The main structure of the cylinder leakage meter consists of two pressure gauges, a pressure regulating valve, and interfaces. Its accessories include a hose with a 14 mm adapter, a 50 cm long hose, and three adapters (10 mm, 12 mm, and 18 mm in size) to accommodate spark plug mounting holes in all gasoline engines. The specific operating method of the cylinder leakage meter is as follows:
(1) Remove the spark plugs from all cylinders and rotate the crankshaft with a tool until the target cylinder reaches top dead center of compression.
(2) Select the appropriate adapter according to the spark plug size and connect the device to the spark plug mounting hole via the hose. Rotate the pressure regulating valve to close the intake passage and connect the air source.
(3) Slowly open the pressure regulating valve to allow high-pressure gas to enter the cylinder. At this time, pressure gauge 1 displays the charging pressure (generally charged to 70 psi~100 psi, 1 psi = 6.895 kPa), and pressure gauge 2 displays the leakage pressure, which can be used to calculate the leakage rate of the combustion chamber. For example, if pressure gauge 1 reads 100 psi and pressure gauge 2 reads 90 psi, the combustion chamber leakage rate is 10%.
Because the piston rings cannot completely seal the combustion chamber, a small amount of gas is allowed to leak between the piston assembly and the cylinder wall. Therefore, the leakage pressure will be slightly lower than the charging pressure. Based on my experience, the normal combustion chamber leakage rate for a cylinder is about 1%. If the leakage rate is much higher than 1%, it indicates poor combustion chamber sealing.
Detail 1: Why should the target cylinder be at top dead center (TDC) of compression?
An engine's working cycle has four strokes. During operation, the positions of the piston and valves constantly change. When the piston is at TDC of compression, both the intake and exhaust valves are closed, and the combustion chamber is in a closed state. Therefore, this is the most suitable time to test the cylinder's combustion chamber sealing.
Detail 2: How to find the cylinder's TDC of compression?
Insert a long, straight iron wire into the spark plug mounting hole until it touches the top of the piston. Rotate the crankshaft with a tool. When the wire moves upward, it indicates that the cylinder is in the compression or exhaust stroke. During the compression stroke, air will blow out of the spark plug mounting hole. Slowly rotate the crankshaft until the wire reaches its highest point; this locates the top dead center of the compression stroke.
Detail 3: Why can it display leakage pressure?
There is a calibrated orifice between the two pressure gauges to limit the flow rate of injected gas. For pressure gauge 2, high-pressure gas is injected through the orifice, while leakage gas leaks through the partially sealed combustion chamber. The orifice's function is to ensure that the gas flow rate injected into pressure gauge 2 equals the normal gas leakage from the cylinder's combustion chamber. In other words, if a cylinder's combustion chamber is properly sealed without any abnormal leaks, theoretically, the pressure readings of the two gauges should be equal. However, in reality, due to differences between engines and errors in tool manufacturing and measurement, the reading of pressure gauge 2 may be lower than that of pressure gauge 1. If you have any questions about the standard leakage rate, it is recommended to compare it with a normal cylinder.
