MAP means manifold absolute pressure. It
pokes right into connects to the intake manifold through an air-tight seal and senses the air pressure inside. At sea level the air pressure is about 15 psi. That’s what the MAP sensor should read if the engine wasn’t running and the car was at sea level. A little less if the car is at a higher altitude. At Denver the air pressure is about 12 psi. With the engine idling the air pressure is less than that, b/c the engine is creating a partial vacuum in the intake manifold with the throttle valve nearly closed. For most engines that idle vacuum is in the 15 -20 inches of mercury range. Inches of mercury is just another unit system to measure pressure. 15 inches of mercury is about 7 psi. So at idle with the engine providing 15 inches of mercury vacuum, the pressure in the intake manifold should be the ambient air pressure minus the vacuum reading, or at sea level 15 psi - 7 psi, or about 8 psi. If you’ve got a higher vacuum than 15 inches of mercury, which is the sign of a well tuned, good compression engine, then the MAP at idle would read a little less than 8 psi; so yes, 5 or 6 psi would be a good reading for a MAP at idle.