ABYC Diesel Practice Exam

If an engine does not achieve full RPM, what test should be performed last?

1. Fuel pressure test

2. Oil pressure test

3. Compression check

4. Coolant flow test

The correct answer is: Compression check

When an engine struggles to achieve full RPM, the compression check is a diagnostic step best reserved for last because it generally indicates mechanical condition issues related to the engine’s internal components, such as rings, valves, or head gasket integrity. Conducting a compression test typically requires more extensive disassembly and intervention than the other tests, making it less convenient to perform earlier in the diagnostic process. In contrast, fuel pressure tests, oil pressure tests, and coolant flow tests can provide immediate information regarding the fuel delivery system, lubrication system, and cooling system. These systems often affect engine performance before any internal mechanical failure. By first assessing these external factors, you can rule out or confirm issues that might be affecting RPM without delving into the more complicated aspects of engine internals represented by a compression check. Therefore, since a compression check may require more time and effort, it is practical to perform it last, following the assessment of fuel, oil, and coolant systems.