A True Statement: “Open the Engine and You’ll Know What Caused the Engine Malfunction”. Leak-Down Tester to Figure Out Where the Problem is Before Opening.

How to

In a 4-stroke engine, compression in the combustion chamber is at its highest when the intake and exhaust valves are closed and the piston reaches top dead center, but if compression leaks for any reason, sufficient power cannot be generated. There are two main reasons for compression leakage: intake and exhaust valves are not closed or piston rings are not airtight. Overhauling the engine will reveal the cause, but a leak-down tester will help you make some assumptions before disassembly.

Roles and differences between compression tester and cylinder leak-down tester 


When using a leak-down tester, the T (top) mark on the flywheel is used to align the piston with the top dead center of compression. Note that there are two top dead center points: the compression top dead center and the exhaust top dead center, and even if the T mark is the same, the valve is open at the exhaust top dead center. When measuring and adjusting valve clearance, be careful not to mistake the top dead center for the top dead center of compression and the top dead center for the top dead center of exhaust.

Good compression is important for an engine. Air and gasoline are sucked into the engine, but if they are not compressed well in the cylinder, the piston cannot be pushed down hard enough to burn the mixture explosively.

There are two ways to think about engine "compression". One is the "compression ratio. This can be calculated from the cylinder volume (piston bore x stroke) and the combustion chamber volume. The greater the ratio of compression of the air mixture drawn into the cylinder, the greater the pressure generated during combustion 5:1-11:In most cases, the range is about 1.

Another concept is "compression pressure. While compression ratio is a calculated value, compression pressure indicates the pressure generated in the combustion chamber when the piston strokes. Compression pressure is an important criterion for determining actual engine condition and engine performance.

Compression pressure depends on the model (engine). Roughly speaking, engines with lower compression ratios tend to have lower compression pressures, and engines with higher compression ratios tend to have higher compression pressures.

In the case of the Yamaha SR400 appearing in the image, according to the service manual, the standard value is 10.5kg/cm2 with a lower limit of 8.5kg/cm2. Compression pressure is measured with a compression gauge, and the value is 8.5kg/If it is less than 1.5 cm2, there is some kind of trouble inside the engine.

This is because low compression pressure = no airtightness between the cylinder and combustion chamber = the high pressure generated by the explosive combustion of the air mixture is not only pushing the piston, but also leaking somewhere. Where the mixture leaks is generally "around the valves" or "piston rings". Compression pressure is measured by attaching a compression gauge to the plug hole and using the starter motor or kick pedal to turn the crankshaft and stroke the piston to open and close the intake and exhaust valves.

If the compression pressure does not increase to the specified value, the possible causes include the intake and exhaust valves not closing completely at the piston compression top dead center, where the pressure in the combustion chamber rises the most, a wide gap between the piston ring fittings, or low tension pressing against the cylinder wall. These can be determined by disassembling the engine.

However, by utilizing a "cylinder leak-down tester," which is different from a compression gauge, the condition of the engine and the cause of compression leaks can be assumed before disassembly. And by utilizing the measurement results of the leak-down tester, engine maintenance may be carried out more efficiently.

POINT
  • Point 1 - There are two types of testers that measure the internal condition of the engine: compression gauges and leak-downt testers.
  • Point 2 - Compression gauge measures compression pressure and leak-down tester measures compression leakage.

Leak-down tester that can measure air tightness in the combustion chamber by the amount of compressed air leakage


The leak-down tester, which can be purchased at any Internet-based tool store site, connects the coupler on the left side of the two gauges to an air compressor and screws the adapter at the end of the black hose on the right side gauge into the plug hole. With the piston at the upper dead center of compression, the regulator under the left gauge is gradually opened to feed air at the measured pressure into the cylinder, and the leak rate is displayed on the right gauge.


Since compressed air is injected through the plug hole, if the piston rotates even slightly from top dead center, it will be pushed down with great force. If the piston is completely at top dead center, the connecting rod and crankshaft will be in line and stick together, so the piston will not move.


