How to do a Compression Test for IC Engines

A close look at Infiniti's VC-Turbo variable compression engine

  It is very vital that the petrol/ air mixture in an IC engine is compressed to the correct pressure during the compression stroke for one to have maximum engine performance. 

  The purpose of a compression test is to determine the condition of piston rings and valves without having to dismantle the engine. Worn piston rings and worn valves will lead to compression loss, and as such, this information is part of a necessary trouble shooting process. That said, nevertheless, loss of compression is not only caused by the above two mentioned.

The following is a guide to performing a compression test. 

Note: It is neccessary that the test be done on each and every cylinder of the engine.

1.  Check with your manufacturer’s specification for  correct compression pressure. This is per standard approximation usually between 8-10 Bar for petrol engine & 18-25 Bar for diesel.

2.  Remove  spark plugs (for petrol). Respectively, remove injectors or heater plugs ( for diesel). 

3.  Fit your compression gauge to cylinder head. 

4.   With trottle in maximum, put to open position (petrol engine)or, with ignition isolated, fuel shut off (diesel engine), Crank the engine by the starter motor for 5-6 seconds and note readings. While doing this, ensure that there is no pressure that escapes the system.

5.  If the readings are within manufacturers' specification range, it is therefore an indication that the engine is in good condition. If, though, the reading for one or all cylinders are below  manufacturers' specification, a wet compression test needs to be carried out in order that you determine the cause of compression loss. 

The procedure for a Wet Compression Test is the same as the above with the exception that this time, before the readings are taken, a few squirts of oil are squirted into the cylinder being tested. Some recommend 5 squirts oil and that is reasonable provided the word FEW will have been done justice to. Squirting the oil into the cylinder helps to seal the piston to the cylinder whilst a "process" of a pressure test is being carried out.

The probability of the outcome is one out of three. It is either that;

 1.  The pressure reading is the same as first test; or 

2.  The pressure reading increases from first test to within manufacturer's specification; or 

3.  The  pressure  reading increases slightly but still  below manufacturer's specification. 

These come up with simple corresponding possible explanations: 

If the reading remains the same, loss of compression is due to worn or leaking valves since a sealed piston will not have improved the pressure. 

If the reading on the guage increases, loss of compression is due to worn rings, since the  oil has sealed the piston and prevented loss of compression. 

Thirdly, if the reading increases by a little margin, so little that it still does not meet the standard specification per manufacturer's detailing, then the loss of compression is due to BOTH piston rings and valves. 

Consequently, the engine will require a major overhaul. However, if the compression test indicates that  the valves are the cause for the loss of compression, valve clearance should be checked first against manufacturer's specification because insufficient valve clearance would cause loss of compression when the valves are not seating correctly.

NB: These two compression tests will work as seen for both two stroke and four stroke engines.

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