Unfortunately, the readings of the vacuum gauge are difficult to interpret, and the results of the analysis of readings can be erroneous, so it is advisable to combine vacuum diagnostics with other methods.
The initial factors by which the readings of the vacuum gauge are analyzed and the most accurate conclusions about the state of the engine are made are the absolute reading of the device and the nature of the movement of the instrument pointer (dynamics of indications). The scale of most vacuum gauges is graduated in mm Hg. Art. As the vacuum increases (and, accordingly, the pressure drop) the instrument reading increases. For every 300 m above sea level, the absolute readings of the vacuum gauge will differ by about 25 mmHg. Art.
Attach a vacuum gauge directly to the intake manifold, but not to other holes through which vacuum is created, separated from the manifold by a channel of a certain length (e.g. to openings in front of the throttle valve).
Warm up the engine to operating temperature before testing. Block the wheels and apply the parking brake. Place the shift lever in neutral (or in Park position on vehicles with automatic transmission), start the engine and let it run at idle speed.
Attention! Before starting the engine, carefully check the condition of the radiator fan blades (no damage or cracks). When the motor is running, do not put 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 serviceable engine, the vacuum gauge should show a vacuum of 430–560 mm Hg. Art., and the arrow of the device should be practically motionless.
Below is a description of the nature of the readings of the vacuum gauge and the methodology for determining the state of the engine based on them.
Vacuum level too low usually indicates a leak in the gasket between the intake manifold and the throttle assembly, the vacuum hose, as well as too late ignition or the wrong time to open and close the valves. Before removing the timing belt covers and checking the alignment of the alignment marks, check the ignition setting with a stroboscope.
If the readings of the vacuum gauge are 75–200 mm Hg. Art. below normal and are unstable (arrow twitches), this indicates a gasket leak at the intake manifold inlet or a malfunction of the injector.
If the arrow regularly deviates by 50–100 mm Hg. st., the cause is valve leaks. To confirm this conclusion, check the compression in the engine cylinders.
If the needle moves irregularly towards low readings or trembles, this indicates a low vacuum. The likely cause is increased resistance to valve movement or interruptions in the operation of the cylinders. Check cylinder compression and inspect spark plugs.
If at idle the needle fluctuates rapidly within 100 mm Hg. Art., and engine operation is accompanied by smoke from the muffler, then the valve guides are worn out. To check this, it is necessary to test the combustion chambers for leaks (with air pumping). If the arrow 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 indications can also be due to burnt valves and interruptions in the operation of the cylinders (ignition failures).
Weak fluctuations of the arrow (within 20–30 mm Hg. Art. round trip) indicate unstable operation of the ignition system. Check all foreseen settings and adjustments, if necessary, connect an ignition system analyzer to the engine.
With large fluctuations of the arrow check the compression in the cylinders, or perform a leak test, as the causes of the malfunction may be an inoperative cylinder or a leak in the cylinder head gasket.
If the instrument readings change slowly over a wide range, then check the cleanliness of the pipelines of the forced crankcase ventilation system (PCV), the correct adjustment of the fuel mixture, the tightness of the gaskets of the throttle assembly or the intake manifold.
Open the throttle sharply, and when the engine speed reaches 2500 min-1, release the damper. The shutter should slowly return to its original position. The vacuum gauge reading should drop to almost zero, then rise and exceed the reference reading corresponding to idling by about 125 mm Hg. Art., after which the vacuum should be restored to its previous level. If the vacuum is restored slowly, and there is no excess of the control reading when the damper is suddenly opened, then the cause may be worn piston rings. If the vacuum recovery is extremely slow, check the cleanliness of the exhaust tract (usually a muffler or catalytic converter). The easiest way to do this is to disconnect the exhaust system components in front of the suspect area and repeat the test.