Understanding Leakdown Tests
Has someone just said you need a leakdown test? Here’s what you need to know.
A leakdown test is a diagnostic procedure used to assess the condition of an internal combustion engine by measuring the amount of air leakage in the cylinders.
It provides valuable information about the health and integrity of the engine's valves, piston rings, and cylinder head gasket. By comparing the measured pressure leakage to acceptable values, mechanics can identify potential issues and determine the need for repairs or further investigation.
To perform a leakdown test, the following steps are typically followed:
Preparation: The engine is brought to top dead center (TDC) on the compression stroke for the cylinder being tested. This ensures that the valves are closed, providing an accurate measurement.
Setup: A specialised leakdown tester is connected to the spark plug hole of the cylinder being tested. The tester consists of a pressure gauge and a regulator that allows controlled airflow.
Pressurisation: The cylinder is pressurised with compressed air to a specified pressure, typically around 100 psi (pounds per square inch).
Measurement: The leakage percentage is determined by comparing the pressure supplied to the cylinder with the pressure retained. The leakdown tester provides a reading indicating the percentage of leakage occurring.
Analysis: The mechanic analyses the results based on the leakage percentage and the specific characteristics of the engine being tested. Higher leakages indicate problems such as worn piston rings, damaged valves, or a blown head gasket.
When should you do a leakdown test?
A leakdown test should be performed under the following circumstances:
Low Compression: If a compression test reveals low compression in one or more cylinders, a leakdown test can help pinpoint the source of the problem. By assessing the percentage of leakage, it can determine whether the issue lies with the valves, piston rings, or other components.
Engine Overheating: If an engine has experienced severe overheating, a leakdown test can help identify potential causes, such as a blown head gasket or damaged valves. It provides a more accurate assessment of the engine's condition compared to a compression test alone.
Engine Rebuilding: During an engine rebuild or overhaul, a leakdown test is often performed to ensure the integrity of the rebuilt components. It helps verify that the valves and piston rings are properly seated and that there are no significant leaks.
A leakdown test provides deeper insights into the internal condition of an engine. Comparing it to a compression test can highlight the added benefits it offers in terms of accuracy and identifying specific problem areas.
Next time you’ve got your car in the shop after a big session at your local drag strip or racetrack and it’s either blowing a bit of smoke or feels like it’s lost power you know what to ask for - a leakdown test.