Automobiles rely on many rotating parts to transfer engine momentum through the transmission and to the wheels. These components are normally made of materials such as hardened steel, cast iron, aluminium or some alloy of metals. Shafts, bearings and other rotating parts are made to exacting specifications, but oil seals are still necessary to keep engine oil and transmission fluid in and foreign materials out of the drivetrain. Oil seals are generally low-cost parts, but can be a big expense in labour cost to replace.
Seals Dry Out
Composed of various natural or synthetic rubber compounds, oil seals dry out if not used for some time. The oil at the seal-shaft interface may slowly dry or drain away, leaving the metal surface exposed to oxidation. A slightly rusted shaft "burrs" the seal when the parts begin rotating, so a seal in this condition may leak when driving the car. An unused seal might develop a dry spot susceptible to weathering and cracking. Sometimes the seal will swell and reseal after some period of reuse, while on other occasions the leak will gradually get worse as the seal disintegrates.
Severe Driving Conditions
Severe cold cracks seals and causes seal failure. Other seals such as brake, transmission and axle seals also experience damage from extreme cold conditions. Off-road vehicles endure conditions that include rocks, thick dust and variable temperatures that the average motorist seldom faces. Off-road driving may also involve crossing creeks or even driving along the beach.
All of these scenarios test the design of all components, but especially oil seals. Dirt in particular will work its way into the gap between the shaft and seal, and gradually wear the seal material away, allowing oil to begin leaking. Differential and axle seals are especially vulnerable to dirt. Salt will corrode the seal body.
A transmission seal can begin leaking if the driveshaft or drive axles become damaged, bent or loose as this causes excess vibration and uneven shaft wear. For instance, a defective U-joint on a rear-wheel drive vehicle can cause this to happen.
Ageing Seals, Bearings
Seals tend to pull away from the surface they mate against as they get older and this effect eventually allows for too much space between the seal and the shaft, allowing oil to escape. Engine and drivetrain bearings gradually loosen through normal wear as they age. Eventually the play between the mating surfaces becomes great enough to cause the shaft to "wobble" and this causes the seal to leak.
Running the engine consistently with a low oil level can make the seal too hot or dry it out so that it warps and starts leaking. Driving with an overheated engine may have the same result. Adding too much fluid can "blow out" a seal, especially on a transmission.
Seals are rather fragile and easily ruined by a ham-fisted mechanic. It is essential to mount seals flush in the recess made for it and for it to be free of dirt or burrs. It's easy to destroy a seal by an errant slip of a tool when working on something else. A gouged, dirty or dented seal will probably leak.
Improper installation or removal of a seal might damage the seal "seat," causing the new seal to leak after installation. Use the proper seal puller and installation tools for any operation involving seal replacement.