Unfortunately, the readings of the vacuum gauge allow for different interpretations, therefore, to confirm the diagnosis, it must be used in combination with other methods of verification.
Both the reading of the vacuum gauge itself and the rate of its change are important for making the correct diagnosis. Most vacuum gauges are calibrated in millimeters of mercury (mmHg Art.). Please note that the readings of the vacuum gauge can be affected by atmospheric pressure and the associated position above sea level. When you rise 1 km above sea level, the pressure decreases by about 75 mm Hg. Art.
Connect a vacuum gauge to the intake manifold (pic. 4.0,a). Make sure that no vacuum hose is disconnected from the manifold, otherwise an incorrect result may be obtained.
Before starting the test, warm up the engine to operating temperature. Place wedges under the wheels and tighten the handbrake. Set transmission to neutral (or PARK in automatic transmission), start the engine and let it idle.
Attention:. Keep your hands and vacuum gauge away from the fan.
Read the vacuum gauge reading. In a serviceable engine, the average vacuum value should be from 400 to 550 mm Hg. Art., and the arrow of the device should almost not fluctuate (see fig. 4.0.b).
Other vacuum gauge readings mean the following:
1. A low constant vacuum usually indicates a leaky gasket between the intake manifold and the cylinder head or throttle body, or a leaking vacuum hose, or late ignition, or camshaft timing problems. Check the ignition setting with a stroboscope and repair vacuum leaks, and if this does not help, remove the timing chain cover and check that the timing marks match.
2. If the device shows 100-200 mm less than normal and the readings fluctuate greatly around this value, then there is reason to suspect a breakdown of the manifold gasket with the cylinder head or a malfunction of the fuel injector.
3. If the pointer of the device periodically twitches down by about 50-100 mm from a stable level, this may be due to a loose fit of one of the valves. To confirm, measure the compression in the cylinders or perform a cylinder leak test.
4. If the gauge pointer makes erratic downward throws, this may be caused by a stuck valve stem or ignition failure. Check compression or perform cylinder leak test, check spark plugs.
5. Rapid vibration of the gauge pointer combined with smoky exhaust can indicate worn valve guides. To confirm this assumption, perform a cylinder tightness test. If the vibration of the arrow occurs when the engine speed is increased, then this may be caused by leaks through the intake manifold gasket or cylinder head gasket, valve spring settlement. burnout of valves or malfunctions of the ignition system.
6. Small fluctuations in rarefaction of the order of 25 mm Hg. Art. up and down from the normal value may indicate a malfunction of the ignition system. Perform all normal ignition checks and, if no fault is found, have the engine checked at a workshop diagnostic stand.
7. If significant vacuum fluctuations are observed, perform a compression check and a cylinder leak test. Possible wear of the cylinder-piston group or breakdown of the gasket under the cylinder head.
8. If the gauge needle fluctuates slowly over a wide range, the causes may be clogging of the crankcase ventilation system, incorrect composition of the working mixture, leakage of intake manifold gaskets or throttle body.
9. Check the change in vacuum in transient mode. To do this, sharply open the throttle and when the engine reaches 2500 rpm. close it abruptly. In this case, the arrow of the device should drop almost to zero, then return to the idle level, exceed this level by about 75 mm Hg. st "then again smoothly return to the idle level and stop. If the vacuum recovers slowly and does not overshoot above the stationary level, this may indicate worn piston rings If the delay is too long, check that the exhaust system is not clogged (most often a catalyst or muffler). The easiest way to do this is to disconnect the exhaust system connection before the suspect element and repeat the test.