ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is performing a compression test on an engine. The specification is 150 psi per cylinder, with no more than a 10% variation between cylinders. Cylinder 1 reads 145 psi, Cylinder 2 reads 130 psi, Cylinder 3 reads 148 psi, and Cylinder 4 reads 115 psi. What conclusion should the technician draw?
- ABoth Cylinder 2 and Cylinder 4 are within acceptable limits.
- BCylinder 2 is faulty and requires further investigation.
- CAll cylinders are within specification.
- DCylinder 4 is faulty and requires further investigation.
Show answer & explanationAnswer & explanation
Correct answer: D. Cylinder 4 is faulty and requires further investigation.
The maximum allowable variation is 10% of the highest reading. The highest reading is 148 psi (Cylinder 3). 10% of 148 psi is 14.8 psi. Therefore, the lowest acceptable reading is 148 psi - 14.8 psi = 133.2 psi. Cylinder 4 reads 115 psi, which is below 133.2 psi, indicating a fault. Cylinder 2 (130 psi) is also below 133.2 psi, but the question asks for the MOST likely cause, and 115 psi is significantly lower.
Why the other options are wrong
- A. Neither Cylinder 2 nor Cylinder 4 are within acceptable limits when considering the 10% variation from the highest cylinder.
- B. Cylinder 2 (130 psi) is indeed below 133.2 psi, but Cylinder 4 (115 psi) is a more significant fault.
- C. Cylinder 4 (115 psi) is well below the acceptable limit based on the 10% variation rule.
Engine Compression Test
A diagnostic test that measures the maximum pressure generated in each cylinder during engine cranking, indicating cylinder sealing integrity.
- Measures piston ring, valve, and head gasket sealing.
- Readings should be within manufacturer specifications, typically 10-15% variation between cylinders.
- Low compression can indicate worn rings, burnt valves, or head gasket leaks.
Memory trick: Compression's high-low dance, check for a 10% chance.