I have often had this problem posed to me. I am driving along with the cruise control set and the air conditioning on. All of a sudden I notice that I am slowing down and that the air conditioning is no longer cooling the car but air is coming out of the heat vents. What is wrong here??
It is a vacuum leak somewhere in the system. Vacuum supplies a lot of functionality in the car. It supplies vacuum for the power brakes, the cruise control servo, the heater controls, the EGR valve, the PCV valve, the carburetor (if you have one) and the transmission modulator. Each of these can be the source of a "vacuum leak". Normally there is also a performance problem associated with a true vacuum leak.
There is a major vacuum leak associated with a wide open throttle and that is the throttle body itself. When you step into it, put the pedal to the metal, floor it, whatever you call it, the vacuum in the intake manifold, the source of vacuum for all that stuff I mentioned above, approaches atmospheric pressure, in other words, there is no longer a source of vacuum to supply these functions.
Well if there is no longer a source of vacuum while I am climbing a steep hill or just plain accelerating where does the vacuum come from to do these things?? Well the engineers thought about that problem and therefore most modern cars have a vacuum accumulator under the hood somewhere. Since a vacuum is really the absence of anything this accumulator collects . . . nothing!! Nothing, that is, but vacuum - sufficient vacuum to supply the heater controls and the cruise control until you resume normal driving conditions once again.
This accumulator is normally a spherical container about the size of a softball and it is connected to the heater controls and the cruise control and the intake manifold. Each of these items do not require any air flow or change of vacuum once they are set to where you want them to be. Therefore there is a check valve in the vacuum system to prevent air from flowing back from the intake manifold to the accumulator when you step into it. If the check valve is missing or faulty then air will flow from the intake manifold into the accumulator until the vacuum level in the accumulator is at the same pressure as the intake manifold. If you have your gas pedal floored then that pressure is atmospheric pressure and all of the controls that depend on vacuum resume their normal position - heat for the heater, and off for the cruise!! Look for a check valve in the line to the accumulator from the intake manifold. It is most likely faulty.
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