How to do a Compression Test on an Engine

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Published: 24th November 2014
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Why do a compression test?

The engine’s compression reveals much about the state of the engine. Low compression caused by a faulty valve or worn piston ring could mean a lack of power or blowing smoke from the exhaust but can also help you rule out some of the more serious faults.

What is a compression test?

If your engine is running smoothly there should be a compression in excess of 100 psi per cylinder with no more than a 10% difference between the highest and lowest cylinder test.

As a general rule a compression of 135 PSI or better is excellent, and a compression of 85 PSI or lower is not good at all. If this is not the case it is likely that your engine valves, their valve seats or piston rings need replacing.

Before Use

1. Compression Tester with Quick Disconnect for Petrol Engines
2. Identify the size of fitting that is required for the engine to be tested.
3. This compression testing kit includes fittings suitable to replace injectors and glow plugs.
4. Refer to the table for details of fittings and thread patterns.
5. The battery and starting system must be in good condition to ensure an accurate test.

Warning:

Always keep well away from any hot / moving engine parts.
Take care to keep compression tester from becoming entangled in moving parts.

Operation

Compression Tester for Diesel Engine

1. Allow engine to reach normal operating temperature. Stop engine, remove all glow plugs or injectors (refer to vehicle manufacturers manual).
2. The compression tester (with correct fitting attached) can now be screwed into a glow plug or injector hole. Ensure that fitting is screwed in securely.
3. Disconnect the fuel shut off solenoid to disable the fuel injection pump.
4. With the throttle in the wide open position, turn the engine over 6-8 revolutions.
5. The dial gauge will automatically record the highest pressure reached.
6. To release the pressure, and reset the gauge, depress the bleed valve button.
7. The compression tester can now be unscrewed, and the next cylinder tested.

Tip

If you find that one of the cylinders has a compression of under 100 psi then pour about a teaspoon full of oil in and retest.

If the reading increases then it’s likely to be a piston ring problem. If not, it’s more likely the valves need replacing.

In summary

For an engine running normally compression builds up quickly and evenly to the specified compression for each cylinder.

If the Piston Rings are leaking: Compression is low on the first stroke but builds up after further strokes but does not reach normal.

However, the compression improves considerably when you add oil.

If the Valves are leaking: Compression is low on the first stroke and does not normally change much after further strokes and does not improve much by adding oil.

If two adjacent cylinders have lower than normal compression and adding oil into the cylinders does not increase the compression, it could be that the head gasket is leaking between the cylinders.

Maintenance

1. Keep your compression tester clean.
2. Ensure that screw threads do not become dirty.
3. If necessary clean with a wire brush.

Conclusion

With the correct equipment a compression test can be done in around 20 minutes but can save you £100s in garage fees.

Author:

Tony Heywood is a ‘Retail Troubleshooter’ with 40 years’ experience in the retail industry, specialising in e-commerce over the last decade. He has his own consultancy firm and is MD of ’e sell it’, an on-line store selling tools and auto-care products worldwide.

For compression testers & other auto-care products at trade prices, please visit: http://www.esellit.com

Or call: 020 8201 0588

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