Correct Response: C. Internal combustion engines operate by the ignition of a fuel-air mixture that is heated and pressurized. This process takes place within the engine cylinders, which need to be well sealed in order maintain pressure. A compression gauge is used to check that the proper pressure levels are reached and maintained. Otherwise, there may be a problem in the cylinders. The ignition coil (A) is an inductive coil that is used to generate the high voltage needed to fire a spark plug. It is not diagnosed with a pressure gauge. The crankcase (B) is an oil bath from which oil is pumped to lubricate various parts of the engine. It is a vented system that is not pressurized. Therefore, it cannot be diagnosed with a compression gauge. The fuel system (D) on many engines is pressurized. It can be diagnosed with a similar, but different, device. This specialized device is called a fuel pressure gauge.
