Anyone wanting to sustain high performance over a long period needs good circulation. As with people, the same applies to automobiles, with the oil circuit playing an important role in the service life of an engine. Find out what functions it performs, what might cause it to fail, and how this can be avoided.


The oil circuit handles a wide range of tasks in the engine bay:


    The oil forms a thin film between the friction surfaces and thus prevents direct contact. By overcoming frictional resistance, it reduces loss of energy and wear.


    The lubricating film is a good vibration and sound damper.


    High-performance components such as valves, pistons, or camshafts do not come into direct contact with coolant. They largely dissipate their heat via the oil.


    Combustion residues, abrasion, and unwanted microparticles are suspended in the oil and discharged. These are then captured in the oil filter or a magnetic separator.


    It is not possible to have a rigid seal for sliding parts. Oil assumes the sealing function, even between the combustion chamber and crankcase.


    Oil prevents rust formation on partly bare components, even during prolonged periods of disuse.


Several components are involved in the forced-feed lubrication process. The lion’s share of the job is assumed by the oil pump: it generates the pressure required to move the oil from the sump to the lubrication points via the channels and bores.


From the oil sump to the lubrication point: the components involved in forced-feed lubrication.


For all of the components in the lubrication circuit to function reliably, it is essential that the oil is changed at set intervals. Caution: always change the oil filter at the same time! Only a clean filter can remove contaminants such as dust, metal abrasion, and combustion residues from the oil.

By the way: Regularly changing the oil is necessary, on the one hand, due to physical contamination. On the other hand, it is also important because of the chemical reactions that occur, for example, through the thermal impact on the oil.

The quality of the oil can be impaired in many different ways.


Despite the oil film between the piston and cylinder wall, combustion gases enter the crankcase. The oil oxidizes and acids are formed, which corrode the component coatings.


Fuel contents do not burn fully, particularly when the engine is cold or the mixture formation is poor. Residues can enter the oil sump via the cylinder wall and cause dilution of the oil. In contrast, thickening frequently occurs in diesel engines because of strong oil oxidation—combined with soot particle deposits.


Resins separate from the oil and combine with fine road dust or dissolved combustion residues, which results in sludging of the oil. This effect is further amplified when traveling short distances: if the engine does not warm up fully, the condensation water from the fuel-air mixture cannot be “boiled off” out of the oil. It is then bound into the circuit due to the constant overturning motion and thus contributes to sludging and constriction, or even blockages in the circuit.


When the oil pressure indicator comes on, it isn’t always because of the oil filter. A malfunction can have many other causes. Taking a closer look at the filter and the seal between oil filter and oil filter flange might help to clarify the issue, especially when high system pressure plays a role.

  • Excessive continuous system pressures or pressure peaks can cause the filter to bloat or the gasket to be pushed outward from the sides.
  • Another indication of overpressure is the deformation of the oil filter’s cover plate. The filter was lifted slightly from the flange, making it light work for the arising oil pressure to push through the gasket.

In all such instances, this causes a loss of oil between the oil filter and oil filter flange. If the engine is not brought to a standstill quickly enough, the inadequate lubrication can lead to major engine damage.

  • Clear signs of excessive oil pressure: a bloated filter …
  • … a pushed-out gasket …
  • … or a deformed filter end plate.


Depending on the engine manufacturer, the oil circuit is designed for a system pressure of 2.3–5 bar. However, oil pumps can generate pressures that exceed well over 20 bar. To ensure adequate lubrication when the engine oil is cold and highly viscous and the engine is running at high speeds, they use a greater amount of oil and generate a higher pressure—more than the oil circuit can tolerate in the long term. When the oil temperature is higher and viscosity is therefore lower, the volume flow increases: the pumps reduce their delivery rate.

The oil pressure regulating valve performs an important task: it restricts the oil pressure in the system. It opens a return channel, if needed, through a controlled-expansion piston, which overrides the resistance of the spring in the event of excessive pressure. This return channel then drains the pressure or oil excess directly into the oil sump. In terms of tribology, defective oil pressure regulating valves are regarded as one of the main causes of engine damage.

Here is a brief overview of the main triggers and their effects.

Sludging Valve sticking when closed All components in the lubrication circuit are constantly exposed to unregulated pressure and pressure peaks from the oil pump.
Sludging Valve sticking when open Inadequate lubrication, as a large proportion of oil drawn in by the pump is being drained into the oil sump directly via the valve.
Sludging or deposits and metal particles (e.g., bearing material) Inadequate lubrication of the controlled-expansion piston in the oil pressure regulating valve The controlled-expansion piston starts to seize. This can lead to temporary jamming in the various positions—and consequently to effects such as those described in 1 or 2.
Loosening of deposits after oil change Irregular oil change intervals lead to the formation of deposits, the operation with low-grade oil can cause excessive formation of resin, which can end up as deposits in the lubrication circuit. If an oil change is delayed or the oil is replaced with a high-grade variant, the deposits will loosen. Effects as described in 1, 2, or 3 are possible because fresh oil has a cleansing effect.


Identifying and rectifying the real cause of problems in the oil circuit can avoid expensive subsequent damage. The problem is often not solved by simply changing the filter. It won’t be long before lubrication is found to be inadequate again!

Therefore, we recommend a close examination of the oil pressure regulating valve and its replacement if there is the slightest suspicion of it being defective. Caution: even with the correct oil pressure, the proper functioning of the valve is not guaranteed, as a fault cannot always be identified when measuring the oil pressure if the valve is only jamming sporadically.


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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