One of the worst things that can happen to your car apart from it stalling in the middle of traffic or failing to start up is leaking. And it is even worse if you have a rear main leak. The rear main leak is synonymous with a potential engine or transmission trouble, the two most critical motor vehicle components.
So how do you diagnose a rear main leak? Keep reading to learn how to tell if a rear main seal is leaking.
What is a rear main seal leak?
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 rear main seal keeps oil sealed inside the rear of the engine, where the crankshaft connects with the transmission.
How can I tell if my rear main seal is leaking?
The location of the rear main seal makes detecting and fixing an oil leak challenging and complicated. Here are some rear main seal leak symptoms;
Oil puddles
The most common sign of a rear crankshaft seal is oil puddles on the garage floor or the driveway. These oil puddles can be seen either when the vehicle is parked for a long time or while driving.
However, the oil may leak from various parts of an engine. That said, it is wise to diagnose where the oil is leaking from before concluding it’s the rear main seal.
Smoke from under the vehicle
A leaking rear crankshaft seal will often drip on the exhaust and cause smoke from under the car. In any case, you should be able to differentiate smoke from under the car and smoke from the tailpipe. Smoke from the tailpipe signifies an engine burning excess oil and not a rear crankshaft seal leak.
Accumulation of dirt and debris
Oil leaks act as adhesive. Therefore, when you have a rear crankshaft seal leak, dirt and debris from the driveway stick to the places covered with oil. Accumulation of dirt and debris is an early stage sign of a leaking component.
If you notice the accumulation of dirt and debris between the transmission and the engine, it is a good indication of a leaking rear crankshaft seal. At this point, you need a rear main seal leak sealer to stop the leak.
Causes of a rear main seal leak
Many factors can cause the rear main seal to leak. Below are the main rear main seal leak causes;
Engine oil condition
One of the factors that can cause the rear crankshaft seal to leak is using the wrong type of engine oil or low oil level. Most engine oils have chemical additives that might affect the seals in the engine. Therefore, an irregular oil change will deteriorate the seals.
Over time, the seals will deplete along with buffers in the oil. As a result, the inner lip that drives on the crankshaft will dry off and become stiff and unable to seal the crankshaft.
Worn out main bearing
If your main engine bearing is bad or worn out, the crankshaft will dangle inside the bearing leading to stretching and moving the rear crankshaft seal while the engine is running. This typically means your engine has a worn main bearing.
If this is the case, you’re in for a significant repair job that involves rebuilding your vehicle engine and replacing many other faulty parts along with the rear crankshaft seal itself.
Crankshaft wear and tear
The rear crankshaft seal lip rides directly on the crankshaft. Therefore, the crankshaft surface that the inner seal drives on is critical.
Any wear or imperfections on the crankshaft can result in a leak. However, the surface can be restored by installing a sleeve kit.
The clogged or defective PVC system
The rear crankshaft seal inner lip rides on the shaft when there is too much pressure from the crankcase; it will drag the seal onto the crankshaft and eventually cause it to swell, resulting in an oil leak.
A clogged or defective positive crankcase ventilation (PCV) system will increase the crankcase pressure and push out the seal.
Note: if you own a turbocharged or supercharged engine, excessive engine blow-by due to defective or worn-out piston rings can increase the crankcase pressure, affecting the seals and resulting in oil leaks.
Transmission misalignment issues
Whether you own a manual or automatic transmission, problems with the transmission’s input shaft or a damaged flex plate can stress the rear crankshaft seal. Ensure you check the flexplate for damage on an automatic transmission. For manual transmissions, check the input shaft.
Improper seal coating application
Some rear crankshaft seals have a polytetrafluoroethylene coating designed to be installed dry. The seal will transmit a PTFE layer to the crankshaft that the inner lip will sit on.
The PTFE will prevent wear and seals better than Viton or silicone materials. However, if the seal is coated with oil during installation, it will cause leaks quickly.
How serious is a rear main seal leak?
Rear main seal leaks are so dangerous because the leak can occur at a high-speed rate. In addition, due to the location, it is subject to high oil pressure. It is also connected to the engine, which must be slick with oil to function correctly.
Critically low oil levels can lead to permanent damage to your engine, and if the oil light ever comes on, you stop driving immediately. Unfortunately, one of the things that makes this repair difficult and costly is the hard-to-access location of this seal.
Usually, your mechanic will have to remove the oil pan as well as either your engine or your transmission to be able to replace the seal. If the seal has a slight puncture, the crankshaft rotation will affect it and cause it to tear more.
This will result in the leakage of a large amount of engine oil and drastically reduce the oil to a dangerous level. Lastly, dangerously low-level oil could cause friction and wear inside the engine resulting in engine Knocking.
Does a rear main seal leak when parked?
Yes. Rear main seal leaks tend to worsen when an engine has warmed to its standard operating procedure. For this reason, a leaking rear main seal is often most evident in the minutes to hours after a vehicle has been parked for the day.
Can a rear main seal leak cause transmission problems?
Yes, it can. The rear main seal keeps oil sealed inside the rear of the engine, where the crankshaft connects with the transmission. However, the seal is usually made of rubber, and it can get corroded or weakened over time. When it starts to leak, this can lead to major problems such as transmission damage.
How much does it cost to replace the rear main seal?
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. The rear main seal usually costs between $600-$900, and the service fee ranges between $550 and $820.
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. The traditional fix for a leaking main seal is to replace it, but some vehicle owners use oil additives designed to restore seals because of the typically high labor cost.
In some cases, that can stop or slow leaks. Other drivers may switch to a heavier oil, such as 10W30 instead of 5W20. Additionally, your mechanic may advise you to replace the rear main seal, even if it isn’t leaking, because much of the disassembly work involved will already be done.
Regardless of the material used in manufacturing the seal, the seal is critical. It is designed to keep the oil where it should be and typically needs replacement when it shrinks or deteriorates.
How long can you drive with a rear main seal leak?
The possibility of a very fast leak is the most significant factor that makes it so dangerous to drive with a rear main seal leak. As we just mentioned, your rear main seal is in a place that gets a lot of oil thrown at it and has the added wear of the crankshaft spinning inside of it.
If the seal tears or cracks at all, your crankshaft rotation can cause the seal to get torn up very quickly, causing a very fast leak. A quick leak can quickly lower your engine oil level to dangerous levels, and dangerously low oil levels can lead to permanent engine damage.