Engine Oil Leak: Causes and What to Do

If you have spotted dark, greasy drips on your drive, the short answer is that the engine is losing oil through a seal, gasket or connection that has aged or failed. A slow seep is common on higher mileage engines and is not always an emergency, but it should never be ignored, because an engine run short of oil can be damaged very quickly.

The trouble is that oil travels. A leak at the top of the engine can run down and drip from somewhere completely different, so the wet patch under the car does not always point to the real source. Below we explain the usual causes, what you can safely check yourself, and what a garage does to trace and cure the leak.

Why is my car leaking engine oil?

Most leaks come from parts that seal two surfaces together. Rubber and cork seals harden with heat and age, then stop sealing properly. The usual suspects are the rocker cover gasket, the sump gasket or drain plug washer, the oil filter seal and the crankshaft seals.

A blocked crankcase breather can raise pressure inside the engine and push oil past seals that would otherwise hold. Overfilling with oil can cause similar trouble.

• Rocker cover gasket worn or hardened
• Sump gasket or loose drain plug
• Oil filter not seated properly after a service
• Crankshaft or camshaft seals worn
• Blocked breather raising crankcase pressure

How can I tell where the oil is coming from?

Start with a clean look. Park on level ground with the engine cold and the bonnet open, and look for wet, shiny or dirty areas around the top of the engine, the filter and the underside. Fresh oil is amber to black and feels slippery. Clear or coloured fluids that drip are probably something else, such as coolant, washer fluid or condensation from the air conditioning.

A sheet of cardboard slid under the car overnight shows where drips land and how often.

How do I check my oil level safely?

Check the level with the engine off, the car on flat ground and the engine having rested for a few minutes so the oil drains back into the sump. Pull the dipstick, wipe it clean, push it fully home and pull it out again. The level should sit between the minimum and maximum marks.

If the level is low, top up with the grade of oil your handbook specifies, a little at a time, rechecking as you go. Topping up is a stopgap, not a repair. If the oil warning light is on, or the level is below the minimum mark, do not drive any further than you must until it is topped up.

What will a garage do about an engine oil leak?

The first job is to find the true source. A technician will usually clean the suspect area, inspect with the vehicle raised, and sometimes run the engine and watch where the oil reappears. The repair then depends on what has failed: a gasket or seal replacement, a new filter or washer, or clearing a blocked breather.

Cost depends on how much has to be removed to reach the part. A rocker cover gasket may be quick to reach, while a rear crankshaft seal can mean removing the gearbox. At Automaster, a garage at Longcombe Cross near Totnes, we inspect first and then explain what is needed before any repair is carried out, so please ask for a quote once the source is known.

Is it safe to drive with an oil leak?

A slow weep that leaves a few spots now and then, with the level staying healthy, can usually be driven on short journeys while you arrange an inspection, provided you check the level often. A leak that drips steadily, leaves a puddle, or produces smoke or a burning smell is a different matter, as oil on a hot exhaust can start a fire.

Stop driving if the oil pressure warning light comes on, if you hear new knocking or rattling from the engine, or if the level keeps dropping between checks. Running an engine with too little oil can destroy it.