The engine is the lifeblood of your car. It powers everything else, and if it starts to fail, you’re in trouble. One common issue that can affect engines is a rear main seal leak.
The most common symptom of a rear main seal leak is oil leaking and burning oil smells from the engine. This is caused by oil leaking onto hot engine parts and smoking. A rear main seal leak can also trigger low oil pressure.
This article will teach you everything you need to know about this problem, from symptoms and causes to fixes and prevention. So read on to learn more!
Table of Contents
- What is a Rear Main Seal? What Purpose Does it Serve?
- What are the Main Types of Rear Main Seals?
- What is Rear Main Seal Made of?
- What are the Signs of a Leaking Rear Main Seal?
- What Causes Rear Main Seal Leaks?
- How to Replace a Rear Main Seal?
- Rear Main Seal Replacement Cost
- Frequently Asked Questions
- Is it worth fixing a rear main seal leak?
- How long does it take to replace the rear main seal leak?
- Is it ok to drive with a rear main seal leak?
- Can you replace a rear main seal without removing the transmission?
- What are the consequences of engine oil leaks from the rear main seal?
- What is an oil stop leak? Should you buy it?
- What is the average lifespan of the rear main seal?
What is a Rear Main Seal? What Purpose Does it Serve?
A rear main seal is a vital component of your engine, located between the crankshaft and the flywheel. Its purpose is to prevent oil from leaking out of the engine.
The rear main seal is basically sitting at the junction of engine and transmission. As you know, that engine produces power, and through the crankshaft, this power is transferred to the transmission. At this junction, a rear main bearing is present.
The purpose of this bearing is to support the crankshaft and to allow it to rotate smoothly. The rear main seal closes the gap between the engine block and the rear main bearing inside the transmission bell housing.
When the engine runs, oil circulates throughout to keep all the moving parts lubricated. The rear main seal has a lip that rides on the crankshaft. As the crankshaft turns, it creates a vacuum that pulls oil up from the pan and circulates it throughout the engine.
If there is a problem with your rear main seal, it can cause oil to leak out of your engine. This can lead to decreased performance as well as damage to your engine.
What are the Main Types of Rear Main Seals?
Many think only three types of rear main seals are available on the market, but there are a few more classifications. Let us have a look at these:
1. Rope or Wick Seal:
This seal has been used since the 1950s and is made from felt or rope. The wick is impregnated with oil, which helps to lubricate the seal as it turns.
However, this type of seal is not very durable and can wear out quickly. It is also not compatible with high-performance engines.
2. Single Piece Seal:
This seal is made from a single piece of rubber and is held in place by a metal or plastic ring. It is more durable than the rope seal but can still wear out over time.
It is also not compatible with high-performance engines.
Compatibility with high-performance engines is important because these seals must withstand the higher temperatures and pressures that these engines produce.
3. Double Piece Seals Seal:
This seal is made from two pieces of rubber held together by a metal or plastic ring. It is more durable than the single-piece seal and is compatible with high-performance engines.
However, it can be more difficult to install than the other types of seals. Installing a double-piece seal requires special tools and skills that not all mechanics have.
4. Double Lip Seal:
A double lip seal is a parallel classification to the other three types of rear main seals mentioned above. These seals have lips on both side surfaces. The spiral groove or lip on the other side helps rotate the seal with the crankshaft.
The two lips work together to keep oil from leaking out of the engine. This type of seal is compatible with high-performance engines and is very durable.
What is Rear Main Seal Made of?
When choosing a rear main seal, it is important to consider the type of engine you have as well as your budget. High-performance engines require a more durable seal, so the material selection of the rear main seal becomes critical in this case.
Silicone is a popular choice for rear main seals because it can withstand high temperatures of up to 480 Fahrenheit. It is also very durable and has good oil retention properties.
However, silicone seals are more expensive than other types of seals. If you have a high-performance engine, silicone is a good option.
Nitrile is a synthetic rubber that is also used in rear main seals. It is less expensive than silicone because it can’t withstand temperatures over 250 Fahrenheit.
It is also not as durable as silicone. However, nitrile seals are still a good option for most older engines.
Polyacrylate provides the best abrasion and heat resistance on a budget. Polyacrylate can also withstand high temperatures of up to 350 Fahrenheit. It is also very durable.
4. PTFE Rubber:
PTFE rubber is probably the best material that can be used for a rear main seal. It has the best sealing ability under high temperatures.
It is best at preventing oil leaks even from engine shafts with slight misalignments and wear. However, PTFE rubber seals are the most expensive type of rear main seal available on the market.
What are the Signs of a Leaking Rear Main Seal?
