Engine Oil Leaks: Where They Come From and How Serious They Are

The usual sources of an oil leak

Oil leaks nearly always come from a seal, a gasket or a connection that has aged. Heat cycles harden rubber and cork, and over many thousands of miles they stop sealing as well as they did. Because oil runs downwards and gets blown backwards by airflow, the drip you see under the car is often some way from the actual source.

That is why finding a leak properly takes cleaning the area, then looking for where fresh oil comes from, rather than guessing from the stain. It also means a leak at the top of the engine can leave the whole underside coated in grime.

• Rocker cover gasket at the top of the engine
• Sump gasket or sump plug washer at the bottom
• Crankshaft and camshaft oil seals at either end of the engine
• Oil filter, filter housing and oil cooler connections
• Turbo oil feed and return pipes on turbocharged engines

Rocker cover and sump gaskets

The rocker cover gasket seals the cover over the top of the engine. When it hardens, oil seeps out around the edge and runs down the side, sometimes onto hot exhaust parts, which can produce a burning smell. It is a common leak and often one of the more straightforward to cure.

The sump gasket seals the oil pan beneath the engine. A weeping sump may be caused by an old gasket, loose bolts or damage to the pan from a knock. Dents on the sump from speed bumps and debris are worth checking, since a punctured sump can lose oil quickly.

Oil seals and the more involved leaks

Crankshaft seals at the front and rear of the engine, and camshaft seals, are rubber rings that keep oil in where shafts pass through the engine. When they fail, oil may appear near the timing cover or at the bell housing between the engine and gearbox.

These can be more involved, because getting at them can mean removing the timing belt or the gearbox, or the clutch. A leak at the back of the engine can also contaminate the clutch. It is worth getting a proper diagnosis before authorising work, so that you know what is leaking and what is involved.

How to check for a leak yourself

Check the oil level regularly on a flat surface with the engine off and settled, using the dipstick if your car has one. Look under the bonnet for wet, grimy areas around the engine and under the car for drips. Placing a piece of cardboard under the engine overnight shows the position and colour of a drip.

A small amount of oil dampness around a gasket may be tolerable for a while, but fresh drips, a falling oil level, blue smoke or a burning smell mean it needs looking at sooner.

• Check the dipstick level on flat ground, engine off
• Look for wet, dirty patches around the cover and filter
• Note any oil smell or smoke from the engine bay
• Watch the oil warning light and never ignore it

Topping up and the danger of low oil

Topping up is a sensible stopgap, using the correct grade of oil for your engine, but it does not fix the leak. If you are topping up between services, the problem is getting worse, and the leak should be dealt with, not just fed.

Running low on oil is one of the quickest ways to wreck an engine. Oil lubricates, cools and cleans, and without enough of it bearings, camshafts and pistons wear or seize. If the oil warning light comes on while driving, stop safely and switch off. Do not carry on to the next junction. On a turbocharged engine, low oil can also damage the turbo.

At Banks of the Tyne in Hebburn we deal with oil leaks as part of engine repairs and servicing, and we will find the source first and tell you what is involved before work starts.

Frequently asked questions