What Does a Good Compression Test Reading Consist of?
As engines are integral to the function of many systems, they necessitate a number of tests to ensure their functionality, one of which includes a compression test. A compression test is a good way of determining the condition of your engine, revealing the state of the valves, cylinders, rings, and valve seats. According to professional mechanics, healthy engines should have compression values of over 100 PSI per cylinder, with a deviation between the maximum and minimum readings remaining within a 10% range.
To perform a compression test, you will first need a fully-charged battery to acquire an accurate reading. Additionally, you will need the right compression gauge for your engine, and if your engine runs on diesel, you will need a gauge with a higher compression tolerance. At this point, you can carry out a compression test with ease.
Begin by removing the spark plugs before warming up the engine. If the spark plug is too difficult to detach, do not remove it by force. Once the engine is all warmed up, turn off the engine and pop the hood. Open the throttle so that the engine gets plenty of air flow as you perform the compression test. Next, detach the air cleaner duct or remove the air filter box from the top of the carburetor, depending on the type of fuel system present.
At this point, you can remove the fuel pump fuse if you utilize an electronic fuel injection system. This prevents the fuel from going into the combustion chambers. With a diesel engine, you must disconnect the fuel shutoff solenoid. Now you can start your car and allow the engine to stall to ensure the remaining fuel is used. After this, you can work on each spark plug, starting with the one closest to the front of the engine. Disconnect it and label it to identify its precise location on the engine. This should be done for all the spark plugs.
It is important to keep cleanliness in mind when performing a compression test. Each spark plug must be cleaned with either a bicycle pump, soft brush, or compressed air to prevent debris from entering the cylinders which can damage the internal walls and cause oil leaks. From here, you can halt the ignition system by disconnecting the electrical connector on the coil pack or remove the spark wire from the coil. Lastly, you can install the compression gauge on the first cylinder in place of the spark plug. Proceed to tighten the gauge connector to prevent compressed air from escaping.
Once everything is set, you can begin the compression test. Ask someone to turn on the engine and observe the pressure build up on the gauge, with the highest reading being attained after 4 or 5 compression strokes. You must note the position of the gauge needle and record the compression reading on the last stroke. Using a release valve, release the pressure on the gauge and reset it to zero. Then, detach the gauge, attach it to the adjacent cylinder, and repeat the test. Once done, compare your results to the highest and lowest compression manufacturer requirements for your engine.
If your results reveal that your engine’s compression values are above the manufacturer’s specifications, there may be a buildup of carbon inside the engine cylinder. In contrast, if it is too low, a wet compression test must be carried out. A wet compression test consists of similar procedures with the exception that oil is added to each cylinder. A compression variation over 15% between cylinders may indicate a blown head gasket, broken valve/springs, or worn out cylinders, valves, and valve rings, while low compression between two cylinders may pinpoint to a leaking head gasket.
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