Unfortunately, vacuum gauge readings are easily misinterpreted and must be analyzed in conjunction with data obtained from other diagnostic tests.
When reading the vacuum gauge indicator, you should pay attention not only to the absolute value of the arrow's deflection, but also to its speed of movement. Most imported gauges show the vacuum depth in inches of mercury. It should be taken into account that all regulatory requirements are given for the case of testing at zero altitude above sea level. An increase in relief for every 300 m after the 600 m mark leads to an underestimation of the instrument's readings by approximately 1 inch of mercury. Note that 1 inch = 25.4 mm.
Attach the vacuum gauge directly to the intake manifold - not to the throttle body. Make sure that all hoses remain connected during the test - otherwise the reading will not be reliable.
Before taking measurements, warm up the engine to normal operating temperature. Chock the wheels and set the parking brake. Shift the transmission into position "P", start the engine and let it run at normal idle speed.
Warning! Before starting the engine, carefully check the fan blades for cracks and other damage. Try to keep your hands and the measuring device away from the impeller. Also, avoid taking a position directly in front of the car!
Read the vacuum gauge readings. On average, the vacuum depth in the intake manifold of a serviceable engine should be fairly stable (without the pointer jerking) and be about 17÷22 inches (432÷560 mm) Hg.
The following paragraphs provide a diagram for interpreting the readings. Low stability of readings may indicate leaks through the gasket between the intake manifold and the cylinder head, or between the manifold and the throttle body. It is also possible that the vacuum hose is not sealed, the ignition timing is incorrect (towards a lag), or the valve timing is incorrect. Check the ignition timing with a stroboscope, then eliminate all other possible causes one by one by performing the checks listed in this Chapter, only then does it make sense to remove the timing cover to check that the timing marks are correctly aligned.
If the measurement result is 3÷8 inches (76÷203 mm) Hg below the norm and there are fluctuations, the cause of such a deviation may be a leak in the intake manifold gasket in the area of the intake port, or a malfunction of the fuel injection injector.
Regular downward deviation of the gauge needle from a stable reading by 2÷4 inches (51÷102 mm) of mercury with a high degree of probability indicates valve leaks. Check the compression pressure in the cylinders or conduct a leak test.
Irregular deviations or resets may be due to valve sticking or misfiring. Measure compression pressure, perform a leak test, check the condition of the spark plugs.
Frequent vibration of the indicator needle within the range of 4 inches (102 mm) of Hg at idle speed, accompanied by smoke emission from the exhaust pipe, indicates worn valve guides. Perform a leak test. If the needle begins to vibrate when the engine speed increases, check for signs of leaks in the intake manifold gasket and cylinder head. Assess the degree of sagging of the valve springs, check the valves for signs of burnout and try to detect misfires.
Minor fluctuations in readings within the range of one to two inches (25÷51 mm) of mercury may be considered evidence of a malfunction of the ignition system. Check the correctness of all normal settings, if necessary, resort to testing using an ignition parameter analyzer.
If the gauge needle fluctuates widely, check the compression pressure or perform a leak test to identify a faulty cylinder or a leaky head gasket.
If the needle on the meter is slowly "wandering" over a wide range of the scale, check the PCV system for leaks and idle mixture composition, and also make sure there are no leaks through the carburetor/throttle body gaskets and intake manifold.
Observe how quickly the vacuum gauge returns to its original value when the throttle is closed after the engine speed has been raised to approximately 2500 rpm by rapidly opening the throttle. The reading should first drop to near zero, then rise above the normal idle speed by approximately 5 in (130 mm) of mercury, and then return to the idle reading. If the vacuum is slow to return and does not spike when the throttle is rapidly closed, check for worn piston rings. If the reading takes a long time to return, check the exhaust system for leaks (the muffler or catalytic converter are often blocked) - the easiest way is to simply disconnect the suspect section of the exhaust system and repeat the check.