum gauge to measure back pressure at a RPM. But the actual pressure and RPM you should use is somewhat vehicle specific and older cars are supposed to be considerably different than new ones. My Haynes manual has no spec for my van. Does anyone here have access to such information.