How to Use a Leak Down Tester to Identify the Cause of Engine Malfunction Before Disassembly

The compression pressure measured by the compression gauge is used as a standard to judge the condition of the engine, but the leak down tester can be used to determine the cause of the problem. Whether it is the intake/exhaust valves or the piston rings, the tester is very useful to identify the cause of engine malfunction.

There are several patterns of compression loss.

This leak down tester can measure the amount of air leakage from the intake and exhaust valves and piston rings by connecting the hose of an air compressor, setting the pressure, and sending air through the plug hole of the cylinder with the piston stopped at the top dead center of compression. Be careful to keep the piston completely at top dead center when measuring, or the piston will be pushed down by the air sent in.

Before measuring the compression pressure and leak down, check whether the tappet clearance is appropriate or not. If the clearance is below the lower limit at this point, it can be concluded that the valve seat is worn out. If the tappet is too thin to fit in the 0.01mm thickness gauge, the valve will not close completely because the tappet is riding on the base circle of the camshaft, resulting in low compression pressure and high leak down.

Good compression is one of the most important factors in keeping an engine performing well. Even if the carburetor or injection system produces a good air mixture and the spark plug fires at the right time, if the compression pressure in the combustion chamber is low, the engine will not produce enough power.

When you feel that the punch power of the engine is not good, knowing the compression pressure can be a clue to solve the problem. If you look at the service manual of each model, the standard value of compression pressure is always indicated.

In the case of the Kawasaki Z1, the standard value for compression pressure is 980 kPa, with an operating limit of 690 kPa, and the variation between cylinders is specified to be within 98 kPa. In the 1970s, the standard value was 10 kg/cm2, the limit of use was 7 kg/cm2, and the variation was within 1 kg/cm2.

The maximum compression pressure is at the top of compression when the intake and exhaust valves are closed and the piston is at the top again. If the piston rings, which stroke in contact with the cylinder, become worn, the gap between the rings widens and the compressed air in the combustion chamber escapes. For this reason, there is an upper limit to the gap between the rings.

If the intake and exhaust valves do not close properly, air will leak from the gap between the valve and the valve seat. The standard tappet clearance for the Z1 is 0.05 to 0.10 mm. The standard tappet clearance for the Z1 is 0.05 to 0.10 mm. If the tappet clearance is worn beyond that, the valve will always be open and the compression pressure will drop.

Even if the tappet clearance is correct, if carbon sludge bites into the contact surface between the valve and the valve seat and reduces the tightness, it will lead to a decrease in compression pressure. If the valve seat sinks and the tappet clearance becomes smaller, you can tell by measuring it with a stickiness gauge, but it's troublesome because carbon bite may not change the tappet clearance. If the tappet clearance isn't getting smaller and compression is leaking, you'll have to disassemble the cylinder head to find the cause.

It is easy to disassemble a single-cylinder engine, but not so easy with a four-cylinder engine. if the compression pressure of all four cylinders is measured and only one cylinder has a lower compression pressure, in the case of the Z1, a difference of 98 kPa or more will require repair.

If the cause of the low compression pressure is the intake and exhaust valves, you may be able to restore compression by removing just the cylinder head and sliding the intake and exhaust valves together or cutting the valve seats. However, if the problem is with the piston rings, the cylinder must be removed.

A compression gauge can measure the absolute pressure, but it can't tell you what is causing the pressure to drop below the standard value. If the cause of the pressure drop is only the carbon bite to the intake and exhaust valves, removing the cylinder to check the piston ring gap may be considered as a waste of time if the engine has reached a certain mileage. However, depending on your point of view, it may be considered as a useless work. The leak down tester is useful in such a situation.

Leak down tester that can measure the air leakage from the combustion chamber

If you measure the leak down on an engine that has just had new piston rings installed and valve seats cut, the leak amount should be less than 10%, which is superb. The gauge on the right that shows the amount of leakage, the lower right it points, the lower the amount of leakage.

Leak down testers, as can be assumed from the inclusion of the word "leak," measure "leakage," and provide insight into the exact opposite of compression gauges, which measure compression.

