Noticed my beater Honda del Sol (177k miles) idling slowly the other day. Normal 45F day, light urban driving. Usually idle is ~800rpm (indicated), was dropping to 600rpm and even 500rpm. Holding her at 1000rpm then releasing the gas pedal yielded ~300rpm and it almost died, followed by ~500rpm. Ran fine at any other speed; it had no hesitation, no noises and no warning lights.
So my GF said “you’re probably low on oil; my Cooper S did that once”. Huh? Why would low oil, other than if it were so low that it starved the engine and caused friction, impact the idle? If anything, I’d think it would idle faster with less fluid to pump, and/or with hotter and hence thinner oil (assuming it’ll run hotter if it has less oil, which it didn’t appear to be).
Sure enough, I checked and it was about a quart low. I’ve never had a car that uses oil, plus my cars usually have oil pressure & oil level indicators (i.e. Porsche 944S2). So I’ve never run those lower than near the bottom of the “normal” range.
Searching “The Googles” only provided unsatisfactory answers about metal-to-metal grinding and galling slowing down the engine, which is not what is happening here. There is still oil pressure, the oil light does not come on, and the engine sounds “normal”. And adding oil immediately cured the slow idle.
Seems like a coincidence. The only thing I can come up with is low oil might increase the temperature of certain parts of the engine and the computer might detect this and decide since the engine temperature is high the idle speed can be lowered. Sort of the opposite of what it does if the engine is cold, where it raises the idle speed. But that explanation sounds pretty bogus, even to me.
The oil pan is just a pool of oil. The pump draws its oil from near the bottom of the pool, through a straw-like “pickup tube”. It then forces the oil under pressure through the engine’s oil channels and in between the critical wear surfaces. From there the oil simply drains back to the pan and the cycle is repeated. As long as the oil level is above the pickup end of the “pickup tube”, the pump has no idea that it’s low, and oil is drawn up through the tube and forced inder pressure through where it needs to go.
That does not mean that it’s okay to allow the oil level to run low. It isn’t. But it doesn’t cause your idle to drop unless it gets so low that the level drops below the straw, the pump sucks air, and the bearings run dry and seize.
The other part of the discussion is, you need to check your oil more often too. The oil pressure is not an indication of oil level until it gets critically low. ALL cars use oil. They just use them at different rates and use more after sustained high rpm driving. That is NORMAL and it is normal for all motors to use more oil as it ages.