What is a thermo vaccum valve do?(1988 MB 420SEL)and what does it do to the engine if it's malfunctioned?

Answer 1

A thermo vacuum valve is a vacuum switching valve that switches on or off based on engine heat. Depending on the function and design of the valve, it may be off when the engine is cool and open up to allow vacuum through it when the engine warms up or vice versa. If it's bad, some item won't operate if it's controlled by the valve such as EGR or air cleaner heated air intake or whatever. A service manual or vacuum diagram or dealer parts dept. can tell you what thermo valves control what items....might be more than a few on your car.