Rear Main seal Leak - Signs, Causes and Replacement Cost

Although there are many causes for oil leaking from your vehicle, the primary reason may be due to an ineffective rear main seal. If your car suffers from such an issue, it is likely that you will need major repairs. How can you tell if your vehicle has a problem with its rear main seal?

Oil may seep onto your driveway at first. You will also need to refill the engine oil because it’s losing the necessary lubrication. You might see oil on the underbody. The oil warning light may also be lit.

This guide will explain the function of the rear main seal as well as what can cause a leak. The top signs of leakage are also discussed and the cost to replace the seal. 

Rear Main Seal Function

Rear main seal prevents oil seepage from the engine's back. It keeps the oil in place where the crankshaft is mounted to the car’s transmission through the flexplate or flywheel. That’s why the rear main seal is often referred to as the rear crankshaft seal.

When the seal isn’t working correctly, the oil can escape from within the crankcase. Leakage can be severe when this happens. 

Locate the Rear Main Seal

At the back of your car's engine is the rear main seal. They are driven into the rear cover and can be concealed in the transmission’s bell-housing, making it more difficult to see. It is almost impossible to see without taking out other important components and sometimes even the transmission.

While the seal is located at the back of the engine, it won’t always be towards the back of the vehicle. Different engines have different faces. The engine faces the car's front, whether it is rear-wheeled or four-wheel-drive. However, all front-wheel vehicles and some all-wheel drives will mount their engine transversely. This engine's front is on one side, the rear will be facing the opposite.  

The location of the rear main seal lies usually opposite where belts or pulleys are. These components are located usually at the rear of the engine. 

Rear Main Seal Water Leakage Symptoms

1. Leaking Oil

Oil can seep from a defective rear main seal. The leak could get worse when the engine is warmed up. You might not see oil leakage every time.

This could happen if you park your vehicle and look at the garage or driveway. If you see an oil stain on the driveway that wasn’t there before, you should be hunting down the source. 

Reported by: How do I fix my car's oil leakage? Common Causes (& How to Fix it)

2. Low oil levels

An oil leakage can occur quickly if the rear main seal is damaged. To maintain the oil levels, it is possible to need to add more oil. It’s even possible that you need to add a quart or more each week. 

While it’s normal to add a small amount of oil from time to time, you shouldn’t need to top off constantly. Before the issue gets worse, get the seal checked. 

3. Warning light for oil

Sometimes the oil warning lamp is the first indicator. The oil warning light is an indicator that your oil levels or pressure are low. This should be taken seriously.

Turn the engine off and pull over. If there’s enough oil in the system to drive home, you may want to try, but use extreme caution. An oil leak in the rear main seal can cause rapid oil loss and engine failure.

Reported: Warning Sign for Low Oil Pressure 

4. Oil for the Underbody

If the oil is not leaking out of the rear main seal, it will begin to collect on the bell housings and engine. The oil may be saturating the underbody due to blowback. 

You should be able see oil behind the engine's back with a quick inspection. You should immediately have your vehicle checked if this has been done. 

Leakage in the rear main seal

1. The Wrong Oil

Leakage can occur if you use the wrong engine oil. The chemical additives in today’s engine oils can affect the seals.

It’s easy to deteriorate the seals if you use a particular oil that shouldn’t be used with your engine. The seals can also dry out from oils, and then become hardened over time. This could lead to a potential leak. Check your owner’s manual and only use the oil that’s recommended. 

You can also get it from too much oil.

RELATED : Full Synthetic Oil vs. Synthetic Blend (Which oil to Use)

2. Wrong Installation

The rear seal contains a coating of polytetrafluoroethylene. It’s meant to be put in place completely dry to create a proper seal. It’s also designed to prevent wear and seal better than silicone or Viton.

Oil on seals is a recipe for disaster. Soon after installation, it could cause leaks. 

3. Bad Main Bearing

The crankshaft hangs from the engine's main bearing, which causes the seal to stretch while it runs. The seal isn’t meant to be stretched and pulled on, so it’s bound to leak.

The main bearing may be worn out and you will know it. There should be an immense amount of noise with this failure and there’s no reason to keep driving your car with this problem unresolved. 

4. Worn Crankshaft 

You can find the rear crankshaft seal around your crankshaft. If the surface of the crankshaft is worn or defective in any way, it won’t be able to seal perfectly. 

For a consistent fit, the surfaces should be exactly identical. If the crankshaft surface doesn’t fit perfectly, it needs to be repaired.

5. PVC Systems that are clogged

On the shaft, is mounted the rear crankshaft seal. It is possible for the seal to leak when there is too much pressure from the crankcase.

An obstruction in the positive crankcase ventilation can cause this pressure to rise. It can push against the seal and cause a leak if it is not checked. 

RELATED : 7 Signs Your PCV Valve is Bad

6. Missalignment

It doesn’t matter what type of transmission your car has, both automatic and manual cars can suffer from misalignment. Any misalignment of the flexplate or transmission’s input shaft can cause stress to the seal. 

The defect may be discovered by quick inspection of the transmission's flexplate. You should inspect the shaft at the input if you're driving an automatic transmission. 

Repair of the Rear Main Seal

A rear main seal replacement costs between $800 to $1,500. The seal can be as low as $50. This is due to the labor involved in this job.

The transmission must be dismantled in many instances. For this reason alone, it’s not a job that is easily performed in the home garage.