A manifold gauge, sometimes referred to as "vacuum" gauge, is a simple yet extremely effective means by which the condition of an internal combustion engine can be determined. A manifold gauge visually indicates the engine's vacuum at the intake manifold. Sometimes these gauges are installed within the interior of the vehicle to monitor gas mileage. Simply put, a high vacuum reading indicates good mileage, and a low reading indicates poor mileage.
Hooking the Gauge Up
Because a manifold gauge does not require electricity, it is quite possibly the easiest type of gauge to hook up to the engine. The first step is to locate an available vacuum port. Vacuum ports are located on top of the intake manifold, and are used to power a variety of accessories throughout the vehicle. Vacuum ports which are unused will have a rubber cap over the port. Pull the cap off, then slide the rubber hose attached to the manifold gauge onto the vacuum port. Turn the engine on, then observe the action of the needle on the face of the gauge.
Steady Needle
If the needle remains steady between 17-22 in./Hg., or "inches of mercury," this steady reading indicates a normal engine in good condition.
Needle Fluctuates as Engine Speed Increases
Have an assistant slowly and steadily press down on the accelerator pedal. If the needle on the gauge fluctuates as the speed of the engine increases, this indicates either an ignition miss, a leaky valve or weak valve spring, or a blown cylinder head gasket.
Needle Quickly Drops, then Returns to Normal
If the gauge's needle remains steady, but then quickly falls before rising again to its steady point, the gauge is indicating that either a cylinder head valve is burnt or the clearance between a valve and the piston is incorrect. The needle on the gauge falls when the defective valve is operated.
Needle Gradually Drops at Idle
If the needle gradually drops as the engine idles, this indicates that the muffler is either choked, or there is excessive back pressure in the exhaust system.
Needle Excessively Vibrates at Idle but Steadies as Engine Speed Increases
Have an assistant slowly and steadily press down on the accelerator pedal. If the needle vibrates excessively while the engine is idling, say within a range of 10 in./Hg., yet steadies as the speed of the engine increases, this indicates that the valve guides have worn out.
