Vacuum gauge:Reading And Interpretation

The complete lack of coverage on Vacuum gauges and their importance ....it isnt our fault ........changing times have prompted us to rely more on OBD scanners to diagnose engine problems where most of us forgot that the engine sitting nicely under our hood was still a hundred year old piece of marvel ......and before the days of the computer wizardry .......a competent mechanic using nothing more than your spark plug (colour) , compression tester and the vacuum gauge could track over a number of engine performance related issues

today we will focus and talk about Vaccum gauges and how to use them.......

Word of advice : Invest in one (buy one)


Vacuum Gauge
Though little understood, the vacuum gauge is probably the best single indicator of your engine's health you can get.


When your car is idling-whether it's fuel injected or carbureted-the throttle plate or plates are restricting the amount of air the engine can breathe in. The pistons are attempting to "suck" the mixture past the throttle. (Of course, in reality, it is atmospheric pressure that is attempting to "push" air into the engine as the pistons travel downward on their intake strokes.) When throttle is closed, vacuum is high in the intake manifold, from the throttle plate(s) to the combustion chambers. By contrast, at wide open throttle there is relatively little restriction to outside air entering the intake manifold, so vacuum in the manifold is very low.

A vacuum gauge reads pressure differences from atmospheric pressure, so the reading is zero in our "normal" sea of air. By convention, vacuum gauges in the read "inches of Mercury."

Now unlike other gauges the Vacuum gauge will keep you entertained for hours and hours ........due to its wide ranging movements across all RPMs, accelerating and decelerating
When you floor the accelerator pedal, you can watch manifold pressure (another word for vacuum) swing from strongly negative to nearly zero (atmospheric pressure). When your engine is "on the overrun," like using engine braking down a steep hill at high RPM, you'll see really high vacuum readings. Naturally, turbocharged and supercharged will show very different results, with readings swinging into the positive at high speed. IAP's vacuum gauge is not designed for turbo or supercharged vehicles . Your vacuum gauge is also a sort of "poor man's" fuel mileage indicator; when vacuum is low, you are burning more fuel.

now for the actual reading (good reading....not the boring kind)