When Auto Mechanic School Graduates Should Conduct a Compression Test on a Vehicle’s Engine

While today’s modern combustion engine may seem invincible due to all the technological advancements they’ve undergone in the past century, an engine is not immune to the effects of wear and tear. During the course of owning a vehicle, it’s almost inevitable that drivers might notice that something is “off” with their engine. How can the problem be diagnosed? Engine issues can be identified with a compression test, a procedure that can pinpoint the source of any problem with a combustion engine. After a mechanic conducts a compression test, they’ll be able to understand what is going on with an engine and take the next step by performing the repairs necessary.  

Those in Automotive School Should Know How A Compression Test Works

A compression test works by using a compression gauge to determine how much compression each cylinder of the engine is creating. As those in automotive school likely already know, an engine creates power by compressing a mixture of air and fuel into a smaller mass inside of the combustion chamber. The spark plug then ignites this vaporized fuel, powering the engine. If compression is reduced for any reason, this could lead to the engine malfunctioning and a loss of power. 

A compression test checks the functioning of the engine’s piston rings and valve-train to make sure that parts such as the intake and exhaust valves, valve seats, and head gaskets have not worn out and are not causing a decrease in compression. This is done by putting a compression gauge inside of the spark plug hole, which reads the compression amount per cylinder in pounds per square inch (PSI) when the engine is cranked. In most cases, there isn’t a problem with the compression if there is less than a 10-percent difference between the lowest and highest amount tested in each cylinder, and the amount is over 100-psi. It’s always good to reference the owner’s manual to make sure that the compression readings are normal.

Signs a Vehicle Could Use a Compression Test

So how does a driver know when it’s time for a compression test? Generally, it’s a good idea to conduct a compression test when a vehicle is experiencing engine trouble of any kind. Better safe than sorry, right? If that’s too vague, here are a few specific signs that a driver should take their vehicle in for a compression test performed by someone with auto mechanic training. 

If a driver notices that there is smoke coming from their exhaust system, especially when accelerating or decelerating, this means that it’s time to get their engine checked. Additionally, if the vehicle is not accelerating normally, glitching during acceleration, or seems to lack power, these are all signs that something bigger is wrong. If a driver finds themselves adding more oil than normal to their vehicle or that their fuel economy is significantly worse, these could all be signs that a compression test is in order. Lastly, a driver should take their vehicle in if the engine appears to be running hot, or if it vibrates during driving. All of these are signs that one of the engine parts responsible for compression could be wearing out, reducing compression and thus affecting the operation of the engine.

Compression tests are a good way to get a read on the internal damage within an engine, and they help mechanics know what needs fixing. Before conducting the test, make sure that the engine has been warmed up, but is totally turned off, and don’t forget to disconnect the spark plugs. Taking these precautions ensures that the engine won’t ignite while the compression test is underway. Once you’ve taken these measures, you’re ready to perform the test and make the repairs necessary.

Do you want to learn more about repairing engines? How about attending auto mechanic school? 

Check out ATC Cambridge’s program options today.