To understand the requirement of running an engine compression test and how it should be done.
Introduction
The power stroke of an engine cycle is the key to how an engine turns explosive energy into mechanical energy. If any of the parts that confine the explosion inside the cylinder becomes worn, engine performance will be affected. The reason for this can be one of the following:
First
The engine cannot compress the fuel/air mixture enough to get the required pressure.
Second:
The exploding mixture can escape past worn valves, seals, piston rings and worn cylinder walls. Due to this, the engine cannot burn the fuel efficiently and produces less power.
Third:
The engine may also begin to burn oil due to leakage of oil into the combustion chamber.
Fourth:
Leakage of fuel/air mixture into the sump and mixing with engine oil thus leaving the less amount of mixture into the combustion chamber and results in the production of less power.
When to run such a test?
If the engine is running rough or is suffering from a lack of power and no faults can be found in the ignition system, fuel system, exhaust system, then there is time to run a compression test.
A compression test can confirm or eliminate the possibility of worn piston rings, worn cylinders, worn crankshaft, worn valves or a faulty gasket.
Procedure
There are four main steps in running an engine compression test.
§ Preparing for the compression test
§ Performing engine compression test
§ Interpreting the results
§ Wet compression test
To compare the results from original cylinder pressure, we have to calculate the maximum cylinder pressure.
Preparing for the Compression Test
§ One should perform this test with a fully charged battery. If battery power decreases as you are testing the last cylinders, you will get a false compression reading on these cylinders.
§ Make sure to have a correct compression gauge. Diesel engine needs a gauge with a higher compression tolerance.
§ Warm up the engine or one can have a 20-minute drive and allow the engine to be at idle. This will allow the valves to sit tight and piston to expand and seal properly.
§ Turn off the engine and open the head.
§ Disable the fuel circuit by turning the ignition off and removing the supply of the fuel pump.
§ Disable the ignition system by unplugging the spark wire from the coil.
§ Install the compression gauge on the first cylinder in the place of the spark plug by hand and tighten the gauge connector snugly to seal the cylinder so that the compression won’t leak.
Performing Engine Compression Test
Engine compression is performed as follows:
Ø Start the engine and allow it to idle until it runs out of fuel and stalls.
Ø Remove the spark plug cover. Disconnect the distributor supply loads.
Ø Fit the compression gauge in the No.1 cylinder of the engine.
Ø Crank the engine using the starter motor for a small time. In compression gauge, there is a non-returning valve. So, the gauge will show the maximum pressure during this interval.
Ø It can be seen that the pressure builds up on the gauge and reaches its maximum point in 4 to 5 compression strokes. One thing should be kept in mind that whether you use
4 to 6 strokes to test the first cylinder’s compression, use the same number of compression of strokes for the rest of the cylinders.
Ø Note how far on the scale the needle on the gauge moves.
Ø Record the compression reading on the last stroke (amount of compression indicated by the needle).
Ø Repeat the above steps for each of the remaining cylinders.
Ø Compare your results to the maximum and minimum compression manufacturer specifications for the particular engine.
Inferring the Results
· If there is less than 66% pressure on the 2nd revolution and less than 80% pressure on the 4th or 6th revolution, this is the sign of piston ring/cylinder bore wear.
· If any cylinder shows more than the maximum pressure, then there is the probability of carbon deposits in the chamber. In this case, head removal and decarburizing may be required.
· If adjacent cylinders show low pressure this could be due and internally cracked cylinder head or an internally blown head gasket between those cylinders. This can also be confirmed by the presence of the coolant in the oil and will be recognized by a whitish coloring to engine oil.
· If not all but any one or two cylinders show fewer pressure readings, then the most probably the reason is a damaged gasket and this issue can be resolved by replacing the gasket.
Common causes of low compression
Common causes of low compression are:
· Cracked block or cylinder head
· Valvetrain problems
· Valve or valve seat problems
· Worn out rings or cylinders
· Blown head gasket
Wet Compression Test
If one or more cylinders registered a compression below the manufacturer specification, perform a wet compression test on those cylinders to narrow down the potential cause of the problem.
The test is basically same as before, except this time, 5 to 10 ml of engine oil is poured down the cylinder through the spark plug hole just prior to fitting the compression gauge. Give some time to allow oil to settle around the piston rings and form a seal.
With this test one the two things will happen:
· Cylinder compression won’t change at all, indicating a compression leak through a head gasket or a valve or-or less common, through a cracked cylinder head or block.
· Cylinder compression will rise, indicating a compression leak through a worn out cylinder or piston rings.
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