When buying a used vehicle we are mostly concerned with the engine condition especially if it is over 100,000 km. The engine compression is the engine's combustion capacity necessary to make the motor run. A compression drop inside one or more cylinders will cause abnormal idling, power loss, higher fuel consumption, and polluting emissions. Most of the time these problems are the result of wear and tear on some parts or other: piston rings, valves, head gasket, or cracks anywhere.
You can find out quite a lot about an engine's internal condition through compression testing, but as long as proper procedures are followed. Many people mistakenly make this test with diluted used oil on a cold engine, or with the throttle valve shut.
How To Read Results?
If you have a compression test performed on a used vehicle you're looking to purchase, it might be interesting to know how to read the results. First of all, the test is made with a compression gauge, which is an instrument that measures pressure produced individually by each cylinder. If you have a 4-cylinder car, you'll get four results. For example you may get the following results:
#1 cylinder: 175 psi
#2 cylinder: 170 psi
#3 cylinder: 175 psi
#4 cylinder: 160 psi
Pressure is measured in "psi" (pound per square inch); in the metric system it is measured in kilopascals (kPa). Usually, manufacturers do not give out detailed specifications on compression numbers but they do offer the following rule: a minimum of 100 psi per cylinder, and a maximum difference of 25% between each cylinder. You should start to worry if one of your cylinders is getting close to 100 psi. Most new engines in good condition compress at around 175 psi. If we look at the example above, we find that the #4 cylinder at 160 psi is O.K. But let's compare it with the cylinder having the highest pressure level at 175 psi.
With a simple rule of three:
175 psi = 100%
160 psi = ?
(160 x 100) / 175 = 91.4% for the #4 cylinder. So 100% ? 91.4% = 8.6% which is well below the 25% limit, thus a good result.
Suppose that #4 cylinder were at 120 psi, the difference ratio would be 31.4% which would indicate excessive wear in the cylinder. When a cylinder has a low compression level, it is possible to find the leak by performing a "liquid test." You do the compression test again but this time you add a little quantity of motor oil (one teaspoon) in the cylinder in order to temporarily increase its tightness. If during this new test the pressure rises by 5% or more it means that the piston rings are causing the trouble. But if the pressure is the same then probably the valves are the source of trouble. Note that this "liquid test" cannot be performed on flat engines such as Volkswagen, Porsche and Subaru. Moreover, if two adjacent cylinders have a low compression level it means you have a broken head gasket. In any case you'll have to open up the engine. And the worst-case scenario would be to have to rebuild the whole motor.
Doing The Test Right
For best results the engine should be operating at normal temperature, or until the radiator fan starts. Disconnect the starter and the fuel supply to prevent any danger of explosion, and remove all the sparkplugs. Keep the throttle valve open to the max throughout the operation. Connect the pressure gauge to the sparkplug hole, and then turn on the starter until the gauge pointer gets up to the maximum. Repeat the operation four or five times while writing down the result each time.
Remember that while checking compression levels you should also watch for unusual sounds in the engine, for any kind of oil leaks as well as smoke in the exhaust system. If you're going to perform this test yourself be sure to follow the recommendations in the car maker's repair manual.
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