The vacuum gauge readings can provide an idea of the condition of the piston group, the tightness of the cylinder head gaskets, the intake and exhaust manifolds, the correct adjustment of the engine fuel system and exhaust gas outlet, the throughput of the exhaust gases, and the condition of the valves (their sticking or burning) and valve springs, and also check the correct adjustment of the ignition timing and the maintenance of valve timing during engine operation.
Unfortunately, vacuum gauge readings are difficult to interpret and the results of reading analysis may be erroneous, therefore, 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 device and the nature of the movement of the instrument pointer (dynamics of readings). The scale of most vacuum gauges is graduated in mm. mercury column. As the vacuum increases (and accordingly the pressure drop) the instrument reading increases.
For every 300 m above sea level, the absolute vacuum gauge readings will differ by approximately 25 mm Hg.
Attach the vacuum gauge directly to the suction manifold, but not to other openings through which vacuum is created, separated from the manifold by a channel of a certain length (for example, to the holes in front of the throttle valve).
Warm up the engine completely before starting testing. Block the wheels and set the handbrake on the car. With the gearshift lever in neutral (or in Park position on vehicles with automatic transmission) start the engine and let it idle.
Caution: Before starting the engine, carefully check the condition of the fan blades (the presence of damage or cracks on them). When the engine 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.
Description of the nature of vacuum gauge readings and methods for determining engine condition based on them
1. Too low a vacuum level usually indicates a leak in the gasket between the intake manifold and the throttle chamber, a leak in the 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 with a timing light and eliminate all other possible causes using the testing procedures described in this Chapter.
2. If the vacuum gauge readings are 75–200 mm Hg 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.
3. If the arrow regularly deviates by 50–100 mm Hg, then the cause is a leak in the valves. To confirm this conclusion, check the compression in the engine cylinders.
4. The arrow irregularly deviates towards low readings, or trembles and shows low vacuum. The probable cause is increased resistance to valve movement, or interruptions in the operation of the cylinders. Check the compression in the cylinders and inspect the spark plugs.
5. If the needle quickly fluctuates within 100 mm Hg at idle speed, and the engine operation is accompanied by smoke from the muffler, then the valve guides are worn out. To verify this conclusion, it is necessary to conduct combustion chamber leak tests (with air pumping). If the arrow fluctuates quickly 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).
6. Weak fluctuations of the arrow (within 20–30 mm Hg in both directions) indicate unstable ignition operation. Check all required settings and adjustments, and connect an ignition system analyzer to the engine if necessary.
7. If the pointer fluctuates greatly, check the compression in the cylinders or perform leak tests, as the cause of the malfunction may be a non-working cylinder or a leaky cylinder head gasket.
8. If the instrument readings change slowly over a wide range, then check the cleanliness of the forced crankcase ventilation system pipes, the correct adjustment of the combustible mixture, the tightness of the throttle body gaskets, or the intake manifold.
9. Open the throttle valve sharply, and when the engine speed reaches 2500 rpm, release the valve. The damper should slowly return to its original position. The vacuum gauge readings should drop to almost zero, then increase and exceed the control readings corresponding to steady-state idle running by approximately 125 mm Hg, after which the vacuum should be restored to the 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 check this is to open the exhaust tract before the suspect section and repeat the test.