When using the leak-down tester, be sure to close the regulator when connecting the air compressor hose to the tester. If the hose is connected with the regulator open, there is a risk of high pressure air flowing suddenly, damaging the gauges or causing the piston to descend too vigorously. Close the regulator, connect the air hose, and check that the piston is at the upper dead center of compression before gradually adding pressure./color]


This gauge indicates a supply pressure of 30-40 psi from the compressor, so adjust it with a regulator and check the leak rate on the right side. A 0% leak rate on the far right indicates zero leakage, and a 100% leak rate on the far left indicates total leakage. When the valve is open, at 100%, air sounds can be heard from the intake or exhaust, and at compression top dead center, the needle on the gauge swings to the right, indicating less leakage. The gauge will determine that a leak of up to 40% is LOW, but if the engine has just been overhauled and the intake and exhaust valves and piston rings are in good condition, it may be 10% or even less.

The cylinder leak-down tester injects compressed air from an air compressor through the plug hole with the piston fixed at the top dead center position of compression, and measures the leakage rate relative to the injected volume. If the intake and exhaust valves are completely closed and the piston ring clearance and tension are sufficient, the amount of injected air leakage is zero or minimal.

In contrast, if the valves do not close tightly due to aging or trouble, or if the piston rings wear out, the airtightness deteriorates and a percentage of the injected air escapes from the combustion chamber. The leak-down tester displays the amount of leakage relative to the injected air on a gauge as the leak rate.

The smaller the leakage rate, the higher the compression pressure measured by a compression gauge because of air tightness, and the larger the leakage rate, the lower the compression pressure because of large air leakage. Since there is a causal relationship between compression pressure and leakage rate, if the compression gauge shows that the compression pressure is below the lower limit, maintenance involving disassembly is necessary, not to mention measuring leakdown.

However, even if the compression pressure is within the default value, the leakage rate may not be small. This is the case when a part of the pressure obtained by combustion is leaking, but the compression pressure itself is within the acceptable range. Even if the compression gauge alone is OK, if the leak rate requires attention, it may be better to perform maintenance. In addition, the leak rate of a single cylinder SR is one, but in the case of a 4-cylinder engine, the leak rate is for all 4 cylinders, so you can see the variation in each cylinder.

It is true that even without a leak-down tester, a compression gauge can tell you the condition of the engine and whether or not maintenance is needed. However, since the leak-down tester measures the engine in a static state, without running the engine, it can tell you where the compressed air that the piston let in through the plug holes at the top dead center of compression has escaped.

For example, if there is carbon embedded in the contact surface of the intake valve, the released air will flow from the intake manifold to the carburetor side (or throttle body side if the car is injected). Conversely, if the exhaust valve is not fully closed, air will flow to the muffler. If the gap between the piston rings is wider than the standard gap or the rings themselves are under tension, air will flow through the gap between the piston and cylinder and into the crankcase.

The "sound" made by the flowing air then helps to locate the leak.

When the leak rate on the tester is high, the cause of air tightness loss can be found at the air cleaner box inlet, muffler outlet, or oil inlet with the cap removed where you hear a "sizzling" sound when you put your ear close to the cap.

If the compression pressure is below the specified value on the compression gauge and there is a leak from the valve, it could be due to foreign objects being caught in the valve seat and valve face, or the seat surface has worn over time, causing the contact surface to widen and the surface pressure to drop, resulting in a loss of airtightness. On the other hand, it is also possible that the valve seat has worn down, causing the valve to sink to near zero or even negative valve clearance.

If the cause is an over-clogged valve clearance, proper adjustment may cure the leak. When compression pressure is lower than specified, it is standard practice to measure valve clearances before disassembling the engine in a panic, but in parallel, measuring leak-down can help pinpoint the cause.

In most motorcycle engines, the crankcase and cylinder are split (in many newer supersport engines, the case and cylinder are one piece), so it is easy to visually check the piston rings when the engine is disassembled.

However, in automobile engines, the cylinder and engine block are one piece, and removing the piston and piston rings requires removing the crankshaft and pulling from the lower part of the block, which is an extremely large job. A leak-down tester can be used to measure the leakage in such an engine, and if there is no leakage into the lower part of the block, the piston may not have to be pulled out, which can be very helpful in identifying the scope of the work.

Although the leak-down tester is not as common a measuring device as the compression gauge, it is good to know that a good use of both will give some idea of engine condition before disassembly.

POINT
  • Point 1 -leak-down tester determines air tightness by injecting compressor air through the plug hole with the piston at the top dead center of compression.
  • Point 2 - By listening to where the air flows, you can identify where the problem is.
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