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My wife has a 2004 Mazda 6, 2.3 liter 4 cylinder. When the car is parked, oil doesn't pool underneath. After driving for about an hour, we need to add about 1-3 quarts. With this issue, where could the oil possible be leaking from?

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Oil has a couple of places it can go.

  1. The ground
  2. The radiator
  3. The tail pipe

It could be a leak that only happens under pressure (IE the engine running) but for that amount the underside of the car would be covered in oil and should be very obvious by looking under the car with a flashlight.

If it's going out the tail pipe you would see obvious blue smoke whenever the car is running.

If it's going into the radiator a simple check of the coolant level, color, and quality with let you know.

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  • Agree. The engine could be burning it as it runs. Check the coolant for a milky color to it, and this is a tell that oil and coolant are coexisting and don't mix. – FossilizedCarlos Jul 31 '11 at 23:11
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    Also, if the vehicle has a drip tray / splash guard, check that for tell-tale signs of leakage under pressure. You'll see either a puddle or evidence that one used to be there. If it's iffy, try taping white paper to the top of the tray - a leak will definitely show up then. – Bob Cross Aug 1 '11 at 21:35
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    The easiest way to find out if it's burning oil might be to drive behind it for a little while and get the passenger to watch the tailpipe. Sometimes cars can blow a lot of oil out the tailpipe before it becomes visible to the driver. – Timo Geusch Aug 1 '11 at 22:00
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If you have a plastic valve cover, it can become slightly warped over time and oil can end up being forced out under pressure. This is unlikely, as @Larry said it should be rather obvious looking under or into the engine compartment.

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I will add two reasons worth investigating, first being worn piston rings, with the oil leaking inside the cylinder(s) and being burnt inside the cylinder(s), but this should not burn such a significant quantity (smoke coming out of the tailpipe is a sign of this). The second one would be your valve cover gasket. Either a low quality or worn one, or even being replaced with silicone, can indeed leak only when oil pressure builds up, and that is with the engine in load, while driving (this one means you will have oil marks on the outside of your engine block, which you can easily spot visually) Good luck with your diagnostic

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