FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A technician observes that an engine's oil pressure gauge reads zero during initial startup, but then quickly rises to normal operating pressure within a few seconds. Assuming the engine was properly serviced, what is the most likely reason for this initial zero reading?
- AInsufficient oil level in the sump.
- BA clogged oil filter bypass valve.
- CA faulty oil pressure relief valve.
- DAir trapped in the oil pressure sensing line.
Show answer & explanationAnswer & explanation
Correct answer: D. Air trapped in the oil pressure sensing line.
Air trapped in the oil pressure sensing line can cause a delay in the gauge reading, often showing zero pressure initially. Once the air is purged by the flowing oil, the gauge will then accurately display the pressure. This is a common transient issue during startup.
Why the other options are wrong
- A. Insufficient oil level would result in sustained low or fluctuating pressure, not an initial zero that quickly normalizes.
- B. A clogged oil filter bypass valve would cause oil to flow through the filter, potentially restricting flow if the filter is also clogged, but wouldn't typically cause a momentary zero reading followed by normal pressure.
- C. A faulty oil pressure relief valve (stuck open) would lead to consistently low or zero pressure, not a quick rise to normal.
Zero Oil Pressure Delay
A temporary period of zero oil pressure indication at startup, often caused by air in the gauge's sensing line.
- Occurs at initial startup.
- Gauge reads zero, then normalizes.
- Caused by trapped air in sensing line.
Memory trick: Air in the line makes the gauge lie, but only for a 'spell' until oil tells.