What is the reason for the high vacuum pressure of the engine?

Application of vacuum measurement in engine fault diagnosis 1 Fault example A Honda Accord car in Japan entered the factory for maintenance. The driver reported that the car suddenly accelerated and turned off, and when accelerating, it made a "beep" sound similar to carburetor tempering and air filter shooting. After preliminary analysis, it is suspected that there is a fault in the intake and exhaust system. Measure the vacuum degree of the intake pipe with a vacuum gauge. When idling, the pointer of the vacuum gauge swings back and forth beyond 45 kPa, and the reading of the vacuum gauge drops to 37 kPa regularly when the engine is running, so it is diagnosed that the valve of a certain cylinder is poorly sealed. When the cylinder head was disassembled for inspection, it was found that the intake and exhaust valves of the second cylinder were bent and deformed, resulting in irregular closing of the intake and exhaust valves of this cylinder. After replacing the valve, the fault disappeared. In fact, by measuring the vacuum degree of engine intake manifold, many faults can be easily analyzed, and the diagnosis range of faults is wider than the usual method of measuring cylinder compression pressure. 2 Application of vacuum measurement in fault diagnosis There are many hoses in the engine of modern imported cars, most of which are directly connected with the intake manifold at one end. The purpose is to use the vacuum generated in the intake manifold when the engine is working as the power source of various auxiliary equipment or the signal source of related sensors. At altitude, the so-called complete vacuum is about 10 1.5 kPa. The vacuum degree and stability of engine intake manifold are related to the number of cylinders, speed, sealing performance, ignition performance, air-fuel ratio of mixed gas and throttle opening. For the normal working engine, the magnitude and change of vacuum in the intake manifold have a fixed range and law. Conversely, if the vacuum degree deviates from the normal value, the engine must be faulty. Common causes of abnormal vacuum reading include misfire of one or more spark plugs, broken air hose or loose hose connector, poor valve seal, air leakage from cylinder head or intake manifold gasket, serious air leakage from piston ring, failure to close exhaust gas recirculation valve (EGR) and full opening of positive crankcase ventilation valve (PCV) due to blockage. Different reasons correspond to different readings of vacuum gauge. It is helpful for fault diagnosis to master the correct readings of vacuum gauge under common working conditions and some abnormal situations caused by faults. 2. 1 When the engine is idling, the reading of the vacuum gauge of the engine intake manifold should be stable between 57.6 kPa and 7 1. 1 kPa. If the reading of vacuum gauge is abnormal during idling test, the following tests should be carried out: a) check the basic ignition timing; B) Check the valve timing; C) Check the cylinder compression pressure; D) Check the positive crankcase ventilation valve. 2.2 Idle speed test was carried out under the condition of air leakage of cylinder gasket, and the reading of vacuum gauge was low, and the pointer swung greatly between 17 kPa and 64 kPa. 2.3 The test of engine sudden acceleration or deceleration can reflect the severity of piston air leakage. When the engine is idling, let the engine suddenly accelerate and decelerate, and observe the reading of the vacuum gauge at the same time. When accelerating rapidly, the vacuum gauge reading will suddenly drop; During the sudden deceleration, the pointer of the vacuum gauge will jump forward several squares in the idling position. For example, when the vacuum gauge is idling, the reading is 60.9 kPa, which will drop to 0 kPa ~ 10.2 kPa when it suddenly accelerates, and then jump to 77.9 kPa~84.7 kPa when it suddenly decelerates. When the piston leaks seriously, the swing amplitude of the vacuum gauge pointer will not be obvious. The greater the swing amplitude of the pointer, the better the technical condition of the engine. 2.4 The ignition timing or valve opening time is too early or too late. If the ignition timing or valve opening time is too late, the pointer of the vacuum gauge will swing slightly between 46.6 kPa and 67 kPa; If the ignition timing or valve opening is too early, the pointer will swing greatly between 46.6 kPa and 67 kPa. 2.5 The exhaust system is blocked. In this case, when the engine is idling, the reading of the vacuum gauge can sometimes reach 53 kPa, and it will soon drop to 0 or very low. When the engine accelerates, the reading gradually drops to 0. 2.6 When the valve is burnt out or the valve clearance is inappropriate, the pointer of the vacuum gauge is stable, but when the faulty cylinder works, the pointer drops, and the drop value is above 6.7 kPa. 2.7 The valve is stuck. In this case, the vacuum gauge pointer will return irregularly. In order to test this situation, the engine can be run at a speed of about 2 500 r/min for about 2 min, and the valve stem can be heated to a certain extent. If the pointer of the vacuum gauge shakes violently for a short time at idle speed, it indicates that the valve is stuck. After the valve is cooled, the jitter of the vacuum gauge pointer will be alleviated. 2.8 Wear of valve guide In this case, when the engine is idling, the pointer of the vacuum gauge shakes violently, but when the speed increases, the pointer is stable. 