What Causes an Oil Pressure Gauge to Be Erratic?by Mike Schoonveld
Unlike many of the gauges on a car which are understandable with common sense and simple logic, the oil pressure gauge sometimes seems to be erratic. Most of the time, there's little to be worried about since breakdowns caused by loss of oil pressure is very low on the list of things likely to go wrong with your car. Still, the gauge is there for a reason and if you understand the possible causes of funky readings on your oil pressure gauge you'll have a better understanding of your car's engine.
The oil pump in your car is a mechanical device that turns faster and pumps more oil when the engine is running at high revolutions per minute (RPM) and turns slower and pumps less oil when the RPMs are lower. At both speeds, it pumps more oil than the parts of the engine needing lubrication needs. The excess oil pumped diverts back to the oil pan. If everything is working perfectly, the by-pass system should keep the oil pressure gauge steady, but it's not uncommon to show slightly more oil pressure at higher RPMs.
Simple physics dictates pumping thicker liquids requires more pressure than pumping thinner liquids. Most manufacturers recommend multiviscosity oil for their engines. Using these helps keep the oil at a constant viscosity. But as oil ages between changes, dirt and contaminants dilute the oil, so expect your pressure gauge to reflect that over time. You may notice a fairly large change in oil pressure readings just after an oil change.
Oil viscosity or thickness is also tied to engine heat. Multiviscosity oil, such as 5W-30, was developed so it would be as thin as 5-weight oil at start up, but give the engine protection of 30-weight at normal operating temperatures. If your engine is extremely cold or running hotter than normal, the viscosity changing chemistry in the engine oil may not be able to compensate and your oil pressure will vary as a result.
There are two types of oil pressure gauges, both of which can be problematic. The mechanical types that work with tiny springs and levers can get dirty, loose, worn or exhibit erratic readings just from engine or road vibrations. Electric ones can corrode and, since the sensor is activated over a tiny range of electrical resistance measures, a bit of corrosion can cause wide changes in readings.
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