2012 Mazda CX-9, 3.7 Liter 6 cylinder engine (105,000 miles - Texas based): oil pressure light comes on and off (remaining off) at cold engine idle. When thermostat opens, oil pressure light comes on at idlel, but goes off when engine is accelerated. Engine idols at 675 rpm (analog indicator) - the light goes out at 775 rpm. Oil remains at normal levels and no sign of leaks. No other indicators of an oil pressure issue. Changed oil with new filter - no contaminants in the oil (antifreeze etc.) Might the oil light be responding to a faulty oil pressure sensor? ($12 at Auto Zone). Thanks much!

 

Oil pressure sensors are a fairly common failure point and there are no OBD-II codes associated with a faulty sensor. Said another way, it is worth a try to see if the light says off when the engine is warm and at idle.

 

I'm thinking you are right in just replacing the sensor. For $12 (US) not a big deal. In older models of Ford engines, I would increase the idle speed so the light would stay off - LOL. If it is the sensor, I'm tempted to put a gauge connection to the driver's-side to monitor the pressure rather than rely on an "unreliable" idiot light. (This supposes there is not a larger issue lurking.)

 

If the oil pressure is actually low, the light comes on at the minimum pressure. Driving at or below minimum oil pressure will cause engine damage.

I would get the pressure tested with a decent oil pressure gauge for peace of mind.

 

Agreed - I'll be gauge testing the oil pressure levels. My question remains: Why doesn't the oil pressure warning light come-on until the thermostat engages? At cold engine temperatures (engine start-up), the oil pressure warning light remains off when the oil pressure is at its lowest? It seems that if the engine's oil pressure was low - a real engine oil circulation issue, it would remain on whether or not the engine was cold or warm. Puzzling!

 

The oil pressure following a cold start is ALWAYS high; as the oil warms up the oil thins and the pressure drops when the engine is at idle.

 

Got it. I was thinking the opposite - my bad. I ran a quick errand today, oil pressure warning light did not come on when thermostat kicked-in for about 5 to 7 minutes - then the light began to flicker rapidly on and off for around 2 minutes at idle, then began the same behavior which is, the light comes on at full idle, and goes out at 775 rpm and remains off until it returns to full idle at a stop. I still wonder if the idle is set too low - perhaps engine conditions change to reduce the idle speed? Thank you for your assistance.

(Note, we purchased the CX 9 new in 2012, and it has had no issues except 2 wheel bearings - until now - again 105,000 miles - not too shabby!! We also have a CX 7( 2007 newly purchased) with 130,000 miles which has only needed some fuel injectors (4 cylinder non-turbo engine).

 

Also keep in mind that the oil temperature is not the same as the water temperature. The water (engine coolant) warms more quickly, but doesn't get as hot. Engine oil warms more slowly and eventually gets hotter.

Put on the $12 sensor. If it shows low oil pressure, find and fix the reason. It could be excessive bearing wear or a worn oil pump. It could be accumulated sludge in the oil pan or pump pickup screen. Or a defective oil pressure regulation valve (part of the oil pump assembly). All will get very expensive quickly!

 

A minor clarification; while the coolant in the engine warms pretty quickly, the coolant in the radiator can take quite a while to come up to temperature and will almost always be cooler than the engine oil.

 

What part of the radiator? The radiator has two tanks, a hot tank and a cold tank. The coolant is directed to the hot tank from the thermostat in the cars' head after the coolant temperature reaches the set point. From the hot tank the coolant flows through the radiator tubing to the cold tank. From the cold tank it flows back into the engine. The hot tank is hot--the temperature determined by the thermostat (I think 190° in our cars). The cold tank is much cooler depending on the rate of flow through the radiator and the air temperature. We also need to keep in mind that the thermostat doesn't click all the way open or click all the way closed. It modulates---moves just enough---to hold very close to the set point. Expect normally warm engine oil to run in the 200°F to 250°F range. Abnormally hot engine oil would be higher than 250.

My old Volvo turbo had the oil cooler inside the radiator hot tank. The transmission cooler was inside the radiator cold tank.