A technician inspects an engine's oil pressure relief valve after a report of low oil pressure at high RPM but normal pressure at idle. Disassembly reveals the relief valve spring has weakened and lost tension. What effect would this have on oil pressure?
- AThe valve would stay fully closed at all times, causing excessively high oil pressure
- BThe valve would fail to open at all, causing oil pressure to rise uncontrollably at high RPM
- CThe valve would open at a lower-than-normal pressure, diverting oil back to the sump sooner and reducing indicated pressure at high RPM
- DThe weakened spring would have no effect on oil pressure regulation
Show answer & explanationAnswer & explanation
Correct answer: C. The valve would open at a lower-than-normal pressure, diverting oil back to the sump sooner and reducing indicated pressure at high RPM
The oil pressure relief valve spring holds the valve closed until oil pressure reaches a set value, at which point it opens to bypass excess oil back to the sump, limiting maximum pressure. A weakened spring allows the valve to open prematurely at a lower pressure, diverting oil sooner and causing lower-than-normal indicated oil pressure, especially noticeable at higher RPM when oil pump output is greatest.
Why the other options are wrong
- A. A weak spring would make the valve open too soon, not stay closed causing high pressure.
- B. A weak spring makes the valve open more easily, not fail to open.
- D. Spring tension directly determines the relief valve's opening pressure, so it has a major effect.
Oil Pressure Relief Valve
A spring-loaded valve in the engine lubrication system that limits maximum oil pressure by diverting excess oil back to the sump once a preset pressure is reached.
- Spring tension sets the opening (relief) pressure
- A weakened spring causes the valve to open at a lower pressure, reducing indicated oil pressure
- A stuck-closed valve can cause excessively high oil pressure
- Located in the oil pump/lubrication system, adjustable on many engines
Memory trick: Weak spring, early escape — oil slips back to the sump too soon.