ASE G1 Auto Maintenance and Light RepairEngine SystemsHard
A technician is performing a relative compression test using a scan tool on a multi-cylinder engine. The results show that cylinder #4 has significantly lower cranking RPM drop compared to the other cylinders. What does this indicate?
- ACylinder #4 has lower compression than other cylinders.
- BCylinder #4 has higher compression than other cylinders.
- CCylinder #4's ignition coil is failing.
- DCylinder #4's fuel injector is stuck open.
Show answer & explanationAnswer & explanation
Correct answer: A. Cylinder #4 has lower compression than other cylinders.
A relative compression test measures the current draw of the starter motor or crankshaft speed as each cylinder fires. A cylinder with lower compression offers less resistance to the starter motor, resulting in a smaller drop (or even a slight increase) in cranking RPM or lower current draw compared to healthy cylinders. Therefore, significantly lower cranking RPM drop indicates lower compression in that cylinder.
Why the other options are wrong
- B. Higher compression would cause a *larger* drop in cranking RPM due to increased resistance.
- C. A failing ignition coil would cause a misfire but wouldn't change the mechanical resistance during a compression test.
- D. A stuck-open fuel injector would primarily cause a rich condition and misfire, not directly affect cranking RPM drop in a compression test.
Relative Compression Test Interpretation
A diagnostic method that compares the cranking effort required for each cylinder, with lower cranking RPM drop indicating lower compression in that cylinder.
- Measures current draw or crankshaft speed fluctuations during cranking.
- Lower resistance (less RPM drop) means less compression.
- Helps identify weak cylinders without removing spark plugs.
Memory trick: RPM drop is low, compression's a no-go!