2.9 When the mixture ratio is inappropriate or the ignition difference of individual cylinders fails, the reading of vacuum gauge is lower than normal at idle speed, and the pointer swings slowly between 44 kPa and 67 kPa when the mixture is rich; When the mixture is thin, the pointer drops and rises irregularly, with a large swing and often idling. Seven, using vacuum gauge to diagnose the fault of gasoline injection engine For gasoline engines, the tightness, ignition and air-fuel ratio of the intake system are the three major factors that affect the performance of gasoline engines, among which the tightness of the intake system is particularly critical. The factors that affect the sealing performance of gasoline engine cylinders are: the state of cylinder, cylinder head, cylinder gasket, piston and piston ring, valve and valve seat, valve guide, valve spring and other related parts. The following four methods can be used to detect the sealing performance of the cylinder: measuring cylinder pressure, measuring cylinder air leakage (or air leakage rate), measuring crankcase blowby, and measuring intake pipe vacuum. See table 1- 1 for the comparative analysis of the test results of the four methods. Table 1 Comparative test method of cylinder tightness Test method Work content Test state coverage content Test result analysis and research Test cylinder pressure Remove spark plug and air cleaner filter element, measure cylinder compression pressure of each cylinder with cylinder pressure gauge, and dynamically test cylinder compression pressure driven by starter. Check the cylinder, cylinder head, cylinder gasket, piston and piston ring, valve and valve spring valve guide, intake pipe gasket, injector sealing ring, pressure sensor hose, intake hose starter speed, excessive oil in cylinder wall and annular groove, carbon deposition in combustion chamber, and high compression ratio caused by repeated grinding of cylinder bore and cylinder head. , and remove the spark plug. Lock the transmission system when the piston is at the top dead center of the compression stroke, apply constant pressure compressed air, observe the no-load static detection of the pressure drop of the instrument, and detect the air leakage of the top dead center, cylinder head, cylinder gasket, piston and piston ring, valve and valve spring. It can be judged from the air leakage at the air inlet and outlet, fuel filler and water tank cover. The valve guide, intake pipe gasket, injector sealing ring, pressure sensor hose and intake hose only detect the air leakage at the top dead center, not the full stroke of the cylinder, and do not cover the influence of "cylinder pulling". It is necessary to use a special instrument (gas leakage meter) to detect crankcase blowby and install a gas leakage flowmeter at the oil filler. Cylinder, piston and piston ring are valves, valve seats and valve springs that comprehensively reflect the total air leakage of each cylinder. No air leakage outside the cylinder can be detected, which cannot reflect the real sealing performance of a specific cylinder. However, the coverage is too narrow, so it is necessary to use other methods to assist in detecting the vacuum degree of the intake pipe. Remove the air filter element and connect the vacuum gauge after the throttle. No-load dynamic detection for measuring the vacuum degree of intake pipe can measure the sealing performance of cylinder, cylinder head, cylinder gasket, piston and piston ring, valve and valve seat, valve guide and valve spring, intake pipe gasket, injector sealing ring, pressure sensor hose and intake pipe under various working conditions, and can sense the air-fuel ratio, ignition timing and ignition quality. The detection value is true and the coverage is wide. It is the main means for dynamic detection of comprehensive index performance without disassembly. It is also a necessary testing instrument for electronically controlled gasoline injection gasoline engines. From the comparison of table 1- 1, it can be seen that the vacuum degree of intake pipe can comprehensively reflect the state, air-fuel ratio, ignition performance and so on. All related components of gasoline engine have high reliability. Therefore, the vacuum detection method of intake pipe should be widely used in modern cars. 1 Mechanism and function of intake pipe vacuum degree 1. 1 Mechanism of intake pipe vacuum degree generation and change. When a gasoline engine is running, a vacuum degree will be generated in the intake pipe, which can be expressed by δ px. Δ px is the sum of the negative pressure on the intake pipe when the cylinders of the gasoline engine alternately intake air. Its numerical value and stability are directly proportional to the number of working cylinders, the speed of gasoline engine, the tightness of intake system, the ignition performance of ignition system and air-fuel ratio, and inversely proportional to the throttle opening. Speed and throttle opening are the basic characteristics of gasoline engine, which directly affect air-fuel ratio and combustion condition. The magnitude and fluctuation range of δ px value reflect the working condition of gasoline engine. For example, under the condition of constant throttle opening, if the speed of gasoline engine decreases, the quality of mixed gas will deteriorate and the combustion conditions will deteriorate, which will slow down the combustion speed of combustible mixed gas and further reduce the speed. At this time, the Δ px in the intake pipe will decrease, which will affect the fuel injection quantity, thus forming a chain reaction. In addition, when the throttle opening, the tightness of the intake system, the ignitability of the ignition system and the air-fuel ratio change, the value of Δ px will also be affected. Therefore, Δ px becomes the "central medium" for causal feedback of gasoline engines. The mechanism of causal feedback of intake vacuum is shown in figure 1-6. Sealing performance of intake system Δ px (rotational speed+opening) Ignition performance Air-fuel ratio A/F diagram 1-6 Mechanism of causal feedback of intake pipe vacuum degree 1.2 Application of intake pipe vacuum degree Using vacuum gauge to detect Δ px of gasoline engine, the sealing performance of intake system can be judged. This method is not only simple and easy to operate, but also covers a large area, so it is an effective detection method. The electronically controlled gasoline injection engine is based on Δ px to measure the fuel injection quantity. Because the installation port of the fuel injector is a potential air leakage point, and the vacuum tubes at the front and rear of the throttle valve are arranged crosswise, it is easy to cause air leakage and installation errors. Therefore, the function of vacuum gauge will be more important when troubleshooting. 