Engine Oil Pressure: Why it Rises on Restart

It's a common observation for many car owners: you turn the ignition key, the engine coughs into life, and for a fleeting moment, the oil pressure gauge or warning light indicates a significantly higher pressure than usual, before settling down to its normal operating range. While it might seem alarming, this phenomenon is often a sign of the engine's lubrication system working as intended, or sometimes, a subtle indicator of a component that could benefit from attention. Understanding why this happens is crucial for maintaining your vehicle's health.

The Basics of Engine Oil Pressure

Engine oil serves a vital role in lubricating moving parts, reducing friction, dissipating heat, and cleaning the engine. The oil pump, driven by the engine's crankshaft or camshaft, circulates this oil throughout the engine, creating pressure to ensure all components receive adequate lubrication. This pressure is critical; too low, and you risk catastrophic engine damage. Too high, and it can also cause issues.

Why the Initial Surge? The Role of Anti-Drain Back Valves

One of the primary reasons for a temporary rise in oil pressure upon restarting is the function of anti-drain back valves. When an engine is switched off, gravity can cause oil to drain back from the oil filter and oil galleries into the sump. This leaves critical engine components, particularly those at the top of the engine, momentarily without lubrication until the oil pump can repressurise the system.

To combat this, many oil filter housings incorporate an anti-drain back valve. This one-way valve is designed to hold a certain amount of oil within the filter and the oilways when the engine is stopped. Upon restart, this retained oil is immediately available, allowing pressure to build much faster. In some systems, this retained oil, being already under some pressure from the pump's initial efforts, can cause a brief, higher-than-normal pressure reading before the system fully equalises.

The M43 Engine Example: A Deeper Dive

A specific instance, as described by an owner of a 318 M43 engine, highlights the complexity of these systems. In this case, after an oil change, the owner experienced a delay in oil pressure build-up, leading them to investigate the oil filter housing. They discovered that the M43 engine, like many, employs multiple valves to manage oil flow and pressure.

The M43 engine's oil circulation involves:

• Oil Pump: Located within the timing chain case, drawing oil from the sump via a pick-up pipe.
• Anti-Return Valve (near the pump): This valve, often referred to as a pressure control valve or bypass valve, is crucial. Its function is to hold oil within the pump itself when the engine is switched off. If this valve is weak or leaks, the pump can become 'dry' on startup, requiring it to draw oil up from the sump and build pressure from scratch.
• Oil Filter Housing Valves: The filter housing itself can contain one or more valves. In the M43 example, two are mentioned:
  • A valve allowing oil into the filter bowl from the pump.
  • A second valve allowing oil past the oil pressure switch and into the secondary oil gallery, feeding the engine.

The owner's investigation revealed that a faulty pressure control valve next to the pump was the culprit for their initial problem (a delay in pressure, not a surge). However, the principle of retained oil and its effect on initial pressure is relevant. If the anti-drain back valves (within the filter housing or elsewhere) are particularly effective at holding oil, or if the pressure control valve is functioning perfectly and retaining a good amount of oil at the pump, the initial pressure reading upon restart could indeed be higher as the system quickly pressurises this pre-existing volume of oil.

Other Contributing Factors to Pressure Surges

Beyond the anti-drain back and pressure control valves, other factors can contribute to a temporary rise in oil pressure:

• Cold Oil Viscosity: When an engine is cold, the engine oil is thicker (more viscous). The oil pump has to work harder to push this thicker oil through the system, resulting in temporarily higher pressure. As the oil heats up and thins, the pressure typically drops to its normal operating range.
• Oil Filter Condition: A new, clean oil filter offers less resistance to oil flow than a partially clogged one. While a clogged filter might restrict flow and lower pressure, a brand-new filter might momentarily allow a slightly higher flow rate until the system stabilises, potentially showing a brief pressure peak.
• Oil Pump Design: Some oil pumps are designed with specific pressure regulation mechanisms that might cause a slight initial over-pressurisation before settling.
• Temperature Fluctuations: Minor variations in ambient temperature can influence oil viscosity and, consequently, the initial pressure readings.

Diagnosing Potential Issues

While a brief pressure surge on startup is often normal, persistent or excessively high pressure, or a noticeable delay in pressure build-up, could indicate a problem. Here’s how to approach diagnosis:

Frequently Asked Questions

Q1: Is a temporary spike in oil pressure on startup bad for my engine?
Generally, no. If it's a brief surge that quickly returns to normal operating pressure, it's usually a sign that the anti-drain back valves are effectively holding oil, leading to a rapid pressurisation of the system. Modern engines are designed to handle these minor fluctuations.

Q2: How can I tell if my anti-drain back valve is faulty?
A common symptom of a faulty anti-drain back valve is a prolonged oil pressure warning light illumination after starting the engine. This indicates that oil isn't being retained effectively, and the system takes longer to build pressure. Conversely, a valve that's stuck partially open could lead to a slight delay, while a valve that's stuck closed might cause pressure issues under certain conditions.

Q3: What kind of oil should I use?
Always refer to your vehicle's owner's manual for the recommended oil viscosity (e.g., 5W-30, 10W-40) and specifications (e.g., API SN, ACEA A3/B4). Using the correct oil is crucial for proper lubrication and pressure regulation, especially in varying temperatures.

Q4: Could a cheap oil filter cause this issue?
Yes, a low-quality oil filter might lack an effective anti-drain back valve or have one that fails prematurely. This can lead to oil draining back into the sump, causing delayed pressure build-up on subsequent starts. It's always advisable to use reputable brands for oil filters.

Q5: Where is the pressure control valve usually located?
The pressure control valve (or pressure relief valve) is typically integrated within the oil pump assembly itself. Its purpose is to prevent excessively high oil pressure by bypassing excess oil back to the sump or intake side of the pump if the pressure exceeds a safe limit. A sticking or weak pressure control valve can lead to either low or dangerously high oil pressure.

Conclusion

Observing a temporary rise in oil pressure upon restarting your engine is a nuanced aspect of automotive mechanics. In most cases, it's a testament to the clever design of valves like the anti-drain back and pressure control valves, ensuring your engine receives lubrication as quickly as possible. However, paying attention to the duration and magnitude of this pressure change, alongside other symptoms like warning lights or unusual noises, is key to identifying potential maintenance needs. By understanding the intricate workings of your engine's lubrication system, you can ensure its longevity and reliable performance on the road.