Measurement of depression is reliable and rather inexpensive way of diagnostics of the engine. According to indications of the vacuum gage it is possible to receive idea of a condition of piston group, of tightness of laying of a head of the block of cylinders, inlet and final collectors, correctness of adjustments of power supply systems and production of exhaust gases, capacity of exhaust gases, a condition of valves (their zalipaniye or a burn-out) and springs of valves and also to check correctness of adjustment of the moment of ignition and gazoraspredeleniye phases at operation of the engine.
Unfortunately, indications of the vacuum gage are difficult for interpreting, and results of the analysis of indications can be erroneous therefore vacuum diagnostics is expedient for uniting with other methods.
Initial factors on which indications of the vacuum gage are analyzed and the most exact conclusions about an engine condition become, absolute instrument reading and nature of movement of an arrow of the device (dynamics of indications) are. A scale of the majority of vacuum gages проградуирована in mm of mercury. In process of depression increase (and respectively pressure drop) instrument reading increases. On each 300 m above sea level absolute indications of the vacuum gage will differ approximately on 25 mm of mercury.
Attach the vacuum gage directly to an inlet collector, but not to other openings through which the vacuum, by the channel of a certain length separated from a collector (for example, to openings before a butterfly valve is created).
Before the beginning of tests warm up the engine to working temperature. Block wheels and tighten the parking brake. Establish the gear shifting lever in neutral situation (or in the situation Park on cars with an automatic transmission), start up the engine and leave it to work at frequency of idling.
Before engine start-up carefully check a condition of blades of the fan of a radiator (absence on them of damages or cracks). In operating time of the engine do not bring a hand too close to the fan, hold the device on sufficient removal from the fan and do not stand on one line with a rotating krylchatka.
Check the vacuum gage indication. On the serviceable engine the vacuum gage should show depression of 430-560 mm of mercury., and the arrow of the device should be almost motionless.
Below the description of character of indications of the vacuum gage and a technique of definition of a condition of the engine on their basis is carried out.
Too low level of depression usually indicates a laying leakage between an inlet collector and throttle knot, a vacuum hose, and also on too late ignition or for the wrong moment of opening and closing of valves. Before removing covers of a gear belt and to check combination of adjusting labels check ignition installation by means of a stroboscope.
If vacuum gage indications on 75-200 mm of mercury. below normal also are unstable (the arrow twitches), it indicates a laying leakage on an entrance of an inlet collector or on malfunction of a nozzle.
If the arrow regularly deviates on 50–100 mm of mercury., that the reason is a leakage of valves. For confirmation of this conclusion check a compression in engine cylinders.
If the arrow irregularly deviates towards low indications or shakes, it indicates low depression. A probable cause is the increased resistance to movement of valves or interruptions in operation of cylinders. Check a compression in cylinders and examine spark plugs.
If idling the arrow quickly fluctuates within 100 mm of mercury., and operation of the engine is accompanied by a smoke from the exhaust silencer then, directing plugs of valves are worn-out. For check of it it is necessary to carry out tests of chambers of combustion for tightness (with an air rating). If the arrow quickly fluctuates and the increase in frequency of rotation of the engine is at the same time observed, it is necessary to check tightness of laying of an inlet collector and elasticity of springs of valves. Such indications also can be caused by a burn-out of valves and interruptions in operation of cylinders (ignition failures).
Weak fluctuations of an arrow (within 20–30 mm of mercury. in both parties) point to unstable work of system of ignition. Check all provided installations and adjustments, if necessary connect to the engine the analyzer of system of ignition.
At big fluctuations of an arrow check a compression in cylinders, or carry out tests for tightness as the idle cylinder or violation of tightness of laying of a head of the block of cylinders can be causes of defect.
If instrument readings slowly change in a wide range, check purity of pipelines of system of compulsory ventilation of a case (PCV), correctness of adjustment of a fuel mix, tightness of laying of throttle knot or an inlet collector.
Sharply open a butterfly valve and when frequency of rotation of the engine will reach 2500 mines-1, release a zaslonka. Zaslonka should come back to a starting position slowly. Indications of the vacuum gage should fall almost to zero, then increase and exceed the control indications corresponding to work on frequency of rotation of idling, approximately on 125 mm of mercury., then depression should be restored at former level. If depression is restored slowly, and at sharp opening of a zaslonka excess of the control indication is absent, wear of piston rings can be the reason. At the slowest restoration of depression check purity of a final path (as a rule, the exhaust silencer or the catalytic converter). The simplest way of such check consists in separation of elements of system of release before a suspicious site and in test repetition.