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Low oil pressure light flashes at idle when the engine is warm. It doesn't do that when the engine is cold and it flashes more frequently the warmer the engine gets. The light doesn't flash or isn't on under load. Oil level is halfway between min and max. I don't see signs of coolant and oil mixing. No obvious signs of leaks and oil level is constant.

The car has another issue as well: lean at idle. STFT values at idle are extremely high, topping at 25%, LTFT is approx 13.5%. Sometimes the ECU goes to "open (?) loop, failure" mode with predfined rich fuel trimms. Oil smells at least some fuel but not more than two years ago. No smoke coming out of tailpipe when the engine is warm.

I know that lean idle should be addressed but the only DTC has been P2187 which to me seems like an expensive diagnosis as I don't have the skills to do it and don't know any mechanics that I could trust to not start the "let's swap some parts" roulette. Besides the problem only manifests at cold weather. The car also has another probably expensive defect, 5th gear is popping out while driving (no leaks, oil change performed approx 50000 km ago), so I'm not willing to spend hundreds of euros on it.

The oil has been changed 9 months and approx 7000 kilometers ago. For the last two months there has been quite a lot short running and idling and I was forced to idle for an hour due traffic accident in minus 25 Celsius weather.

My question is, can the oil be so badly diluted with fuel that it lowers the oil pressure so much that the pressure is below the minimum for oil pressure switch to trigger? Would it be worth to try to change the oil and use thicker viscosity?

The oil is 5W-30 synthetic, the thinnest that manufacturer recommends. The oil pressure light is on while ignition on and engine off so doesn't that indicate that the switch is probably functioning correctly?

The car is Mazda 3 1,6 L gasoline engine with approx 156000 kilometers on it. There might have been a period approx four to seven years ago that no oil service and proper driving took place. There's no receipt or such to indicate that oil has been changed during that time. Since 2021 the oil has been changed on yearly basis, under 10000 km intervals.

Is it paint to get to the oil pressure switch to measure pressure and change the switch if needed considering this car model and the fact that the splash shield bolts and such are probably badly rusted?

EDIT: The engine makes ticking sound at idle which removes under load but it's been like that since I bought it so I've thought that is normal for it.

EDIT2: Car is from 2005. No timing chain or oil pump maintenance performed.

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  • Welcome to Motor Vehicle Maintenance & Repair! What year is your Mazda 3? Commented Mar 4 at 13:29
  • It's 2005. To my knowledge no timing chain or oil pump maintenance has been performed.
    – Masza
    Commented Mar 4 at 13:49

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First, change the oil to new proper grade oil as soon as possible.

See if the problem is still there. I myself prefer changing oil at 5000 km no matter what the manufacturer says about the intervals and I keep my vehicles for 20+ years.

If the problem still occurs, get the oil pressure testing kit, on Amazon they start at around 30 bucks or so. It screws into the block instead of the oil pressure sending unit. That will tell you if you just need to replace the oil pressure sensor or you have a bigger problem. On some vehicles changing the oil pump is a really big deal (not sure about Mazda 3).

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  • Something else I saw on the maintenance for the 2005 Mazda 3 was at a point the valves may need adjustment. I don't recall the mileage this needs to occur, but do remember when I looked at them the OPs vehicle was past that. I think this could be why it has the ticking sound. Agree with changing the oil soonest. You shouldn't be able to smell gas on the dipstick. And definitely fuel in the oil will thin it and cause issues. Too much fuel in the oil will wash down the cylinder walls and cause wear. Commented Mar 6 at 12:37
  • So I have a major issue if the oil smells like gasoline soon (couple hundred kms) after oil change? The smell is not like pure gasoline, like when I open the gas tank or fill it. But it's not like the smell of fresh oil in a bottle either.
    – Masza
    Commented Mar 6 at 14:14

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