Use vacuum gauge to judge engine failure

Carbos column uses vacuum gauges to determine engine faults. □ Wang Zhixin The first condition for the formation of combustible gas mixtures is that the gas in the float chamber can only be sucked out of the nozzle by creating a sufficiently large vacuum at the nozzle of the carburettor. During the intake stroke, the intake valve opens, the piston descends from top dead center, the cylinder volume increases, and the air pressure in the cylinder Pa is less than the atmospheric pressure Po. The difference AP=P−Pa is the so-called vacuum degree. It can be seen that the seal between the piston and the piston ring and the cylinder wall, between the cylinder head (pad) and the cylinder block, between the valve and the valve seat, between the intake manifold (pad) and the cylinder block will affect the intake air The degree of vacuum in the tube. In other words, the degree of vacuum in the intake manifold will be affected no matter where a failure occurs from the intake pipe to the exhaust port. Vacuum test the degree of vacuum, you can from the side to reflect the quality of the various parts of the engine seal, help to quickly eliminate engine failure.

In order to better use the vacuum gauge, the initial ignition timing and the idle limit must be adjusted strictly according to the technical requirements before the test. If these operations can be performed accurately, any deviation from the normal vacuum value indicates that the engine is faulty.

When measuring, the vacuum of the vacuum gauge must directly originate from the intake manifold, because only the vacuum of the intake manifold is directly derived from the vacuum of the engine. In order to distinguish the faults reflected by the vacuum values ​​under different operating conditions, the vacuum degree of the test engine intake manifold can be divided into three basic types: idle test, rapid acceleration test and exhaust system obstruction test.

When the engine is working normally, the vacuum level measured with a vacuum gauge under idle conditions should be 50~70kPa. If it is not within this range, it should be checked and analyzed according to different conditions to determine the fault.

The vacuum gauge reading during idle test is not normal, and the following inspection should be performed: 1 check the initial ignition timing; 2 check the timing of the gas distribution; 3 check the cylinder pressure; 4 check the crankcase forced ventilation control valve.

During the idle test, the vacuum gauge pointer is regularly dropped by 6~9kPa, and the following inspections shall be carried out: 1 detecting the non-working spark plug; 2 detecting the burned valve (pressure test); 3 detecting the burned piston (pressure test) .

If the vacuum gauge reading drops irregularly to 10,27 kPa, the following checks should be made: 1 Check spark plug; 2 Look for stuck valve; 3 Look for stuck valve lifter or hydraulic tappet.

If the vacuum gauge pointer slowly swings between 27~34kPa, the following work should be done: 1Adjust the carburettor (mixed gas may be too strong); 2Check the spark plug (gap plug clearance may be too small).

Between the rapid swing, indicating that the inlet valve tappet and the catheter wear, with the loose; if the vacuum gauge pointer slowly swing between 34,76 kPa, and the increase in the engine speed as the rotation speed increases, indicating that the valve spring elasticity is insufficient.

A large swing between them is caused by air leakage from the cylinder liner.

If the engine idling speed is too high, the test manifold vacuum is less than 40 kPa, which means that the manifold or the main pipe behind the engine throttle leaks, and the leaked parts are mostly manifold pads and pipelines connected to the manifold (such as vacuum booster trachea Wait). For example, once we repaired a Mercedes-Benz 560 sedan, we used this method to detect an air leak, and used the elimination method to determine the location of the fault in the control valve, and finally determined that a power fuse that controls its function was blown. After replacing the fuse, troubleshooting.

If the engine is difficult to start and the stable idling cannot be guaranteed, the vacuum level of the test engine is above 50 kPa, indicating that there is no problem with the engine intake line, and the fault is caused by poor ignition or fuel injection (such as ignition coil failure) caused by the electronic control system.

Tested during rapid acceleration, it can show the degree of piston leak. In case of rapid acceleration, the reading of the vacuum gauge should suddenly drop; in the case of rapid deceleration, the vacuum gauge pointer will jump significantly forward in the position of the original idle speed.

That is, when quickly opening and closing the throttle, the vacuum gauge pointer should swing between 7, kPa.

If the piston leaks badly, the swing of the vacuum gauge pointer will be less obvious. The larger the swing of the vacuum gauge pointer, the better the technical condition of the engine.

If the vacuum gauge pointer is lower than normal during idling, the pointer will fall back to *0* near rapid acceleration, and the pointer will not rise to approximately 86kPa when the throttle is suddenly closed, usually due to the piston ring, intake pipe or carburetor liner. Leakage caused.

This test was performed under exhaust system obstruction test conditions, and the vacuum gauge reading was carefully observed. If the reading gradually decreases, it indicates that there is an obstruction in the exhaust system.

When using the vacuum gauge to test the vacuum, you should also pay attention to the following points: 1) When the engine is idling, if the valve is stuck, the vacuum gauge pointer will be returned at irregular intervals.

2) If the valve clearances are all too small, the vacuum gauge reading will be low, approximately between 44~47kPa, and the pointer will oscillate back and forth; if only one valve gap is small, the vacuum gauge pointer will be on each ignition of the cylinder. There will be a regular decline.

When the flammable mixture is too rich, the vacuum gauge pointer will oscillate back and forth, and the exhaust pipe will emit black smoke or even fire. At this time, adjust the carburetor idle adjustment screw.

The above values ​​are all measured at sea level. We know that the degree of vacuum in the intake manifold decreases with increasing altitude. Normally every altitude increase by 500m, the vacuum degree will decrease by 5.5kPa. Therefore, when determining the degree of intake manifold vacuum, it should be based on altitude. The situation is converted. mouth

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