The use of a vacuum gauge at a relatively low cost allows you to get enough capacious information about the internal state of the engine. Based on the results of the measurements, it is possible to get an idea of the degree of wear of the piston rings and cylinder mirrors, to identify signs of failure of the cylinder head gaskets and intake manifold, violations of the carburetor adjustments and the patency of the exhaust system, jamming or burnout of valves, sagging of valve springs, failure to set the angle ignition advance or valve timing, ignition system failures, etc. and so on.
Unfortunately, vacuum gauge readings are easily misinterpreted and therefore must be analyzed in conjunction with data obtained from other diagnostic tests.
When reading the indications of the vacuum gauge indicator, one should pay attention not only to the absolute value of the deviation of the arrow, but also to the speed of its movement. Most imported meters show the depth of vacuum in inches of mercury. However, it should be taken into account that all regulatory requirements are given for the case of testing at zero altitude above sea level. Relief increase for every 300 m after the 600 m mark leads to an underestimation of the instrument readings by approximately 25 mmHg Art.
1. Connect a vacuum gauge directly to the intake manifold - not to the throttle body. Make sure that all hoses remain connected during the test, otherwise the reading taken will not be considered reliable.
2. Before starting measurements, warm up the engine to normal operating temperature. Chock the wheels and apply the parking brake. Move the transmission to position «R», start the engine and leave it running at normal idle.
Before starting the engine, carefully inspect the fan blades for cracks or other damage. Try not to bring your hands and the meter closer to the impeller. Also avoid taking a position directly in front of the car!
3. Read the vacuum gauge (see illustration Examples of possible vacuum gauge readings). On average, the depth of vacuum in the intake manifold of a healthy engine should be fairly stable (no jerky arrows) and be about 430-560 mm Hg Art.
The scheme of interpretation of the readings taken
Stable low readings may be evidence of leaks through the gasket between the intake manifold and the cylinder head, or between the pipeline and the throttle body. It is also possible that the tightness of the vacuum hose is broken, the ignition timing fails (backward), or violations of the installation of the gas distribution phases. Check up installation of a corner of an advancing of ignition by means of a stroboscope, then one by one exclude all other possible reasons, carrying out the checks listed in the present Chapter, only after that it makes sense to remove a cover of a GRM drive for the purpose of check of correctness of alignment of setting marks.
If the measurement result is 80-200 mm Hg Art. below the norm and at the same time there are fluctuations, the cause of such a deviation may be a violation of the tightness of the inlet pipe gasket in the area of the inlet port, or a malfunction of the fuel injection injector.
Regular deviation of readings down from a stable value by 50-100 mm Hg. Art. with a high degree of probability indicates leakage of valves. Check the compression pressure in the cylinders or perform a leak test.
Irregular deviations and resets of readings may be due to stuck valves or misfiring. Measure compression pressure, test for leaks, check the condition of the spark plugs.
Frequent vibration of the meter needle with an amplitude of 100 mm Hg. Art. at idle speed of the engine, accompanied by a smoke emission from the exhaust pipe, indicates wear of the valve guides. Do a leak test (see previous Section). If the needle starts to vibrate as the engine speed increases, check for signs of leaks in the intake manifold and cylinder head gaskets. Assess the degree of valve spring sagging, check for signs of burnt valves and try to detect misfires.
Minor fluctuations within the range of 25 to 50 mmHg. Art. can be considered as evidence of a malfunction in the functioning of the ignition system. Verify that all normal settings are correct and, if necessary, test with an Ignition Analyzer.
At deviations of meter readings in a wide range check compression pressure or perform a leak test to identify a defective cylinder or head gasket failure.
If meter needle slowly «walks» over a wide scale range, check the patency of the crankcase ventilation system (PCV) and idle mixture, also check for leaks at the carburetor/throttle body gaskets and intake manifold.
Estimate the rate of recovery of the vacuum gauge readings when the throttle valve is closed after it has been fully opened quickly. The indication should first fall to almost zero, then rise above the value characteristic of normal idle speed by about at 130 mm Hg. Art. and return to idle speed. If the depth of depression is restored slowly and does not form a peak throw when the throttle is closed abruptly, check for worn piston rings. With a long delay in the return of readings, check the patency of the exhaust system (often the muffler or catalytic converter is blocked) - the easiest way is to simply disconnect the suspect section of the exhaust system and repeat the test.
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