Usually, engine diagnosis in modern power plants starts with the same basic methods that they have always been used for testing. Engine problems may be in electronic control parts and high technical systems. Nevertheless it is recommended to start with the basics of your engine's health When it comes to drivability diagnosis like using any kind of scan tools. This engine diagnosis gives you a good indication of the true condition of your engine. Performing an engine vacuum test and compression test are quick and simple and they give valuable results. Cost of test equipment is very low. Performing these tests your way to diagnosis will be obvious for any particular problem. Let's talk for instance about compression test. All interior combustion engines rely on compression of fuel and air mixture to make the most of the power produced by the engine. The rising movement of piston on compression stroke applies pressure on the air /fuel mixture in combustion chamber. Overhead valves close with the intention that this compression process can occur at the right time. The exhaust and intake valves must be sealed properly in order to achieve maximum compression. The air-fuel mixture becomes hotter when compressed. Being hot mixture makes it easier to ignite. And when ignited it generates more power than the same mixture at a low temperature. If initial engine diagnosis recommends that the reason of the problem is poor compression, a test should be performed. Typical compression gauge specifies pressure in pounds per square inch and compression testers have a vent valve that holds the highest pressure reading on the meter.