How To Test For A Compression Problem If Your Car Is Not Starting

23 December 2014
 Categories: , Blog


A low or no compression is one of the things that can make your car fail to start normally (if the battery is okay). This is especially the case if the engine is cranking despite not starting. The good news is that there is a simple test you can use to confirm whether your car has a compression problem.

First, an understating of the importance of the compression will help you to understand the problem better. The air-fuel mixture needs to be compressed so that it gets hot enough to ignite when the spark plug fires. If there is no compression, or if it is too weak, then the mixture will not burn even if the spark plug fires.

There are different methods for testing for low compression depending on your car's engine. Testing for low compression in a diesel engine is a bit complicated and may require a mechanic's input. If you are lucky enough to have a gasoline engine, then all you need to have is a handheld compression gauge and then follow the steps below:

  • Warm up the engine
  • Disable the ignition module
  • Insert the tester into each cylinder's spark plug one by one
  • Hold the throttle to full open position – this provides the engine with adequate air.
  • Crank up the engine for about five full revolutions
  • Record each cylinder's reading
  • Compare the readings and note their variations

If the readings vary by more than ten percent, then there is a problem in at least one of the cylinders. In that case, you will need a mechanics input to identify the route of the problem. Generally, your engine may fail to provide adequate compression if:

  • There is a timing problem in the camshaft (perhaps due to excessive wear)
  • The timing belt or chain is broken
  • The cylinder sealing is broken
  • The starter motor is slow

If all the readings are within ten percent of each other, then the no-start problem is not due to low or no compression. It might be that the ignition system is defective or that the air/fuel ratio is not optimal. All these are things that your mechanic can diagnose with ease.

As you can see, there are several problems that may cause your car to fail to start. Knowing how to test for low compression, however, helps you to eliminate or confirm one source of the problem. That way, you can at least contact a mechanic from an auto repair shop with a rough idea of your car's fault.