Specifically, it measures the airtightness when compressed air from an air compressor is pushed into the combustion chamber through the plug hole with the piston stopped at the top dead center of compression. While compression gauges, which measure compression pressure by turning the crankshaft with a cell motor or kick pedal, are dynamic, leak-down testers, which measure the pressure without moving the piston, are static.

The leak down tester has two gauges: one shows the pressure after the compressor air pressure has been regulated with a regulator, and that pressure, adjusted to 200-300 kPa, is fed into the combustion chamber. The other gauge shows how much of the air sent into the combustion chamber is leaking, or leaking down.

The leak down amount is not an absolute value like the compression pressure, but a percentage of the leakage against the amount of air put into the combustion chamber. If the valve is completely airtight, the amount of leakage is 0%. On the other hand, if the valve is open, all the air you put in goes out of the combustion chamber, so the amount of leakage is 100%.

At compression top dead center, the intake and exhaust valves are closed, but the piston ring mating gap is not zero, so the air you push in will leak slightly. There will be a slight difference depending on the gauge, but with the gauges I have, if the leakage is in the range of 10-40%, I consider the leakage to be small.

When the amount of this leak is in the 40-70% range, it is considered MODERATE. However, this is a static measurement, so this level of leakage may be acceptable when the engine is actually performing. If the amount of leakage increases to more than 70%, it can be judged that the leak is too large and needs to be repaired.

What makes the leak down tester so useful is that it allows you to understand the condition of the engine, which is often misunderstood with compression pressure alone. Some of the latest engines have a standard compression pressure of over 1500kPa. If you look at the Z1's pressure of 980 kPa with that in mind, it is not unreasonable to conclude that compression is leaking from somewhere.

However, if the leak down amount is less than 20% on a compression pressure of 980kPa, it can be concluded that there is no compression leakage from the intake and exhaust valves or piston rings.

The cause of the malfunction can be identified by where the air flows out.

While the compression pressures of the other cylinders were all at 1000kPa, only one cylinder had almost zero compression. This was most likely due to a human error during assembly.

I set up a leak down tester and it shows that 90% of the air is leaking.

It turned out that the cause of the problem was the intake valve, because the air put in through the plug hole was leaking heavily from the intake manifold. This also meant that there was no need to remove the cylinder = there was nothing wrong with the piston rings.

The leak down tester can be used in combination with a compression gauge to get a more accurate engine condition, but what makes it even more useful is that you can see where the leak is coming from when the leak is large.

It's obvious from the logic of the gauge, but since it forces the compressed air through the plug hole into the combustion chamber, it leaks into a less airtight area and makes the air flowing sound. If the leak is 50%, if the piston rings are worn, you will hear a whooshing sound of air flowing when you remove the oil filler cap. If the exhaust valve is not closed tightly, you will hear the sound from the exhaust system, and if the intake valve is open, you will hear the sound from the air cleaner case.

In fact, this is the biggest advantage of the leak down tester. That is, if the leak is coming from the intake or exhaust valve, you don't need to pull the cylinder, and if you remove the oil filler cap and hear air flowing, you don't need to remove the valve from the cylinder head. Of course, you can do both valve seat cutting and piston ring replacement if you don't feel comfortable with just one or the other, but the role of the leak down tester in letting you know where the cause is for the phenomenon of low compression pressure is not small.

In the example of the Kawasaki KZ900LTD shown here, only the No. 4 cylinder had almost zero compression pressure, indicating a complete problem of some kind. When we connected the leak down tester, we found that 90% of the leak was at the top dead center of the piston compression, and the air was coming out of the intake manifold where the carburetor was removed.

After removing the cylinder head and checking the intake valves, it was discovered that the valve stems had been bent during assembly and were not fully closed. By simply measuring the compression pressure, there are many possibilities, but by using the leak down tester, we were able to find the cause.

The Leak Down Tester is a measurement tool that is sure to become an ally in engine maintenance for older and out-of-print vehicles, as it allows you to identify problems before they are actually disassembled.

The stem of the intake valve (left) that leaked a lot of air is slightly bent. The intake and exhaust valves were installed with the bent stems, and they fit in the valve guides well at that time, but when the cam opened the valves by turning the cell, they could not be closed. I was able to locate the leak with a tester, so I was able to fix it in the shortest time possible.

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