Rear Main Seals

The rear main seal is a synthetic rubber ring that stops engine oil from leaking out of the rear crankshaft bearing. Like all engine seals, it is subjected to high temperatures and vibrations that will eventually cause it to fail. Oil leaks around the area between the engine and transmission are usually a sign that the seal has failed. Replacing the seal is not a simple task but it can be done at home, saving hundreds of dollars in repair costs.

The rear main seal is located just inside the hole where the crankshaft exits the crankcase. It can be difficult to see when the clutch and transmission are located right next to it. The seal is several inches in diameter and has a rectangular cross-section about half an inch wide. It is one of the largest seals used in a car engine and often comes in two halves for easy installation. Most seals are black or gray but some have other colors, like orange or green.

The primary function of the rear main seal is to stop engine oil leaking out of the rear crankshaft bearing. The seal can fail if a low oil level causes it to harden and crack, or if a worn bearing allows the crankshaft to deform it. Even a slight gap can result in a large amount of oil leaking out of the seal in a short space of time, especially when the engine is running. The other important function of the seal is to prevent contaminants from entering the engine through the bearing.

An illuminated oil warning light is one sign that there may be a problem with the rear main seal, but the warning may also be caused by other problems. Replacing the seal can be a long and difficult task, so it is best to check for the other problems first. A loose sump plug, broken gasket, or a crack in the oil pan could also be causing the oil loss. Even an oil leak from the valve cover can appear to be coming from the seal if it flows down the back of the engine. The sensor may even be faulty, so check the dipstick as well.

Replacing the seal is a task that many people want to put off for as long as possible, and there are a couple of temporary measures that may help do this. The first measure is to perform a normal oil change but use an oil with a higher viscosity. It should take longer to leak out of the seal but will also reduce engine performance slightly. The second measure is to introduce additives to the engine oil that cause the seal rubber to swell and hopefully reduce the size of the gaps.

These temporary measures will only last a short time and will not stop the seal from deteriorating. The cracks in the seal will quickly grow larger and the rate of oil lost will increase. The engine oil should also be changed along with the seal to remove any contaminants that may have entered the engine, and the oil pan gasket should also be changed as well. Always consult the service manual or a mechanic before attempting to change the seal.

The first step is to drain the engine oil and remove the oil pan. The engine will need to be supported if any mounting bolts also need to be removed. The end bearing cap holds the rear main seal and only this cap needs to be removed. The other bearing bolts may need to be loosened slightly to remove both halves of the seal. Before installing the new seal, the seat should be cleaned and lubricated with fresh oil. After putting all the components back in place, run the engine and leave the vehicle standing for a while to check for any new oil leaks.

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