Causes and solutions of low compression pressure of cylinder

The reason why the compression pressure of the cylinder is small is that the piston ring and the inner wall of the cylinder have a gap due to friction, which leads to poor sealing between the valve and the valve seat. Install the cylinder pressure gauge at the spark plug position, and after the piston is moved by external force to complete the stroke, the cylinder pressure can be read from the gauge. In actual automobile maintenance, vehicles are often difficult to start because of carbon deposition in the valve and insufficient cylinder pressure. In order to solve these problems, it is usually only necessary to clean up the carbon deposits in the valves and combustion chambers. Cause analysis of low compression pressure in cylinder When the engine is working normally, gasoline and engine oil that normally enter the combustion chamber cannot be completely burned in the cylinder under the condition of insufficient oxygen supply, resulting in soot and burnt particles of lubricating oil. If the engine continues to run, it will be further oxidized into colloid. Firmly bonded to piston crown, piston ring, valve back, inner surface of intake pipe, throttle body, combustion chamber and other parts. Under the repeated action of high temperature, colloid is transformed into asphaltene, colloid and carbon, forming carbon deposit. These colloids with high colloid content (poor gasoline quality) enter the combustion chamber together with gasoline through the fuel supply system of the vehicle. After burning gasoline, the fuel injector, the combustion chamber of the engine, the piston ring groove, the spark plug and the fuel entering the fuel supply system are always stopped in the case of urban road congestion, so the engine cannot run at high speed, and the fuel and lubricating oil escaping from the combustion chamber cannot burn 100%. Some unburned engine oil forms colloid under the action of high temperature and oxygen, adheres to the surface of internal parts of the engine, and forms carbon deposit under the action of high temperature. Processing and replenishing high-quality gasoline. Impurities such as wax and colloid in gasoline are the main components to form carbon deposits, so gasoline with high cleanliness has a weak tendency to form carbon deposits. Please note that high symbols are different from high quality. That is to say, 97 oil is not necessarily less than 93 oil. The symbol only indicates the octane number of the oil, not the quality and cleanliness. Adding gasoline detergent to gasoline can effectively prevent the formation of carbon on metal surface, gradually activate and remove the original carbon particles, and protect the engine from damage. However, be careful when adding gasoline cleaner. If you put in inferior products, the effect will be counterproductive. Don't sit around for a long time. Ordinary EFI vehicles do not need preheating. If the cold car is a hot car, the idle time is too long, the time for the engine to reach normal temperature will also be extended, and the evaporation rate of gasoline sprayed on the back of the valve will slow down and carbon will be deposited. At the same time, it has been idling, so the amount of air entering the engine is less, and the cleaning effect of carbon deposits is also weakened, which promotes the accumulation of carbon deposits. So please don't be idle for a long time. Run at high speed and try to improve the shift speed of manual gear. The purpose of high-speed operation is to prevent carbon deposition by using the scouring effect of airflow on the inlet. In addition, improving the shift speed is similar to running at high speed. Replacing the original 2000 rpm gear shift with 2500 rpm can not only effectively prevent the generation of black smoke, but also improve the power of the car, avoid the explosion caused by too low gear shift speed and protect the compression pressure of the engine cylinder. Fault diagnosis and analysis:) 1) Verify and reproduce the fault and identify the fault. The user went to the station to check the fault at noon. The weather was very hot and the temperature reached 39. The user repeatedly said that it could not be solved after checking the fault. Technicians verified the fault, and the engine started well and ran smoothly, and the fault could not be reproduced. Through the user's description of this fault phenomenon, this fault is relatively rare. In order to determine whether the fault is true and consistent with the user, first put the car in the station and verify the fault the next morning. The next morning, the technicians started the engine several times, and there were signs of a car, but the engine could not be started. 2) Check and confirm the cause of the failure. Use the resistance DC voltage of multimeter to detect the battery voltage as 14.8V and the starting voltage as 12.5V, so as to eliminate the reasons of insufficient battery power, broken wiring and excessive starting wiring resistance. When starting, the starter acts strongly to confirm that the starter functions well. When the ignition key is turned on, it doesn't work, and the sound of fuel flow is heard instantly. Connect the fuel pressure gauge, check that the fuel pressure is 30 1kPa, and confirm that the fuel pump is in good condition. The high-pressure fire is checked well, and the fault still exists after replacing the ignition coil and engine ECU. Remove the four spark plugs respectively, check that the spark plug electrode gap is normal and the combustion is good, and use the cylinder pressure gauge to detect the cylinder pressure. The cylinder pressures of 1~4 are 505kPa, 482kPa, 493kPa and 5 19kPa respectively. The cause of the failure is that the cylinder pressure is too low, and the normal cylinder pressure should not be lower than 1000kPa. There are two reasons for the decrease of cylinder pressure: piston ring fracture wear or gas pipe wear; The valve clearance is too small, the valve cannot be closed, or the valve is stuck in the pipeline and cannot be reset. According to the maintenance experience, this car has just traveled 2739km, which is not the cause of cylinder wear. The intake valve clearances of 1~4 cylinder are 0.4mm, 0.5mm, 0.53mm and 0.43mm, and the exhaust valve clearances of 1~4 cylinder are 0.3mm, 0.3 1mm, 0.29mm and 0.3mm respectively. If the intake valve clearance is too large, remove the cylinder head and have a look. It is understood that after the cylinder head is removed, the carbon deposit is attached to the conical surface of the intake valve with a thick thickness. Hard and smooth carbon deposits are attached to the intake manifold and the intake part of the hood. Causes of failure: too much hard carbon deposit on the conical surface of intake valve, compressed air mixture during cold start, leakage of gap between intake valve and valve seat, too low cylinder pressure, too high temperature during daytime in summer or softening of carbon deposit during hot start, and poor valve seal. However, the fault diagnosis of engine cold start: remove the hard carbon deposits on the intake manifold, intake valve and cylinder head intake, and clean them with cleaning agent. After assembly, the engine intake valve clearance is normal, and the engine starts and runs well. Two days later, I paid a return visit to the user and confirmed that the fault did not appear again, and the fault was completely eliminated. Case summary: The reason of carbon deposit in the tank is that users use inferior fuel when they go out, which leads to hard carbon deposit in the engine intake system. Please pay attention to refueling at large regular gas stations. What is the content? Teach you to be a first-class car mechanic (second edition) is sold in Dangdang, Tmall, JD.COM and Taobao, and edited by Xiaofei Zhou.