Normal manifold vacuum on an engine running at idle speed is around 18 to 20 inches. If you have an engine at idle and your vacuum gauge reads very low, or no vacuum, you are probably connected to ported vacuum.

What is a good vacuum reading at idle?

about 18 to 22 in.-Hg Idle vacuum for most engines is about 18 to 22 in. -Hg, but some may produce only 15 to 17 inches at idle. (Remember what we said about experience.) If vacuum is steady and within these ranges, the engine and fuel and ignition systems are operating normally.

What is normal vacuum pressure?

Vacuum pressure is measured relative to ambient atmospheric pressure. It is referred to as pounds per square inch (vacuum) or PSIV. The electrical output of a vacuum pressure transducer is 0 VDC at 0 PSIV (14.7 PSIA) and full scale output (typically 5 VDC) at full scale vacuum, 14.7 (0 PSIA).

What PSI should a MAP sensor read?

At sea level, atmospheric pressure is about 14.7 psi (pounds per square inch). When the engine is off, the absolute pressure inside the intake equals atmospheric pressure, so the MAP will indicate about 14.7 psi. At a perfect vacuum, the MAP sensor will read 0 psi.

How do you increase manifold pressure?

The manifold pressure will increase as throttle is increased, due to the greater flow of fuel/air mix into the manifold (greater mass of air flowing into a fixed volume -> higher pressure). So the pressure difference will get larger as the throttle is increased.

What should inHg MAP sensor read at idle?

With the engine not running and the key on, the MAP sensor should read around 28-inHg. When the engine is started and idling, the vacuum of the engine should reduce atmospheric barometric pressure by 20-inHg. The reading on the scan tool scan tool should be approximately 8- to 9-inHg.