The rear main seal keeps oil sealed inside the rear of the engine, where the crankshaft connects with the transmission. It can be expensive to repair, often $600 or more, because it typically involves removing the transmission and, in some cases, all or part of the engine. On most front-wheel-drive vehicles the engine is mounted transversely, so the rear of the engine is the end opposite the one with the pulleys and drive accessory belt. An oil leak from the main seal, also known as the rear crankshaft seal, will usually drip from where the transmission connects to the engine, though it may show up elsewhere, such as on the oil pan. Rear main seals can be made of rubber or silicone, and they can wear out because of age, the rotational forces of the crankshaft, corrosion from road salt and other environmental factors. The traditional fix for a leaking main seal is to replace it, but because of the typically high labor cost some vehicle owners use oil additives designed to restore seals. In some cases, that can stop or slow leaks. Other drivers may switch to a heavier oil, such as 10W30 instead of 5W20. Mechanics frequently advise that when a transmission has to be removed for any reason, that’s a good time to replace the rear main seal, even if it isn’t leaking, because much of the disassembly work involved will already be done.
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