Engine cylinder pressure testing
n group deteriorates, resulting in poor cylinder sealing performance, and the engine power and economy are degraded. Cylinder compression pressure detection is to check the seal quality of the piston ring, valve and combustion chamber. If the cylinder pressure of the engine is too low, the engine will have a power drop, the fuel or oil consumption will increase, the discharge will exceed the standard, and the starting will be difficult. If the engine cylinder pressure is too high, it will cause the engine to deflagrate and start-up difficult. The Uneven pressure in each cylinder of the engine can cause the engine to run rough or lack cylinders. When the above failure phenomenon occurs, the cylinder pressure detection should be performed on the engine. Through the engine cylinder pressure detection, the diagnosis and diagnosis of engine cylinder tightness and whether the intake and exhaust systems are unobstructed, and other detection and analysis to find the fault point to determine which repairs are needed.
1.1 Prerequisites for cylinder pressure detection
(1) The battery is fully charged. Loss of battery power can cause the engine speed to be too low.
(2) The engine coolant temperature is normal. The engine coolant temperature is at a normal operating temperature (above 80 °C). The cold car and the hot car combustion chamber have different sealing degrees, so the measured values are different.
(3) Remove the air filter element.
(4) Remove all spark plugs. The cylinder pressure of each cylinder is measured cylinder by cylinder. Without disassembling the spark plug, the measured cylinder pressure will be higher than the actual cylinder pressure. If one is to be measured, the higher the cylinder pressure will be measured.
(5) For the EFI car, the oil pump insurance or other relay insurance must be removed and tested again, otherwise, it will lead to “draining cylinder” and low cylinder pressure.
(6) The test hole must not leak. Before the measurement, tighten the hose joint of the cylinder pressure gauge and the spark plug hole, and do not leak. Before each measurement, the cylinder pressure gauge should be returned to zero. When the crankshaft is rotated by the starter, the accelerator pedal is fully depressed to the end, and the throttle valve is throttled. Maintain 3 to 5 s in the fully open position (engine speed is above 2500 r/min, engine speed is too low, cylinder pressure will be too low).
(7) Calculation of cylinder pressure. Each cylinder is tested twice, with the average being the cylinder pressure of the cylinder.
1.2 Gasoline engine cylinder pressure check steps
(1) Warm up and stop the engine.
(2) Remove the engine cover.
(3) Disconnect the positive electrode of the ignition system from the primary ignition coil so that it does not work, and insulate other wires with electrical tape so that they cannot come into contact with the ground wire. For the ignition system of the distributorless unit, remove the four ignition coils.
(4) Remove the four spark plugs and blow off all the dust in the pits with an air gun.
(5) Turn off the engine’s fuel pump so that it does not function or disconnect the four injector connectors. Note: When the cylinder compression pressure check is performed, if the injection system does not stop working, the fuel injected from the fuel injection engine will enter the cylinder. If fuel vapor is ejected from the spark plug hole, it may cause a serious explosion and cause personal injury.
1.3 Diesel engine cylinder pressure detection method
The method for detecting the compression pressure of a diesel engine cylinder using a cylinder pressure gauge is as follows:
(1) Start the engine and run it in place. When the engine coolant temperature reaches about 80 °C, stop the engine.
(2) Firstly clean the dust and dirt at the installation hole of the blower.
(3) Disassemble the high-pressure oil pipe and oil return pipe joint on the injector, remove the injector, and install the 6 MPa pressure gauge in the threaded hole of the injector (do not use the hand pressure gauge). A gasket should be added to the joint between the pressure gauge joint and the injector seat to prevent air leakage.
(4) The engine is driven by a starter, and the maximum reading of the pressure gauge is the cylinder compression pressure value.
The measured value should not be lower than 20% compared with the original factory standard value, and the pressure difference of each cylinder should not exceed 8%.
2.cylinder pressure calculation
The cylinder pressures of the cylinders are added, divided by the number of cylinders as the average cylinder pressure. The average cylinder pressure of the engine must not be less than 75% of the standard, otherwise, it needs to be repaired.
(maximum cylinder pressure – minimum cylinder pressure), the average cylinder pressure = average cylinder pressure difference
The cylinder pressure of the overhauled engine shall comply with the original design regulations. The pressure difference of each cylinder of the gasoline engine shall not exceed 8% of the average pressure of each cylinder, and the diesel engine shall not exceed 10%. The cylinder pressure in the vehicle should not be lower than 25% of the original design.
3 inspection results analysis
(1) When the detected value is higher than the specified value by more than 10%, it indicates that there may be accumulated water or oil inside, or too much carbon in the combustion chamber, or the cylinder head gasket is too thin, or the cylinder block and the cylinder head are excessively worn.
(2) When the detected value is lower than the specified value, 20 to 30 mL of fresh oil can be injected from the spark plug hole or the injector hole to the top of the piston, and the crankshaft is rotated several times and then retested. If the pressure rises significantly, it indicates the piston ring and the cylinder. Badly worn.
(3) If there is no obvious change after oil filling, the valve movement flexibility can be checked, such as no card issuance phenomenon, and the adjustable atmospheric door gap retest. If the pressure rises to the specified value, the original valve clearance is too small.
(4) If there is no obvious change after adjusting the valve clearance, the pressure of the adjacent cylinders can be measured. If the pressure value is also low, the cylinder head gasket between adjacent cylinders may be burned through. If the cylinder head gasket is not burnt through, it may be that the valve or valve seat is in poor sealing condition and should be dismantled.
(5) A cylinder that is completely free of pressure may be stuck in the valve, burnt in the gap or burned through the piston, and the piston ring is stuck in the ring groove. The cylinder pressure value changes from high to low in 2 to 3 measurements, probably because the valve is not tightly closed.
(6) If the compression pressure of the cylinder is low, a small amount of engine oil is injected into the cylinder from the spark plug hole, and then checked again. If the pressure increases after the oil are added, the piston ring and cylinder bore may be worn or damaged. If the pressure is still low, it may be that the valve is stuck or improperly seated, or the cylinder head gasket is leaking.
(7) The pressure in the adjacent two cylinders is low, while the other cylinders are normal, which may be due to the leakage of the adjacent two cylinders or the cylinder head bolts are not tightened; the pressure reading of one cylinder or multiple cylinders is high at the same time. It may be because the engine is overheated or knocked, which is a malfunction caused by a change in compression ratio.
4 Check cylinder compression pressure precautions
(1) The cylinder pressure cannot be measured during a cool car. Due to factors such as temperature and atmospheric pressure, the cylinder pressure measured only when the engine reaches the normal operating temperature has a substantial reference value.
(2) For the EFI diesel engine, the fuel pump insurance or other relay insurance must be removed and tested again, otherwise, it will lead to “draining cylinder” and low cylinder pressure.
(3) During the test, the throttle choke must be fully opened, otherwise, the cylinder pressure will be low due to insufficient intake air in the combustion chamber.
(4) Since the cylinder pressure measurement has a certain contingency, it is often inaccurate to measure only once. Only after 2 to 3 tests and then take the average value, the test result is effective and reliable.
(5) The starter running time in the test should not be too long or too short. Excessive time will consume too much power and damage the starter. If it is too short, it will not meet the test standards.