Engine Cranking Sound Diagnosis

A lot can be learned from testing the engine with a cranking sound diagnosis test. Compression is vital to the generation of power, and losing compression in one or multiple cylinders will cause those cylinders to not generate as much power, and the engine will not run properly. In this scenario, replacing basic parts like spark plugs and other tune-up items will not fix the problem.

Compression can leak past the intake or exhaust valves, the piston rings, or head gasket. A cracked engine block or cylinder head can also cause compression leaks, but are rarer causes as well.

Preparing For The Test

When you perform a cranking sound diagnosis, you need to disable the engine so that it will not start. This can be accomplished by pulling the fuel pump relay. If you leave the fuel pump able to run during the test, it may put a lot of fuel into the cylinders and this will risk hydro-locking the engine. Even if you don't hydro-lock the engine, the test results may be skewed if the compression loss is located in the piston rings. You may use the "clear flood" mode if the vehicle is equipped with it. With the ignition key in the off position, hold the throttle to the floor, then crank the engine. If it starts, the clear flood mode was not engaged, so let up on the throttle and turn the key off.

Performing a Cranking Sound Diagnosis Test

Using the key, crank the engine over and listen to the cranking sounds it produces. During engine cranking, the engine will make a uniform and rhythmic sound if the compression is close to the same across the cylinders. As each piston comes up on compression, it resists the force of the starter motor and slows the cranking speed. If the compression of each cylinder isn't the same, the engine will make an uneven sound.

A cylinder that has low compression will be easy for the starter to turn, so when a low compression cylinder is on its compression stroke, it will allow the engine to spin more freely. While cranking, that will sound like the engine speeds up for a moment, and returns to the previous speed. The sound of this speed change will become more complicated when there is more than one cylinder with low compression.