4.3.4. Diagnostics of a condition of the engine with use of the vacuum gage

Use of the vacuum gage at rather small monetary expenses allows to obtain rather capacious information on internal state of the engine. By results of the taken measurements it is possible to make idea of degree of wear of piston rings and mirrors of cylinders, to reveal signs of failure of laying of a head of cylinders and the inlet pipeline, violation of adjustments of the carburetor and passability of system of production of the fulfilled gases, jammings or a progar of valves, flashes of valvate springs, failure of installation of a corner of an advancing of ignition or phases of gas distribution, failures of the system of ignition, etc., etc.

Unfortunately, it is easily wrong to interpret results of the readings taken by means of the vacuum gage, and therefore, they have to be analyzed together with the data obtained during performance of other diagnostic checks.

When reading indications of the indicator of the vacuum gage it is necessary to pay attention not only to an absolute value of a deviation of an arrow, but also to the speed of its movement. The majority of measuring instruments of import production show depression depth in inches of a mercury column. At the same time it is necessary to consider that all standard requirements are provided for a case of performance of checks at zero height above sea level. Increase in a relief on each 300 m after a mark of 600 m leads to understating of instrument readings approximately on 1 inch hg. Let's notice that 1 inch = 25.4 mm.

Connect the vacuum gage directly to the inlet pipeline - not to the throttle case. Track that during check performance all hoses remained connected, - otherwise the read instrument cannot be considered reliable.


Before starting measurements, warm up the engine up to the normal working temperature. Prop up wheels antiretractable boots and cock the parking brake. Transfer transmission to situation "P", start the engine and leave its working at turns of normal idling.

Before start of the engine attentively check fan blades for existence of cracks and other damages. Try not to bring closer to a krylchatka of a hand and the measuring instrument. Also avoid to take a position just before the car!


Consider indications of the vacuum gage. On average depression depth in the inlet pipeline of the serviceable engine has to be rather stable (without breakthroughs of an arrow) and to make about 17 ÷ of 22 inches (432 ÷ 560 mm) hg. The scheme of interpretation of the taken readings is provided in the following paragraphs:


Low stability of indications can be the evidence of leaks through laying between the inlet pipeline and a head of cylinders, or between the pipeline and the case of a throttle. Also the probability of violation of tightness of a vacuum hose, failure of the moment of ignition (towards lag), or violations of installation of phases of gas distribution is not excluded. Check installation of a corner of an advancing of ignition by means of a stroboscope, then serially exclude all other possible causes, carrying out the checks listed in the present Chapter, only after it it makes sense to uncover the GRM drive for the purpose of check of correctness of combination of adjusting tags.

If the result of measurement appears on 3 ÷ 8 inches (76 ÷ 203 mm) hg lower than norm and at the same time fluctuations take place, violation of tightness of laying of the inlet pipeline near the inlet port, or malfunction of an injector of injection of fuel can appear the reason of such deviation.

The regular deviation of an arrow of the measuring instrument down from the stable indication on 2 ÷ 4 inch (51 ÷ 102 mm) hg with high degree of probability demonstrates leakages of valves. Check compression pressure in cylinders or carry out the test for leaks.

Irregular rejections or dumpings of indications can be connected with jamming of valves or admissions of ignition. Measure compression pressure, carry out the test for leaks, check a condition of spark plugs.

The frequent vibration of an arrow of the indicator within range in 4 inches (102 mm) hg at single turns of the engine which is followed by smoke emission from an exhaust pipe speaks about wear of the directing plugs of valves. Carry out the test for leaks. If the arrow begins to vibrate at increase in turns of the engine, check for existence of signs of leakages of laying of the inlet pipeline and a head of cylinders. Estimate degree of a flash of valvate springs, check valves for existence of traces of a progar and try to reveal admissions of ignition.

Insignificant fluctuations of indications within range from one to two inches (25 ÷ 51 mm) hg can be considered as the evidence of violation of serviceability of functioning of system of ignition. Check correctness of all usual adjusting installations, in case of need resort to testing with use of the analyzer of parameters of ignition.

In the presence of fluctuations of an arrow of the measuring instrument in the wide range check compression pressure or carry out the test for leaks for the purpose of identification of the defective cylinder or violation of tightness of laying of a head of cylinders.

If the arrow of the measuring instrument with a slow speed "walks" in the wide range of a scale, check passability of the PCV system and composition of mix of idling, also make sure of lack of leaks through laying of the carburetor/case of a throttle and the inlet pipeline.

Estimate return speed to a reference value when closing a butterfly valve after rise in frequency of rotation of the engine up to the size about 2500 rpm at the expense of a bystry priotkryvaniye of the gate. The indication at first has to reduce practically to zero, then rise over the value characteristic of normal turns of idling approximately by 5 inches (130 mm) hg and again to return to the indication of single turns. If depth of depression is restored slowly and does not form a peak throw at sharp closing of a butterfly valve, it is necessary to check whether piston rings are worn-out. At a long delay of return of indications check passability of system of production of the fulfilled gases (the muffler or the catalytic converter are often blocked) - just it is the simplest to disconnect the suspected section of system of release and to repeat check.