Unfortunately, the readings of the vacuum gauge are difficult to interpret and the results of the analysis of readings can be erroneous, therefore, 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. rt. pillar. 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 mm. rt. Art.
Attach the vacuum gauge directly to the suction manifold, but not to other vacuum openings 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 completely before testing. Block the wheels and put the car on the handbrake. When the gear lever is in neutral position (or in the Park position on vehicles with automatic transmission) start the engine and let it idle. Warning! Before starting the engine, carefully check the condition of the fan blades (the presence of damage or cracks on them). 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. rt. Art., and the arrow of the device should be practically motionless.
The following 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.
1. Too low a vacuum usually indicates a leak in the gasket between the intake manifold and the throttle chamber, the vacuum hose, as well as too late ignition or the wrong opening and closing of the valves. Before removing the timing belt covers and checking the alignment of the timing marks, check the ignition setting with a stroboscope and eliminate all other possible causes, guided by the verification methods described in subsection 3.4.3.
2. If the vacuum gauge readings are 75–200 mm. rt. Art. below normal and are unstable (arrow twitches), then this indicates a leak in the gasket at the inlet of the suction manifold or a malfunction of the injector.
3. If the arrow regularly deviates by 50–100 mm. rt. Art., then the cause is valve leaks. To confirm this conclusion, check the compression in the engine cylinders.
4. The pointer fluctuates irregularly towards low readings or, trembling, indicates 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.
5. If at idle the arrow quickly fluctuates within 100 mm. rt. Art., and the operation of the engine is accompanied by smoke from the muffler, then the valve guides are worn out. To verify this conclusion, 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, 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).
6. Weak fluctuations of the arrow (within 20–30 mm. rt. Art. round trip) indicate unstable ignition operation. Check all foreseen settings and adjustments, if necessary, connect an ignition system analyzer to the engine.
7. In case of large fluctuations of the arrow, check the compression in the cylinders or carry out leak tests, as the cause of the malfunction may be an idle cylinder or a leak in the cylinder head gasket.
8. If the readings of the device change slowly over a wide range, then check the cleanliness of the pipelines of the forced crankcase ventilation system, the correct adjustment of the combustible mixture, the tightness of the gaskets of the throttle body or the intake manifold.
9. Open the throttle sharply, and when the engine speed reaches 2500 rpm, release the throttle. The shutter should slowly return to its original position. The vacuum gauge reading should drop to almost zero, then rise and exceed the control reading corresponding to stationary idle by about 125 mm. rt. 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 open the exhaust tract in front of the suspicious area and repeat the test.
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