Leak-Down Test vs. Compression Test

Leak Down vs Compression Test

The engine's primary function is to compress fuel and air, then initiate ignition, ultimately generating heat energy that propels mechanical motion. If the engine fails to compress the fuel and air mixture adequately, it cannot generate the required power for regular operation.

In cases where the engine cannot withstand the pressure it attempts to create, it escapes from unintended points, leading to undesirable outcomes.

While modern engines come equipped with sophisticated computers that detect malfunctioning cylinders, the process differs for older engines. To ensure proper compression, mechanics rely on a compression and leak-down check.

The compression check is more prevalent, and you can rent a tester from your local auto parts store's loan-a-tool program. Conversely, the leak-down tester is less common and requires a reliable tester for an accurate evaluation.

Let’s look at the differences between leak-down vs. compression tests.

What Do Leak-Down Tests and Compression Tests Do?

Both instruments are engineered to gauge cylinder pressure to identify and troubleshoot engine malfunctions.

The compression tester operates independently, utilizing the engine's internal compression to build up pressure within the cylinder, whereas the leak-down tester relies on an external source of compressed air.

For those lacking technical expertise, a compression tester is an obvious choice.

In contrast, using a leak-down tester necessitates access to an air compressor or nitrogen tank, which may require a significant additional investment that may be prohibitive for those new to engine repair.

Engine Compression

The ambient pressure at the sea's surface is roughly equivalent to 14.7 psi, denoting the force exerted by the weight of the atmosphere per unit area at that particular location.

A cylinder with a compression ratio of 9:1 compresses the mixture of air and fuel to a pressure of approximately 132 psi at sea level (9 times 14.7 equals 132.3).

Upon ignition of the fuel and air mixture, the pressure within the cylinder can surge to 1,000 psi or beyond. To accomplish this feat, the engine is heavily reliant on specialized components.

These include piston rings, valves, valve seals, cylinder head gaskets, and valve seats, which are engineered to effectively contain the pressure within the cylinder until it can be released through the exhaust valve.

No engine can completely seal off the compression; however, once the engine has reached its optimum temperature, it should exhibit a high degree of sealing ability.

How Do You Use a Leak-Down Tester and a Compression Tester?

To utilize a compression tester, it is necessary to extract the spark plugs, introduce the tester's hose into one of the spark plug orifices, and rotate the engine until it completes around 4 to 8 revolutions.

After the revolutions, the pressure can be read off the gauge, which can be repeated on each engine cylinder.

Alternatively, when operating a leak-down tester, the spark plugs are removed, and the engine is rotated until the piston reaches the top dead center for the particular cylinder under scrutiny.

The hose of the leak-down tester is then introduced into the spark plug orifice and connected to a compressed air source with a minimum pressure of 100 psi.

Many leak-down testers include a regulator to adjust the air pressure to precisely 100 psi.

Finally, the second gauge on the leak-down tester measures the pressure in the cylinder. By subtracting this measurement from 100, the percentage of leak-down can be deduced.

Leak-Down Test vs. Compression Test

Below are the differences between a leak-down and compression test, including their pros and cons.

1. Compression Test

The compression test entails running the engine through its cycles with the compression tester replacing the spark plug in the cylinder.

The gauge will move and maintain the highest reading as the engine creates pressure.

Measurements should be taken for all cylinders and compared to one another. For a healthy engine, there should be no more than a 10 percent variance between the readings.

It is important to note that conducting the compression test on a hot engine will produce a more accurate reading within the normal temperature range of the engine due to improved sealing.

In contrast, a cold engine will yield the worst-case scenario as the metal has not yet expanded.

Additionally, engine compression readings may fluctuate depending on altitude and other variables, making it more crucial to compare the cylinders with one another rather than relying on the supposed compression number.

If the readings from one or more cylinders fall below the 10 percent range of the others, one may apply some oil to the cylinders and repeat the test.

If the pressure rises significantly on the retest, the issue may be worn piston rings. However, if the pressure remains the same, the problem may lie with the valves or the head gaskets.

Pros and Cons of Compression Test


The compression test is a widely recognized and conventional diagnostic method utilized by most auto technicians, who keep a compression tester in their toolbox and are proficient in its use.

The tester comes in various styles, such as the screw-in and hand-held types, which facilitates rapid testing by holding it against the spark plug hole during the test.

Another benefit of the compression test is that it can be carried out anywhere as it doesn't require a compressed air source.


Nevertheless, an inadequately executed compression test could produce misleading findings, demonstrating a low compression level.

If the vehicle's battery or starter motor is faulty, leading to low cranking speed, the test would be inaccurate and exhibit low compression. Similarly, an erroneous low reading will be recorded if the throttle is not kept in the wide-open position.

Upon obtaining a genuinely low compression reading, it's crucial to detach the gauge and add oil to the cylinder before redoing the test to detect the cause of low compression.

In other words, this is a "Wet Compression Test," where a high compression reading after the oil addition indicates worn piston rings.

2. Leak-Down Test

The leak-down test assesses the sealing capability of the cylinder by infusing compressed air, around 100 psi, at TDC and determining the loss from the seals, which is typically some amount.

The leak-down tester consists of two gauges that aid in measuring the leakage. Like the compression test, it is advisable to have the numbers within a percentage of each other. Additionally, this test can assist in pinpointing the problem before dismantling the engine by detecting the cylinder from which air is escaping.

Once the cylinder is pressurized, listen for the sound of air escaping and its source. The following are the sounds to listen for:

  • Air escaping from the PCV, oil cap, oil dipstick, or road draft tube: Indicates worn piston rings in that cylinder, cylinder damage, or pressure getting past them.
  • Bubbles or pressure in the coolant: Indicates a faulty head gasket or a warped or cracked cylinder head.
  • Hissing or whistles from the intake: Indicates a bent valve or improperly seating intake valve seat.
  • Hissing or whistling from the exhaust outlet or manifold: Indicates a worn exhaust seat or bent valve.
  • Sounds from a cylinder neighboring the tested one indicate a warped head or a bad head gasket.

Suppose it is discovered that the cylinders can make and maintain compression.

In that case, it confirms that any power-related problems lie in the essential three components of power production: fuel, air, or spark. Such components are less expensive than issues in one of the cylinders, resulting in a teardown and rebuilding.

Compression and leak-down tests can determine the engine's internal health without requiring a computer or teardown.

Pros and Cons of Leak-Down Test

One of the primary advantages of a leak-down test is that the engine can be tested without relying on other systems that may produce false readings.

Unlike a compression test, a starter motor is not required to crank the engine, which makes it possible to perform the test while the engine is removed from the car.

This feature is handy when testing a used or unknown replacement engine, as the leak-down test provides conclusive results, and the source of a combustion leak is always known by the end of the test.

For instance, when an intake valve leaks, air pressure can be felt coming through the intake system, while an exhaust valve leak results in air being felt or heard coming out of the tailpipe.

If the rings leak, air will come from the oil filler cap. It is worth noting that some air pressure will always come out of the crankcase as piston rings never have a 100% seal.

On the downside, the leak-down tester requires compressed air to work, which may not be as accurate as when the test is performed in a shop, depending on the compressor size.

The test also takes longer since the engine needs to be set at TDC compression for each cylinder.

However, it can be challenging to locate TDC correctly on all cylinders. If the cylinder is not precisely at TDC, it will rotate when air pressure is applied, requiring resetting the cylinder back to TDC and starting the test again.