The Role of Motor Oil
Motor oil plays a vital role in the health of your vehicle's engine. It lubricates the moving components, reduces heat and prevents the accumulation of sediment from inside the engine. Like coolant, motor oil is contained in a closed system, meaning you shouldn't have to "top it off." If you discover your vehicle's oil levels are dropping, though, it may indicate a breach in this otherwise closed system. The first step to fixing this breach is to determine from where exactly the oil is leaking.
Oil Pan Gasket
One of the most common areas from which motor oil leaks is the oil pan gasket. The oil pan is a metal pan that's found below your vehicle's engine, typically in the undercarriage. It's responsible for holding excess oil when the engine is off. When engaged, a pump moves oil from the pan to the engine. If the gasket securing your vehicle's oil pan has gone bad, it may allow oil to leak.
Oil Pan Drain Plug
A bad oil pan drain plug can also cause oil to leak from your engine. The drain plug is used to remove old oil during an oil change. You typically twist off the drain plug, using a socket wrench, at which point gravity will pull and drain out the oil. If the drain plug has been stripped or otherwise damaged, though, it may cause oil to leak.
Rear Main Seal
If you haven't done so already, inspect the area of your vehicle's engine where the crankshaft connects to the flywheel. Most automotive engines have a rear main seal here that can leak, especially when the vehicle has been neglected. When this happens, you'll usually see a pool or puddle of oil underneath your vehicle. However, the presence of oil underneath your vehicle may indicate leakage from several other places, you'll need to perform a more thorough evaluation before proceeding.
Even a bad oil filler cap can cause oil to leak from your engine. When your vehicle's engine is running, it pressurizes the oil chambers to about 30 to 45 psi. All of this pressure essentially pushes against the filler cap. And if the cap is loose or not creating a complete seal with the engine, some of the pressurized oil may leak out.
Finally, a bad oil filter can cause oil to leak from your engine. The filter should be replaced every time you change your vehicle's oil. But if the mechanic doesn't tighten the filter all the way -- or otherwise doesn't install it properly -- oil can leak from here.