How our Oil Pressure Gauge works by Mike C.
Posted 14 October 2015 - 07:52 AM
The factory/dash oil pressure gauge is not calibrated very well. The display itself is pretty good... it's the sensor unit that varies considerably. Some sensors will start indicating with only a few psi oil pressure; others (most) seem to require about 20 to 25psi before they'll make the needle budge. On most StarQuests, each 1/4 of the display is about 25psi. In theory then half-scale would be 50psi IF the sensor was one of the "good ones" that starts working at only a few psi. For sensors that take 20 to 25psi to kick in, half-scale is more like 70 to 75psi oil pressure... which is normal oil pressure on most healthy StarQuests. Oil pressure goes up/down in sync with RPMs too - that's why many modern engines use oil pressure in the variable valve timing system. Dash oil pressure readings lag real oil pressure by several seconds: when you rev the engine, oil pressure will build up as quickly as the RPMs do even though the gauge will take several seconds to do anything. Proof: start the engine and let it idle... wait a minute until the gauge is holding steady. Now stall the engine, or quickly shut it off and then turn the key back to ON. Notice the oil pressure gauge will still be showing pressure? The pressure inside the engine will be pretty much zero within a second of it being shut off despite the dash gauge saying "there is still significant pressure."
Higher idle oil pressure on a cold engine is normal; the oil is thicker and thus easier to pressurize. 15psi at idle is the factor minimum spec; most StarQuests have at least 25psi at idle. 50 to 75psi at "cruise" speed/RPMs is normal.
Given the wide variation (car to car) in StarQuest oil pressure gauge readings, the following rules of thumb should be used:
1: learn the "typical" gauge readings (all gauges!) for your car when it's running right. Then, when any gauge starts reading differently, you know something has changed.
2: oil pressure: on the 87-later cars, or any StarQuest converted to the auto-lash adjusters (automatic valve lash adjustment "pills") low oil pressure will manifest itself as rapid ticking/clattering from the top of the engine. A little clatter is normal on initial start-up if the engine has been parked for a while... the oil has been squeezed out of the adjusters (also commonly called "lifters" since they do look like dinky versions of old fashioned lifters on pushrod engines); it takes a few seconds for oil pressure to re-pressurize the lifters so they can do their jobs.
3: expect different oil pressure gauge readings if you change oil viscosity or if you change brands of oil filter. Some filters have a lot more restriction than others. I noticed a significant increase in oil pressure when I switched to the NAPA Gold filters which are really Wix filters.
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