Unfortunately, vacuum gauge readings are difficult to interpret, and the results of reading analysis can be erroneous, so it is advisable to combine vacuum diagnostics with other methods.
The initial factors by which the vacuum gauge readings are analyzed and the most accurate conclusions about the engine condition are made are the absolute reading of the instrument and the nature of the movement of the instrument pointer (dynamics of readings). The scale of most vacuum gauges is graduated in mmHg. As the vacuum increases (and, accordingly, a drop in pressure) the instrument reading increases. For every 300 m above sea level, the absolute vacuum gauge readings will differ by approximately 25 mmHg.
Attach the vacuum gauge directly to the intake manifold, but not to any other vacuum-producing ports, separated from the manifold by a channel of a certain length (for example, to the holes in front of the throttle valve).
Before starting testing, warm up the engine to operating temperature. Block the wheels and apply the parking brake. Place the gear shift lever in neutral position (or in the Park position on vehicles with an automatic transmission), start the engine and let it idle.
Caution: Before starting the engine, carefully check the condition of the radiator fan blades (absence of damage or cracks on them). When the motor is running, do not place your hands too close to the fan, keep the appliance at a sufficient distance from the fan and do not stand in line with the rotating impeller.
Check the vacuum gauge reading. On a working engine, the vacuum gauge should show a vacuum of 430–560 mm Hg, and the instrument needle should be practically motionless.
Below is a description of the nature of vacuum gauge readings and the method for determining the engine condition based on them.
Too low vacuum level usually indicates a leak in the gasket between the intake manifold and the throttle body, a vacuum hose, as well as too late ignition timing or incorrect valve opening and closing timing. Before removing the timing belt covers and checking the alignment of the timing marks, check the ignition timing using a stroboscope.
If the vacuum gauge readings are 75–200 mmHg below normal and are unstable (the arrow twitches), then this indicates a leak in the gasket at the inlet of the intake manifold or a faulty injector.
If the arrow regularly deviates by 50-100 mmHg., then the reason is leaky valves. To confirm this conclusion, check the compression in the engine cylinders.
If the arrow irregularly deviates towards low readings or trembles, then this indicates low vacuum. The probable cause is increased resistance to valve movement or interruptions in cylinder operation. Check the compression in the cylinders and inspect the spark plugs.
If at idle the needle quickly fluctuates within 100 mm Hg, and the engine operation is accompanied by smoke from the muffler, then the valve guide bushings are worn out. To check this, it is necessary to conduct combustion chamber leak tests (with air pumping). If the needle fluctuates rapidly and at the same time an increase in engine speed is observed, then it is necessary to check the tightness of the intake manifold gasket and the elasticity of the valve springs. Such readings can also be caused by burnt valves and interruptions in the operation of the cylinders (ignition failures).
Weak fluctuations of the needle (within 20–30 mmHg in both directions) indicate unstable operation of the ignition system. Check all required settings and adjustments, and connect an ignition system analyzer to the engine if necessary.
With large fluctuations of the arrow check the compression in the cylinders, or perform a leak test, as the cause of the malfunction may be a non-working cylinder or a leaky cylinder head gasket.
If the instrument readings change slowly over a wide range, then check the cleanliness of the positive crankcase ventilation (PCV) system lines, the correct adjustment of the fuel mixture, and the tightness of the throttle body or intake manifold gaskets.
Open the throttle valve sharply, and when the engine speed reaches 2500 min⁻¹, release the valve. The damper should slowly return to its original position. The vacuum gauge reading should drop to almost zero, then rise and exceed the idle speed control reading by approximately 125 mmHg, after which the vacuum should return to its previous level. If the vacuum is restored slowly, and when the valve is opened sharply the control reading is not exceeded, then the cause may be wear of the piston rings. If the vacuum is restored extremely slowly, check the cleanliness of the exhaust tract (usually a muffler or catalytic converter). The simplest way to perform this test is to disconnect the exhaust system components before the suspect section and repeat the test.