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 gain an impression about a condition of piston group, about 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, the capacity of exhaust gases, a condition of valves (their sticking or a burn-out) and springs of valves, and also to check correctness of adjustment of the moment of ignition and phases of gas distribution during the operation of the engine.
Unfortunately, it is difficult to interpret indications of the vacuum gage, and results of the analysis of indications can be wrong therefore it is expedient to unite vacuum diagnostics with other methods.
Initial factors on which indications of the vacuum gage are analyzed and draw the most exact conclusions about a condition of the engine, absolute instrument reading and the nature of the movement of an arrow of the device (dynamics of indications) are. The scale of the majority of vacuum gages is graduated in mm of mercury. In process of depression increase (and according to 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 vacuum, is created by the channel of a certain length separated from a collector (for example, to openings before a butterfly valve).
Before tests warm up the engine up to the working temperature. Block wheels and tighten the parking brake. Install the gear shifting lever in neutral situation (or in situation Park on cars with the automatic transmission), launch the engine and leave it to work at idling frequency.
Before launch of the engine carefully check a condition of blades of the fan of a radiator (lack on them of damages or cracks). In operating time of the engine you do not bring a hand too close to the fan, you hold the device on sufficient removal from the fan and do not stand on one line with the rotating krylchatka.
Check the indication of the vacuum gage. On the serviceable engine the vacuum gage has to show depression of 430-560 mm of mercury., and the arrow of the device has to be not mobile almost.
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 leakage of laying between an inlet collector and throttle knot, a vacuum hose, and also too late ignition or for the wrong moment of opening and closing of valves. Before uncovering a gear belt and to check combination of adjusting tags check installation of ignition by means of a stroboscope.
If indications of the vacuum gage on 75 - 200 mm of mercury. below normal also are unstable (the arrow twitches), it indicates leakage of laying on an entrance of an inlet collector or on malfunction of a nozzle.
If the arrow regularly deviates on 50–100 mm of mercury., leakage of valves is the reason. For confirmation of this conclusion check a compression in engine cylinders.
If the arrow irregularly deviates towards low indications or shakes, then it indicates low depression. The increased resistance to the movement of valves or interruptions in operation of cylinders is a probable cause. 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 followed by smoke from the muffler then, the directing plugs of valves are worn-out. For check of it it is necessary to carry out tests of combustion chambers for tightness (with an air rating). If the arrow quickly fluctuates and increase in frequency of rotation of the engine is at the same time observed, then it is necessary to check tightness of laying of an inlet collector and elasticity of springs of valves. Such indications can be also caused by a burn-out of valves and interruptions in operation of cylinders (failures of ignition).
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 the analyzer of system of ignition to the engine.
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 the wide range, then check purity of pipelines of system of compulsory ventilation of a case (PCV), correctness of adjustment of fuel mix, tightness of laying of throttle knot or inlet collector.
Sharply open a butterfly valve and when the frequency of rotation of the engine reaches 2500 min.-1, release the gate. The gate has to come back to a starting position slowly. Indications of the vacuum gage have to reduce almost to zero, then increase and exceed the control indications corresponding to work at an idling rotation frequency, approximately on 125 mm of mercury., then depression has to be restored at the previous level. If depression is restored slowly, and at sharp opening of the gate excess of the control indication is absent, then wear of piston rings can be the reason. At extremely slow restoration of depression check purity of a final path (as a rule, the muffler or the catalytic converter). The easiest way of such check consists in separation of elements of system of release before the suspicious site and in test repetition.