This article is part of a 6-part series on engine lubrication systems. Part 4 covers how oil pumps create flow and pressure, the different pump designs including gear, rotary, internal gear, and vane types, and how pressure is regulated with relief valves. Watching an oil pressure gauge drop at a stoplight tells you something about pump wear or bearing clearance that you cannot see any other way.
Positive Displacement and Vacuum Generation
Oil pumps are mechanical displacement devices. As the internal pumping elements, which are gears or rotors, move apart, they create a partial vacuum at the inlet, drawing oil into the housing. As the elements mesh or move together, the volume decreases, forcing the oil out of the discharge port under pressure. The pump produces flow; pressure is the result of that flow meeting resistance from the engines internal bearing clearances and galleries.
Oil Pickup Engineering
Rigid pickups use a fixed pipe extending to the bottom of the sump. They utilize a screen to exclude large debris and are positioned to remain submerged even during minor oil level fluctuations. Floating pickups are designed to pivot and float on the top layer of the oil. This ensures the pump draws the cleanest oil, avoiding the sludge and heavy contaminants that settle at the bottom of the sump. Many pickup assemblies include a small relief or bypass valve. In the event of a screen clog due to heavy sludge or ice, the vacuum increase forces this valve open to maintain oil flow to the pump, prioritizing lubrication over filtration.
Gear Pump
A gear pump consists of a driving gear fastened to a shaft and a driven gear. The drive shaft is typically geared to the camshaft via spiral or helical gears. Oil is trapped in the spaces between the gear teeth and the pump housing. The oil is carried around the outside of the gears to the outlet side. As the teeth mesh at the center, the oil is squeezed out and forced into the discharge gallery. Gear-to-housing and gear-to-cover plate clearances must be minimal to prevent internal leakage, also called slip, which reduces pumping efficiency at high temperatures.
Rotary Pump
A rotary pump features an inner star-shaped rotor and an outer rotor. The inner rotor is mounted off-center. As it rotates, its rounded points walk around the inner surface of the outer rotor. The spaces between the rotor lobes increase at the inlet, creating a vacuum. The spaces decrease as the lobes move toward the outlet, compressing and discharging the oil. The inner rotor fits snugly against the outer rotor at all times, creating a continuous seal that allows for efficient high-pressure output.
Drive Train Loading
The oil pump is a parasitic load on the camshaft. The drive gear on the camshaft must be properly meshed with the pumps spiral gear to ensure timing and torque transfer. The clearance between the gears or rotors and the pump housing is critical. Excessive wear in the housing allows oil to bypass the pumping elements, causing a drop in system pressure, especially at low RPM or idle.
Pressure Regulation and Relief
An oil pump in optimal condition is engineered to produce flow and pressure far exceeding the engines operational requirements. Without regulation, excessive pressure would cause oil filter rupture, seal failure, and erratic lubrication. The pressure relief valve, or PRV, is a spring-loaded plunger or ball located on the discharge side of the pump. When oil pressure exceeds the mechanical tension of the relief spring, the valve opens. Excess oil is diverted back to the suction side of the pump or directly into the sump. The spring rate or tension of the relief valve determines the maximum system oil pressure.
Internal Gear and Vane Designs
Internal Gear Pump (G-Rotor Variation)
This design utilizes an internal driving gear meshed with an external driven gear. A stationary crescent-shaped filler block is often positioned between the gears. As the gears rotate, the volume between the teeth increases at the inlet, drawing oil in. The oil is carried past the filler block in the gear teeth. As the gears mesh at the outlet area, the oil is compressed and forced into the discharge gallery. This pump type is typically driven directly by the crankshaft or via a drive spline connected to a distributor or intermediate shaft.
Vane Pump
A vane pump features a rotor placed off-center, or eccentric, within a round housing. The rotor contains sliding vanes. Centrifugal force and spring tension hold the vanes in constant contact with the housing wall. As the rotor turns, the vanes create expanding volumes at the inlet, creating a vacuum, and contracting volumes at the outlet, creating pressure. The sliding vanes automatically adjust for minor housing wear, maintaining a high-efficiency seal throughout the components service life. This is called self-compensation.
Lubrication and Filtration Circuit
A filter bypass valve is a secondary relief valve integrated into the filter housing or adapter. If the oil filter element becomes restricted or clogged, or if the oil is too viscous for cold-start flow, this valve opens to allow unfiltered oil to reach the engine. The engineering trade-off is that the system prioritizes the presence of contaminated lubricant over the total absence of lubricant at the bearing surfaces. O-rings and gaskets, such as the oil pump body gasket and the cylinder block to pump gasket, are critical for maintaining the vacuum on the inlet side. Any air leak, which causes aeration, will lead to pressure cavitation and lubrication failure.
Assembly Alignment and Drive Integration
Oil pump drive shafts often feature specific tangs or slots that must align with the distributor or crankshaft drive gear. The clearance between the pump gears and the cover plate must be maintained to prevent end-play leakage, which significantly reduces low-speed oil pressure. Fastener torque is critical to preventing housing distortion, which can bind internal gears or rotors.
A pump that cannot hold internal clearances will lose prime and deliver aerated oil instead of a solid film. Proceed to Part 5.
Local Shop Note:
This is similar to something a technician on E Main Rd in Le Roy, N.Y. told me about — a repair where the symptoms pointed one way, but the real cause was somewhere else. He was at an ATTS seminar, and he was telling me about a sedan that came in with a complaint that the oil pressure light would flicker at hot idle and the pressure gauge would drop to near zero. The customer had already replaced the oil pump and the oil pressure sender. Still had low pressure.
He hooked up a mechanical gauge and confirmed the pressure was dropping to 4 psi at hot idle. He dropped the oil pan and checked the pickup screen — it was clean. Checked the pump clearances — within spec. Then he started looking at the pressure relief valve. He pulled the spring and found it was broken, allowing the valve to stay partially open. That bypass was dumping oil back to the sump instead of letting it build pressure in the galleries. The pump was fine — the relief valve was the problem.
He replaced the relief valve spring, and the oil pressure came back to 7 psi at hot idle.
After seeing enough repairs like that, you start to realize the pressure relief valve is the one thing that sets the maximum oil pressure. If the spring breaks or weakens, the valve opens too early, and you lose pressure. Always check the relief valve when you’ve got low oil pressure and everything else looks good. Sometimes the pump is pumping fine — the valve just won’t let it build pressure.