If an engine needs topping up with fresh oil more than usual, or if you see a pool of oil under the engine where the car has stood, there is an oil leak. Trace the source immediately.
The engine could be damaged if the leak is allowed to become serious.
Finding the exact source of the leak is easier if you first clean the outside of the engine thoroughly with a proprietary degreasing fluid and a stiff bristle brush. Protect electrical parts with plastic bags, or plastic sheets held on by sticky tape, then hose the degreaser away with water until the engine is reasonably clean and oil-free. Leaking oil then shows clearly.
Timing-cover oil seal
A common source of leakage is the timing-cover oil seal at the crankshaft-pulley end of the engine. Generally, oil leaks most from the seal when the engine is running fast, less when it is idling.
Check for signs of oil sprayed out sideways in line with the oil seal or pulley on to adjacent parts of the bodywork or engine.
Look also at the underside of the engine beneath the seal, and on the sump pan for oil streaks starting at the lower edge of the seal.
The only cure for a leaking oil seal is replacement, and ensuring that the rotating parts with which it is in are smooth and free of burrs or raised metal.
Checking a crankcase side-cover gasket
On some engines — such as those fitted to certain Leyland cars — there is a cover over the camshaft tappet chamber on the side of the crankcase about halfway down the engine.
The cover is hidden by the inlet or exhaust manifold, so leaks from its gasket may be hard to spot.
With the engine running, look for oil coming from the lower edge of the cover. You may need a small mirror and a torch to see.
Tightening the cover securing screws slightly may cure the leak. If not, replace the gasket (See Replacing gaskets and oil seals).
Looking for leaks on the crankshaft rear oil seal
The crankshaft rear oil seal is usually hidden by the flywheel and clutch housing, so the only visible sign of a leak is a drip from the bottom of the clutch housing. Where the seal is leaking badly, there may be clutch judder or slip caused by oil spraying on the clutch.
Replacing the seal is the only cure. This means removing either the gearbox or the engine — a job best left to a garage.
Checking the rocker cover
Run the engine when you check for oil leakage from the rocker or cam cover, so that the moving parts beneath it are spraying oil around the inside. Keep clothes and hair well clear of the pulley(s) and belt(s).
Look carefully for oil seeping out from around the gasket flange, particularly at the front and rear ends of the cover where the gasket may not be sealing properly.
Leaks may be due either to the cover screws being unevenly or excessively tightened, or to the gasket being broken or distorted.
Check the gasket side near the source of the leak; look for breaks or distortion. Use a small mirror to see under the rim of the cover.
Some gaskets are held in place on the cover flange by small tongues. Others may be stuck to the cover, or just held in position by the cover flange.
A broken gasket should be replaced. A distorted gasket can usually be sealed temporarily, but a new one should be fitted as soon as possible.
Slacken the cover screws and straighten the gasket with tweezers or thin-nosed pliers. Retighten the cover screws and check again for oil leakage.
The screws should be tightened just enough to press the cover on to the gasket firmly, but no more. Overtightening distorts both the cover and the gasket, causing more leaks.
Leaks may also occur around a cover which is secured by nuts and bolts, if the washers are wrongly fitted or the nuts not tight enough.
On some engines, special fibre or plastic washers are used. Make sure that you fit washers of the same type, and that the screws are correctly tightened.
Inspecting the sump
To check the sump pan thoroughly, run the car up on ramps so that you can get underneath the engine end. Apply the handbrake and chock the wheels still on the ground.
With the engine running, check for oil leaks around the outside of the sump-pan mounting gasket flange, and also from the drain plug at the bottom of the pan.
Look closely at the sump mounting flange around the crankshaft: sometimes the gasket is in several parts, or there are separate gaskets at the front and rear, which are liable to distortion. Leaks here can be confused with those from a crankshaft front or rear oil seal.
If the sump gasket is leaking, it may be only because it has settled and contracted slightly in service. The leak may be stopped by tightening the sump mounting bolts or nuts.
Make sure you tighten all the fixings evenly, working gradually around the mounting flange.
Beware of overtightening, which can distort or destroy the gasket. If tightening fails, the sump gasket should be replaced.
An occasional oil drip from the sump drain plug shows that the plug is not tightened enough, that the washer is leaking, or that the plug is not fitted correctly.
If there is a steady drip from the plug, fit a new washer. Otherwise, the washer should be replaced at the next oil change (See How to drain engine oil and remove filter).
Checking the head gasket
With the engine running, check for an oil leak at the cylinder-head gasket. This is a sign that the rocker shaft or camshaft oil-supply passage is leaking where it passes through the head gasket.
Oil may leak from the head gasket if the head nuts or bolts are loose, or if the head gasket, cylinder-head face or block face is faulty (See How to remove a cylinder head).
Oil-filter or pump adaptor
Depending on the position of the oil-filter or oil-pump adaptor on the engine, you may need to raise one end of the car on ramps and get underneath to see clearly. Apply the handbrake and chock the other wheels.
With the engine running, a serious oil leak is clearly visible, streaming from the oil-filter seal or pump-adaptor housing on the side of the engine, which is where the oil is under more pressure than anywhere else in the system.
If the leak is serious, stop the engine immediately to prevent damage, and replace the gasket (See Replacing gaskets and oil seals). A slight seepage can sometimes be cured by checking the tightness of the filter or pump-adaptor fixings.
However, the best way of curing a leak is to remove the component and replace the gasket or seal.
The distributor flange
With the engine running, check carefully for leaks around the distributor mounting flange where it is attached to the crankcase or camshaft cover.
On some engines there is a thin gasket between the mounting faces; on others there is an '0-ring rubber seal fitted over the distributor housing between the mounting faces.
Replacing the gasket or seal is the only cure for an oil leak there. You need to remove the distributor to do this (See Removing and refitting the distributor).
Fuel-pump mounting gasket
Fuel-pump gaskets are fitted only to mechanical fuel pumps that are mounted on the engine.
Locate the pump, and with the engine running check for oil drips coming from the underside of its mounting flange or gasket.
On many engines there is a thick spacer plate sandwiched between two thin paper gaskets. This thick plate may crack if the pump mounting nuts or bolts have been overtightened.
Check first that the pump mounting nuts or bolts are not loose, but take care not to overtighten them as this will distort the mounting flange and make the leak worse. If the leak persists, check the underside of the mounting and gasket with a mirror, looking for the exact source of the leak. Replace the gasket if necessary smeared with a non-setting sealant. Make sure that the pump flange face and the mounting face are both clean before fitting a new gasket.
Note that the thick spacer plate (if fitted) also governs the distance the fuel-pump lever travels. If you fit a new plate, it must be the same thickness as the old one. Compare the new with the old before fitting, and use parts approved by the car manufacturer.