Low oil level
Checking the oil level is necessary before deciding to repair the engine. When the oil level is low, it is difficult for the oil pump pickup tube to draw enough oil to maintain the correct pressure in the engine lubrication system. Low oil pressure under these conditions is a small evil compared to the damage that low oil pressure can cause. Probably the best preventive measure in this case would be to check the engine oil level at least once a week and add oil as needed.
Clogged oil filter
The simplest or most obvious cause of low oil pressure is a clogged oil filter. Unfortunately, there is no way to say for sure that this is usually the result of inattention or mistakes. If the oil has not been changed within the time that the engine suffered from low oil pressure, then the oil and filter should be changed.
Liquid or diluted oil
The idea of acquiring an oil that is too thin (those. low viscosity), pretty stupid. If you are using oil that was purchased with an automotive engine lubrication circuit, it is almost impossible to purchase oil that will cause low oil pressure on its own. Some people may disagree with the statement that using a thick oil with a high viscosity will cause the oil pressure to rise. This is often true. However, if the engine lubrication system, oil pump and bearings are in good condition, even a low viscosity oil will be able to provide the correct oil pressure.
Another thing is diluted oil. The oil becomes diluted in several ways. Perhaps the most common way is described below. When the engine is running, especially after starting, some of the exhaust gas will blow past the piston rings and into the crankcase. These gases contain some unburned fuel. Unburned fuel dilutes the engine oil. This dissolved oil can cause low oil pressure. But an even more critical phenomenon can occur, which is that unburnt fuel, combined with moisture in the crankcase, can form an acid that can damage bearings and their surfaces long before the low oil pressure caused by its dilution, may damage the engine.
Another way: engine oil can become diluted due to coolant (coolant). If the cylinder head gasket begins to leak, or if the cylinder head or cylinder block cracks, coolant may leak into the combustion chamber and/or crankcase and dilute the engine oil.
Although low pressure due to diluted engine oil can be detected by a simple oil change, the question is: does the engine need major repairs due to a problem caused by diluted engine oil? Another question is this: how much damage was the result of diluted engine oil? A simple oil change can increase oil pressure, but the engine may already be damaged and must be repaired before catastrophic consequences occur.
Oil pump pressure relief valve stuck open
All engines are equipped with an oil pressure reducing valve in the oil pump. The purpose of the valve is to limit the oil pressure within limits that prevent the oil filter from turning into a frag grenade. Occasionally, the relief valve will stick open, allowing oil to be pumped by the oil pump and returned back to the oil sump. This fault is very difficult to identify and may require a complete engine overhaul.
If time is not a critical parameter, remove the oil pan and install a new oil pump. Although it is possible to repair the pressure reducing valve, there is not much point in removing the oil pan and making repairs «just in case».
Damaged oil pump pickup tube
When the oil pump is removed for a new pump installation operation, check the feed tube for possible damage. If the tube is cracked or loose, the oil pump will not be able to draw oil from the oil sump. This will cause low oil pressure. Before replacing the oil pump, make sure the feed tube is in good condition.