What Makes a Car Leak Oil?

by Tom Lutzenberger
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classic car image by MAXFX from Fotolia.com

Leaking oil can be an irritating car problem, leaving unsightly stains in your garage and driveway. It can also be a sign of problems that will get worse over time if not addressed in a timely manner. However, such leaks can often be resolved quickly if you know where to look.

Types of Oil

A car runs with two kinds of oil: transmission fluid and engine oil. The transmission fluid helps keep the transmission, gears, differential and clutch (if you have a manual transmission) working. It provides a lubricant for moving parts to mesh with each other without grinding and destroying each other due to friction. Engine oil helps keep your engine pistons, crankshaft, and related parts working. It keeps the engine parts lubricated so the engine doesn't overheat with friction.

Transmission Leaks

Transmission oil leaks tend to occur between the transmission and the differential. These parts are bolted together with gaskets in between them to ensure tightness and a seal. As this seal wears down or has a break, the transmission oil will leak. Differences in outside temperature will exacerbate the gap and oil will leak more often when the car is cold than when it is hot and the metal parts have expanded.

Engine Leaks

Engine leaks happen much the same way as transmission leaks. The engine casing is a series of parts bolted to each other. In between these engine casing parts there are gaskets, again to help maintain a seal. When these gaskets fail, engine oil will leak out. However, unlike transmission oil that leaks when the vehicle is cold, engine oil leaks tend to happen when the engine is hot. This is due to pressure inside the working engine, which causes the oil to become very thin. Engine oil can also leak from the hoses and lines that pump the oil into the engine and drain it out. This can be due to bad clamps or to small breaks in the oil hosing.


Oil leaks are usually indicative of a car problem that will get worse over time. If the leak is due to a gasket failure, the oil will eventually drip out. While a minor transmission leak just causes stains, a major leak will lower the transmission fluid too low and allow gear parts to grind themselves to the point of failure. An engine leak will allow oil to seep out onto a hot engine. This can cause a fire. Alternatively, if the engine oil leaks out to the road, enough leakage will drain the engine oil level and the engine could seize up due to lack of lubrication.

Benefits of Troubleshooting

Knowing which oil is leaking in a car can help pinpoint where a growing problem is occurring in your vehicle. While some repairs can be costly, if a leak is due to a simple gasket failure you may be able to get the repair done under warranty if your car is fairly new. Main transmission and engine gaskets should not fail within the first five years of operating a new car. Identifying and fixing leaks as they occur can prevent them from ballooning into a serious engine or transmission failure.

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