Oil sump:
An oil sump is mounted under almost every petrol and diesel engine. This is called a “wet sump system”. Engines without an oil sump use a dry sump system. The oil sump is, among other things, the storage place for oil. When the engine is stationary, 90% of the oil collects here. The rest is found on components such as the cylinder head, oil pump, turbo, etc. Click here for more information about engine oil. The oil strainer (through which the oil is drawn in by the oil pump) is located at the lowest point of the sump.
On the underside of the sump there is almost always a drain plug. This is at the lowest point and is intended for draining the oil during, for example, a service. There are engines (e.g. of an old type Smart) that do not have a drain plug. The oil then has to be extracted through a tube with a special suction device.
The oil sump in the image below is from a BMW. It is quite flat. Some other versions are much deeper. That is entirely dependent on the engine construction with the crankshaft etc.
Between the engine block and the top of the oil sump there is always a gasket. This can be made of paper, cork or rubber, but in most cases it is liquid sealant. This sealant cures when it comes into contact with outside air.
A so‑called “crankcase pressure” builds up in the oil sump. The undersides of the pistons/cylinders open into this area. The pressures are caused, among other things, by blow‑by gases past the piston rings. These crankcase fumes must be extracted by the engine. See the page crankcase ventilation.
Oil strainer:
The oil pump draws the oil from the sump and pumps it to the lubrication channels. At the beginning of the suction pipe there is also a strainer (sometimes also called a sump filter). This strainer is intended to stop coarse dirt particles before they enter the oil pump. The strainer can become clogged, for example by black sludge (when driving too long with old engine oil) or by sludge caused by burning of the engine oil. The latter can happen if the exhaust runs underneath or very close along the oil sump. When the engine is heavily loaded and the exhaust is therefore red‑hot and the engine is switched off too quickly, the heat from the exhaust is drawn into the sump. The oil, which is already very hot, can burn in the area of the oil strainer. This is, for example, a known problem with certain types of VW Passat, Audi A4 and Skoda Superb with a 1.8 turbo engine where the engine is mounted longitudinally.
The oil pressure warning lamp (red) may then light up under certain conditions. It is then necessary to check the strainer for contamination, clean it if required and preferably replace it.
Removal/installation of the oil sump:
In case of leakage of the sump gasket or damage to the sump, the sump will need to be removed. First, the engine oil must be drained. Depending on the type of engine, part of the exhaust system (if it runs underneath it) will also have to be removed. Sometimes the subframe must be lowered. If there is enough space under the sump, all bolts can be loosened. The sump will be stuck to the engine block by the gasket that bonds both parts together. Carefully remove the sump.
Once the sump is off, the old sealant residues will still be stuck everywhere. Alternatively, there may be a loose rubber gasket, which is the easiest to replace. With loose sealant, all old sealant residues must be removed from both the sump and the engine block. Do not use coarse sandpaper for this, as it will make (deep) scratches that can fill with engine oil. Both surfaces must of course remain as flat as possible.
After degreasing everything, it is time to apply the new liquid gasket. Definitely do not apply too much, because after assembly pieces of sealant can end up in the sump and clog the oil strainer. Spread the sealant a bit over the entire mounting surface of the sump and remove the excess sealant. After assembly, engine oil can be added fairly soon. However, it is recommended to let it dry for at least an hour before driving the car.