Leak-down tester

A leak-down tester is a measuring instrument used to determine the condition of internal combustion engines by introducing compressed air into the cylinder and measuring the rate at which it leaks out.

Compression testing is a crude form of leak-down testing which also includes effects due to compression ratio, starter/battery condition and other factors. Leak-down testing confines the results to cylinder leakage alone.

Testing is done on an engine which is not running, and normally with the tested cylinder at top dead center, although testing can be done at other points in the compression and power stroke. Pressure is fed into a cylinder via the spark plug hole and the flow, which represents any leakage from the cylinder, is measured.

Leakage is given in wholly arbitrary percentages but these “percentages” do not relate to any actual quantity or real dimension. The meaning of the readings is only relative to other tests done with the same design of tester. Leak-down readings of up to 20% are usually acceptable while greater than that requires a repair. Racing engines would be in the 1-10% range for top performance.

In the United States, FAA specifications^[1] state that engines up to 1,000 cu in (16 L) engine displacement require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. The input pressure is set for 80 psi (550 kPa), and 60 psi (410 kPa) minimum cylinder pressure is the accepted standard.

While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate. For example, a leaking exhaust valve will make a hissing noise in the exhaust pipe while a head gasket may cause bubbling in the cooling system.

How it works

This schematic shows the component parts of a typical leak-down tester. The gauge on the right is held at a standard pressure by adjusting the pressure regulator while the gauge on the left shows an example reading of 85-or 15% leakage.

A leak-down tester is essentially a miniature flow meter similar in concept to an air flow bench. The measuring element is the restriction orifice and the leakage in the engine is compared to the flow of this orifice. There will be a pressure drop across the orifice and another across whatever leaks in the engine. Since the meter and engine are connected in series, the flow is the same across both. (For example: If the meter was unconnected so that all the air escapes then the reading would be 0 or 100% leakage. Conversely, if there is no leakage there will be no pressure drop across either the orifice nor the leak, giving a reading of 100 or 0% leakage).

Gage meter faces can be numbered 0-100 or 100-0, indicating either 0% at full pressure or 100% at full pressure.

There is no standard regarding the size of the restriction orifice for non-aviation use and that is what leads to differences in readings between leak-down testers generally available from different manufacturers. Most often quoted though is a restriction with a .040in. hole drilled in it.(Some poorly designed units do not include a restriction orifice at all, relying on the internal restriction of the regulator. A very unstable standard.). In addition, large engines and small engines will be measured in exactly the same way (compared to the same orifice) but a small leak in a large engine would be a large leak in a small engine. A locomotive engine which gives a leak-down of 10% on a leak-down tester is virtually perfectly sealed while the same tester giving a 10% reading on a model airplane engine indicates a catastrophic leak. The non standard size of the restriction orifice determines the reading which therefore differs for each design.

Some manufacturers use only a single gauge. In these instruments maintaining the input pressure is (hopefully) maintained automatically by the pressure regulator alone. Any error in the input pressure will produce a corresponding error in the reading.

In instruments with two gauges the operator manually resets the pressure to 100 after connection to the engine guaranteeing consistent input pressure and greater accuracy.

Most instruments use 100 psi (690 kPa) as the input pressure simply because ordinary 100psi gauges can be used which corresponds to 100% but there is no necessity for that pressure beyond that. Any pressure above 15 psi (100 kPa) will function just as well for measurement purposes although the sound of leaks will not be quite as loud. An engine pressurized to 100psi must be locked at exactly top dead center or it will rotate under the pressure. This presents a serious danger to the operator. Using less pressure is less dangerous and opens the possibility to test at positions other than top dead center.

Due to the simple construction, many mechanics build their own testers and those instruments function perfectly well.

References

1. ^ Advisory Circular 43.13-1B, chapter eight