A healthy engine is a precision piece of machinery. When its tolerances are within specific limits, seals are tight, spark is adequate, and components are well oiled, it is reliable and operates with utmost efficiency. Sealing is especially important, and taking care of it properly doesn’t just mean making sure the head gasket keeps coolant and oil only where they should be. Proper sealing of the piston rings and valves is also essential, so that combustion occurs properly when peak power is achieved – and the engine also remains healthy.
However, as engines take on runtime and mileage, the rings and valves inevitably wear out, causing a drop in compression. Some degradation is okay – the engine will continue to run healthy. However, low compression may be a sign of a failing cylinder head or some other problem that requires substantial work to fix. If the problem is not resolved, it can destroy the engine. Therefore, it is a good idea to run a compression test, a leak-down test, or both. The first is easy and involves simply plugging a gauge into the cylinder and turning the engine over. The latter involves a hair more, but you’ll get more information from it – and a lot more confidence about what exactly the problem is.
Compression testing is not complicated
Before diving too deep, it’s a good idea to test one (or both) in several situations. Maybe you have purchased a used car and want to check its health. Or, your engine is showing signs of wear and tear, whether it’s due to lack of power, strange noises, misfire, mysterious coolant loss, or any other sign of poor health.
Compression testing is the easiest test to do, and it can tell you a lot. Remove the spark plug, then take a compression tester, thread it into the spark plug hole of the cylinder, and crank the engine (without turning it over – so, disable the fuel pump as well). Keep an eye on the gauge, wait until it reaches maximum, then stop and write down the reading. From there, rinse and repeat on every other cylinder. All readings should be within reasonable range of each other. Any cylinder that is clearly short is going to be a problem; 130 psi is considered ideal, and the cylinders should be within 10% of each other.
However, some variables must be taken into account to get the most accurate reading. The battery should be healthy, and – ideally – a charger should be connected to its terminals. The starter should also not get too hot, as it will not rotate the crankshaft as fast as it should. Air density and valve lash can also have an impact on the results, so keep those aspects in mind when performing your tests. This test is not as involved as the leak-down test, but it still requires good preparation, and it cannot tell you everything you need to know.
Leak-down to avoid crying
Leak-down testing is more accurate and controlled; Not cranking the engine more than necessary. However, it requires more equipment and preparation. An air compressor is required to fill the leak-down tester with air, which is then fed into the cylinder. For best results, you must also ensure that each piston is at top dead center (TDC). The air supplied to the tester from the compressor must also be tightly sealed.
Here’s a quick overview of this method: Set the piston at TDC, turn on the compressor to build air pressure into the first bit of the device, plug the other end into the cylinder, and open the valve to see the variation. Leak-down testers have both a compressor gauge and a cylinder gauge, and the difference between them will give you the percentage of leak-down. Additionally, listen for any air escaping from the engine, and look for bubbles in the coolant reservoir. As told by Gareth Foley fcp euroThese signs help figure out what the problem is that is causing the compression leak. Foley also notes what is considered healthy, acceptable, and bad leak-down. Less than 5% indicates excellent health, 10% is good, 15% is decent, and 20% or more means major work must be done to correct the problem.
It is never a bad idea to perform both tests with the proper equipment. However, compression testing takes less equipment and time, so it’s quite easy to do in a pinch. That being said, leak-down testing does give a better overall picture of what’s going on, so if you can’t let any issues slip by your attention then this is the approach you’ll want to take.