1.2 What can be covered by detecting the vacuum degree of intake pipe 1.3. 1 Sealing performance of intake system Internal factors of cylinder: cylinder, cylinder gasket, cylinder head, piston, piston ring, valve and valve seat. External factors of cylinder: valve guide, valve spring, hydraulic support rod, intake pipe gasket, injector sealing ring, throttle body gasket, intake hose, etc. It is worth noting that air leakage outside the cylinder has a greater influence on Δ px than air leakage inside the cylinder. If the external seal becomes worse, the gasoline engine will not work normally. 1.3.2 blockage of exhaust system in order to reduce emission pollution, most modern cars are equipped with three-way catalytic converters. In the process of use, the internal part or random blockage is caused by glue accumulation, carbon accumulation, damage and other reasons. When exhausting, the back pressure will increase, and the δPX will be too low, resulting in incomplete exhaust and insufficient intake. 1.3.3 air-fuel ratio A/F Whether it is carburetor or electronically controlled gasoline injection engine, the preparation of combustible mixture is controlled by δ PX. If the A/F value is not accurate, the combustion conditions will deteriorate, which will further affect the speed and the value of δ px. 1.3.4 ignition performance, valve timing, ignition time, spark energy, misfire or misfire of each cylinder, etc. Will affect the speed and δ px value. Valve timing is not only the premise of ignition timing, but also directly affects the rotational speed and the value of Δ px.1.4. Theoretical analysis of adjusting ignition timing according to vacuum gauge shows that when the tightness of intake system and air-fuel ratio are normal, the dynamic optimal ignition advance angle should correspond to the maximum value of Δ px. Practice has proved that when a single cylinder is on fire, if the cylinder works normally, the δ px value will decrease obviously; If the cylinder does not work originally, the δ px value will not change. When the optimal ignition advance angle increases or decreases, Δ px decreases. Monitoring ignition timing and ignition quality with vacuum meter is not only simple, but also as accurate as using ignition timing lamp and tachometer. The influence of ignition advance angle on Δ px is shown in figure 1-7. Figure 1-7 Influence of ignition advance angle on Δ px When adjusting the ignition advance angle, if it is found that the value displayed by the vacuum meter is lower than the normal value, the position corresponding to the maximum value of Δ px can be found by rotating the distributor housing, and then the distributor corresponds to the optimal ignition advance angle. 2. Detection method of intake pipe vacuum degree and failure mechanism analysis 2. 1 Detection method Detect intake pipe vacuum degree, connect the vacuum gauge behind the throttle valve (as shown in Figure 1-8), normally make the gasoline engine run at the specified idle speed, and check the reading and indicating status of the vacuum gauge. Figure 1-8 Inspection position of sealing performance of intake system1-cylinder 2-piston and piston ring 3-cylinder head and cylinder gasket 4-valve and valve seat 5-valve spring and valve guide 6-hydraulic strut 7-intake pipe gasket 8-fuel injector sealing ring 9-vacuum pipes before and after throttle valve 165438. When idling, the pointer should be kept at 64 kPa ~ 7 1 kPa (swing amplitude and swing speed are related to sealing, air-fuel ratio and ignition performance). If it is suspected that a cylinder is not working properly, the single cylinder fire-extinguishing method can be used for diagnosis. Quickly open and close the throttle. If the pointer swings sensitively between 6.7 kPa and 84.6 kPa, it means that Δ px has good following performance to the throttle opening, that is to say, all components have good sealing performance under all working conditions. B. poor sealing. When idling, Δ px is lower than normal and obviously unstable. When the throttle valve is opened quickly, the watch hand will drop to zero, and it will not return to 84.6 kPa after it is closed. C the ignition time is too early or too late or the spark energy is insufficient. When the ignition time is too early or too late, the spark energy is insufficient or the valve timing is inconsistent, the combustion conditions will deteriorate, the power loss of the gasoline engine will increase, the rotational speed will not be improved, and a high vacuum degree cannot be formed, resulting in unstable idle speed and weak acceleration. When idling, the pointer swings between 46.7 kPa and 57 kPa. The ignition time is too early, and the watch hand swings greatly; If the ignition time is too late, the hands will swing slightly. D. the exhaust system is blocked. Due to the high back pressure of the exhaust system, δ px can sometimes reach 53 kPa at idle speed, but it immediately drops to very low or even zero. When the traffic jam is serious, the gasoline engine can only keep running at a low speed. See table 1-2 for the fault contents of gasoline engine in abnormal state. Table 1-2 Fault Contents of Abnormal State of Gasoline Engine