In a high distance engine, low oil pressure is usually due to a combination of worn main and rod bearings and crankshaft journals. The oil pump does not generate pressure. It makes flow and the resistance to that flow causes pressure. Resistance is the product of the orifices in the engine block through which the oil flows, and the amount of gap between the bearings and crankshaft journals. As the bearings wear out, gap increase allowing increased flow which reduces pressure.
Note here that it doesn’t take much of an increase in bearing clearances to cause a noticeable drop in oil pressure as well as noise. This applies to all cars from brand new engines to high mileage ones.
Excessive bearing gap or clearance ( usually more than about .001 inch per inch of diameter of the crankshaft journal) can create up to a 20 percent or greater drop in oil pressure, which may have an adverse effect on lubrication elsewhere in the engine ( the camshaft and upper valvetrain, usually in overhead cam engines). Excessive bearing gaps will also increase engine noise and pounding, which over time can lead to bearing stress and failure.
Recommended bearing clearances may vary a great deal depending on the engine application, however many engine producers today aim for about .001 to .002 inch clearance in the main and rod bearings.
Excessive gaps somewhere else in the engine can also reduce oil pressure. These are wear in the lifter bores, excessive gaps between the camshaft journals and cam bearings, and excessive end play in the cam. Usually , any cracks in the oil galleys, leaking galley plugs, or leakage between the oil pump and block will also affect the oil pressure.
One major solution for low oil pressure due to excessive bearing gap is to reduce the clearances by changing the bearings or overhauling the engine. Having some new oil pump or a higher pressure pump installed won’t help because the bearings have too much a leakage rate to hold the required pressure. Installing a higher volume oil pump can actually increase flow and regain a little lost pressure. But the underlying clearance problem will still be there, which will speedup bearing noise, wear and fatigue.
Another common cause of low oil pressure is wear or excessive gaps in the oil pump itself. Though specifications vary, as a rule gear type oil pumps should have less than about 0.003 inches of end play between the gears and the cover. The gaps between the teeth and pump unit should usually be less than about 0.005 inches. With rotor style pumps, the gap between the outer rotor and pump housing unit should usually be less than 0.012 inches, with no more than about 0.010 inches between the inner and outer rotor lobes. Excessive clearance inside the pump will limit the pump’s ability to pump oil efficiency, which reduces flow and pressure.
Because of the close tolerances that are required inside the oil pump, particle of any kind can create problems if it gets sucked into the pump. Anything bigger than the minimum internal clearances can score or jam the pump. particles such as pieces of old valve stem seals, gasket material, plastic chips from a worn timing chain gear, bearing material, casting flashing, sand, dirt, etc., may be harmful if taken in.
But how is it possible that these particles come in? The screen that is on the oil pump pickup tube in the crankcase only stops relatively big pieces of debris from being taken into the pump, but even at that it does not always do that because most pickup screens have some type of bypass valve or vent that allows oil to bypass the screen if the screen becomes plugged or the oil is too thick to pass through the screen. The holes in the screen itself measure about 0.040 inches square, which are huge openings as far as small particles are concerned. However the holes are large by design so the screen will flow an adequate amount of oil when the engine is cold and the oil in the crankcase is thick. This implies that the oil pump is the only engine component that is continually given unfiltered oil! The oil does not pass through the filter until after it leaves the pump. So any particle that finds its way into the crankcase will first go through the pump before it is trapped by the filter. A reason why oil pumps wear out and break.
Restrictions in the pickup tube screen can limit the flow of oil into the pump, reducing flow and pressure. Even a relatively small amount of dirt buildup on the screen can limit oil flow at higher engine speeds. A coating only .005 inch thick on the screen will reduce the total “open” area of each hole to .030 inches, creating an amazing 44 percent reduction in oil flow!
The pressure relief valve, which is mounted on the pump body or elsewhere on the engine, can be yet another reason of low oil pressure if the valve sticks open or is held open by a small piece of debris. The relief valve is designed to limit oil pressure as engine speed increases. The valve opens when pressure reaches a preset value (typically 40 to 60 psi). This sends oil back into the crankcase and reduces maximum oil pressure in the engine. The reason for doing so is to stop oil pressure from reaching dangerous levels. Too much oil pressure can be just as lethal as too little because excessive pressure can rupture the oil filter or even blow out pressed-in oil galley plugs in the block.
Low oil pressure may also be caused by air in the pump. If there is too little oil in the pan, air can be drawn into the pump. But this can also happen if the crankcase is overfilled. The oil can become filled with tiny bubbles because it is having contact with the spinning crankshaft and is being churned into foam.
Sometimes the engine may become oil starved for oil at higher rpms because the oil is not returning quickly enough to the crankcase. The underlying cause here is usually severe buildup that restricts the oil return holes in the head.
Leakages between the oil pickup tube and pump, as well as between the pump and block can also invite air into the pump. It is not uncommon to find engines where the pickup tube has fallen completely off, creating a complete loss of oil pressure.
A plugged oil filter oil can be yet another reason for a low oil pressure. When the oil leaves the pump, it journeys through the filter before going on to the bearings and oil galleys. All filters create a certain amount of resistance to flow that in turn increases with the rate of flow. But the amount is not much, typically only a couple of pounds. But as the filter gets clogged with debris, the restriction created is increased. Eventually it may get to where no oil will pass through the filter element. So to prevent such a situation, a pressure relief valve located in the filter or where the filter mounts to the block is designed to open if the pressure differential across the filter exceeds a preset value (typically 5 to 40 psi). This allows the oil to bypass the filter and keep on flowing. But the engine’s oil pressure will be reduced to that of the bypass valve. Changing the plugged filter is the solution.