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How to Interpret Compression Tests

Last Updated: Sep 21, 2018 12:10PM NDT
Note: This information is general for all motors - your vehicle may exhibit different behavior or naturally have higher/lower compression values - when in doubt refer to the factory manual for your vehicle.
 

Factory/Spec Cylinder Pressure

Your vehicle/motor may have a published standard 'ideal' cylinder pressure during the compression stroke.
If available, you will find this value in the factory/service manual for your vehicle but you should be aware that this will represent a theoretical pressure which you are unlikely to see during real world testing.  

Except in cases where the measured pressure is vastly different from the spec value, the pressure of each cylinder compared to the others is a more important factor in troubleshooting compression issues.

If a factory value is not available you can estimate it by multiplying your intake pressure by your engine's compression ratio - for example a 2005 Silverado with a 5.3L V8 has a compression ratio of 9.9:1 - if you measure an intake pressure of 17 psi you can expect a cylinder pressure in the 170 psi range.
 

Relative Cylinder Pressure

The pressure of each cylinder should be measured and compared to the values for each of the remaining engine cylinders. No cylinder should have a pressure lower than 75% of the highest recorded value.

For example, consider a V6 with the following compression test results
 
Cylinder Pressure
#1 160 psi
#2 128 psi
#3 168 psi
#4 154 psi
#5 120 psi
#6 159 psi

In this example the highest pressure is 168 psi, so the minimum allowable value will be 0.75 * 168 = 126 psi. In this scenario cylinder # 5 is considered to have low compression, while #2 is just passing.
 

I Have Abnormal Compression - Now What?

 

Result Likely Culprit
Pressure starts low and stays low
Adding 1tbsp of oil does not increase pressure
Valves
Pressure builds up, but not high enough
Adding 1tbsp of oil increases pressure
Piston Rings
More than one adjacent cylinder show low pressure Headgasket
All pistons show low pressure (<100 psi)
Adding 1tbsp of oil does not increase pressure
Timing
No Pressure Piston, Other Engine Damage
High Pressure Carbon Buildup


Low Compression
If a test indicates low compression:
  1. Try the test again - loose fittings or other issues could cause you to read an incorrect low value
     
  2. Add roughly 1 tbsp of the appropriate motor oil for your vehicle to the cylinder and try the test again. If the pressure increases this points towards damaged/leaking piston rings - if the pressure does not increase then you might want to have a look at the valves.
     
  3. Check neighboring cylinder pressures - if two side-by-side cylinders are showing abnormally low pressure you might have a headgasket leak
     
  4. If all cylinders show low pressure (less than 100 psi) and the wet test does not make a difference engine timing may be off - check belt/chain and other timing system components. 

High Compression
An abnormally high compression test may be indicative of carbon buildup in the cylinder.


No Compression
If a cylinder shows no compression it may be a more serious issue (piston damage, missing valve, etc) requiring direct examination

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