Engine Vacuum Testing

Engine Vacuum Testing

Engine vacuum is produced by the piston drawing air from the intake manifold during the intake stroke. The throttle plate keeps air from freely flowing into the manifold creating manifold vacuum. When the throttle body is opened there is a collapse in vacuum. This collapse in vacuum is actually an increase in air pressure.

A quick snap of the throttle plate should drop the vacuum to around 5 Hg and then it should recover to a steady 21 Hg. When this air is mixed with atomized fuel from the fuel injectors engine speed is increased. It's important to remember for every 1000 ft. of increase in altitude 1 Hg is subtracted from the reading. For instance, if specifications required 21Hg at an altitude of 3000 ft. above sea level a reading of 18 Hg would be considered normal.

To check manifold pressure with a vacuum gauge you need to locate a port in the manifold or throttle body. Manufacturers put ports on their manifolds for lots of different reasons: Brake Booster, PCV tube, EGR Switch, A/C vents, and on and on. You simply need to find one small enough for the vacuum gauge line to slide onto firmly.

Gauge Readings:

Normal Engine: On a normal engine accelerate to around 2000 rpm and then quickly release the throttle plate. The engine should snap right back to a steady 17- 21 Hg.

Steady low between 5-10 Hg: This indicates that the engine has a leak in the intake manifold or the intake gasket. This leak should be pretty easy to find because it would be making a loud hissing noise.

Steady low between 10-15 Hg: This reading indicates late valve timing. There's a good chance the vehicle has jumped timing. Check the timing belt or chain depending on the application.

Steady low  between 15-18Hg: This just low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem.

Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve or a there's an engine misfire.

Needle drops during acceleration: If the needle drops steady during acceleration there's a restriction in the exhaust or intake. This is typically due to a clogged CAT or muffler.