There is a vacuum port connected to the number 6 runner on my 225 that runs the heater controls under the dash. Any chance you could have some kind of a vacuum leak? What's connected to that port on your car? Have you done a vacuum test?
There is no vacuum fitting on the intake manifold. It is not hooked up to anything. Heater is not vacuum operated. I have been working on cars for 12 years and have never heard of a vacuum test.
