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Compression test

Car culture term

Compression test
Photo: Flygvapenmuseum / Stina Hedvall / Wikimedia Commons · CC BY 4.0

What is Compression test?

Measuring cylinder compression pressure to assess engine health. Compares each cylinder peak pressure to identify problems.

Compression test is a diagnostic procedure that measures the maximum compression pressure in each engine cylinder during the compression stroke. The test reveals problems with valve seating, piston rings, or head gasket sealing. Each cylinder is measured individually; comparison between cylinders identifies which cylinders may have problems.

The test is performed with the engine warm but not hot. The spark plug is removed, a compression gauge is installed in the spark plug hole, and the engine is cranked while measuring the peak pressure. Healthy cylinders typically reach 150-180 PSI (varies by engine compression ratio). Differences between cylinders larger than 10-15% suggest problems. Cylinders below 100 PSI typically indicate serious problems requiring further investigation.

The compression test alone doesn't diagnose specific problems but indicates which cylinders need investigation. A wet compression test (oil added to the cylinder before testing) helps differentiate between piston ring problems (compression improves with oil) and valve sealing problems (compression doesn't improve with oil). Combined with leakdown tests and other diagnostics, compression testing helps identify specific engine repair needs. The cost of a compression test is typically $50-$150 in shop labor, or DIY with the right gauge.

Related terms

Leakdown testHead gasketValve adjustment

Frequently asked questions

What does a compression test measure?
The maximum pressure achieved in each cylinder during the compression stroke. The pressure depends on the engine compression ratio, the cylinder size, the valve sealing, the piston ring sealing, and the head gasket condition. Measuring the peak pressure provides a snapshot of engine health.
What is normal compression pressure?
Varies by engine. Typical naturally aspirated engines: 150-200 PSI. Forced induction engines (turbo, supercharger) typically have lower compression ratios and lower compression test pressures (130-160 PSI). High compression engines (12:1+): 180-220 PSI. The exact specification depends on your engine; consult the service manual for normal range.
What does low compression mean?
Several possibilities. Worn piston rings (compression escaping past the rings to the crankcase). Worn or damaged valve seals (compression escaping through the valves). Damaged head gasket (compression escaping to coolant or another cylinder). Burned valve (compression bypassing through the burned area). The compression test alone doesn't differentiate; further diagnostics (leakdown test, visual inspection) are needed.
Can I do a compression test myself?
Yes, with proper tools. A compression gauge (about $30-$80) and basic tools are required. The test takes about 30-60 minutes. The work is straightforward but requires care to avoid damaging spark plug holes. Many DIY mechanics use this test for diagnostic purposes before paying for shop labor.
Is low compression always serious?
Major problem if cylinder is below 100 PSI. Mild low compression (cylinders within 10% of each other but slightly low) may be acceptable depending on engine and use. The pattern of which cylinders are low (specific cylinders, all cylinders, etc.) indicates the type of problem. Significant variation between cylinders is the key warning sign.

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