Several signs may indicate you have a leaking rear main seal:
- Low Oil Pressure Light
- Engine Oil Appears Dark
- Oil Puddles in Driveway
- Smoke From Vehicle’s Underbody
- Running Low on Engine Oil
- Difficulty Changing Gears
Let us dig deeper into each of these signs:
1. Low Oil Pressure Light:
If you have a rear main seal leak, you may notice that your low oil pressure light comes on more frequently. This is because the leaking oil is not circulated through the engine properly, which causes the pressure to drop.
The job of the oil pressure sensor is to measure the oil pressure in the engine. So, if it is not functioning properly, it may give false readings.
2. Engine Oil Appears Dark:
If you check your engine oil and notice that it looks darker than usual, it could indicate a rear main seal leak.
When you have a leaking rear main seal, dirt and debris from outside may enter through these tiny openings. These particles not only promote the process of engine oil degradation but also reduce its efficiency.
As a result, the oil loses its original color and starts to appear dark.
3. Oil Puddles in Driveway:
To be sure that your rear main seal is the reason behind the dark color of your engine oil, check for oil leaks around the seal area.
A sure sign of a rear main seal leak is oil puddling on your driveway or garage floor. Remember that a small amount of seepage is normal and nothing to be alarmed about.
Oil can leak from the valve cover gasket or other places. However, if there is a large puddle of oil or oil stain, it is definitely an indication of a problem.
4. Smoke From Vehicle’s Underbody:
Another sign of a rear main seal leak is smoke from the vehicle’s underbody.
When oil leaks from the rear main seal, it drips onto the hot engine parts such as the exhaust manifold or catalytic converter. This causes the oil to smoke.
Plus you may sense a burning smell of engine oil through vents.
5. Running Low on Engine Oil:
If you find that you constantly have to top off your engine oil, it is a sign that you have a rear main seal leak.
Leaks can range from tiny seepages to large gushes. So, if your car is leaking oil, it will definitely affect the engine’s oil level.
The engine will start to run less smoothly as the oil level gets lower. You may also notice a decrease in fuel efficiency.
6. Difficulty Changing Gears:
Another symptom of a rear main seal leak is difficulty changing gears.
When the oil leaks from the rear main seal, it may enter the clutch housing. As a result, the clutch may start to slip or get stuck.
This will make it difficult for you to change gears, and you may even hear a grinding sound when you try to do so.
What Causes Rear Main Seal Leaks?
There are several reasons why your rear main seal may start to leak. Let’s dive into the most relevant ones:
1. Wear and Tear:
Wear and tear are a normal part of a car’s life cycle and are inevitable. Over time, the rubber seal starts to harden and crack. This makes it less effective at doing its job of preventing oil from leaking.
As a result, you may notice small seepages that gradually become larger over time. However, there are ways to extend the life of your rear main seal and prevent it from leaking. We will discuss this in more detail later on.
2. Worn Main Bearings:
Another reason for a rear main seal leak is worn main bearings. The main bearings are responsible for supporting the crankshaft. They are located between the engine block and the crankshaft.
Over time, these bearings can become worn out due to friction. When this happens, it puts more pressure on the rear main seal, causing it to leak.
To prevent this, regularly check your main bearings and replace them if necessary.
3. Harsh Weather:
Harsh weather conditions can also cause the rear main seal to leak.
When it is cold, the rubber seal contracts and becomes less effective. Similarly, when it is hot, the rubber expands and may start to crack.
Either way, extreme weather conditions can put more strain on the rear main seal and cause it to leak.
4. Clogged PCV Valve:
The PCV (positive crankcase ventilation) valve regulates the pressure in the engine. How the PCV system works is that it releases the pressure build-up in the engine by venting it into the atmosphere.
However, over time the PCV valve can become clogged with dirt and oil. When this happens, it doesn’t work as effectively. When pressure builds up in the engine, it puts more strain on the rear main seal and causes rear main seal failure.
5. Low or High Engine Oil Level:
One of the most common causes of rear main seal leaks is often driving with low engine oil levels.
If the oil level is too low, it can cause the seals to dry out. This will make them less effective at doing their job and more likely to leak.
On the other hand, if the oil level is too high, it can cause the seals to swell. This will also make them less effective at doing their job and more likely to leak.
To prevent this from happening, make sure to regularly check your engine oil level and top it up if necessary. You should also use the type of oil recommended by your car’s manufacturer.
6. Rough Crankshaft Surface:
Another common cause of rear main seal leaks is the rough crankshaft surface. The crankshaft is the part of the engine that converts the up and down motion of the pistons into rotational energy.
Over time, it can become bent due to wear and tear. As one lip of the rear main seal sits on the crankshaft. When the surface of the crankshaft becomes rough, it can cause the seal to leak.
7. Misaligned Shafts:
If the engine’s shafts are not properly aligned, it can also cause the rear main seal to leak. The shafts in an engine are responsible for transferring power from one component to another.
If you have a bad driveshaft, it puts extra strain on the rear main seal and causes it to leak. This is often caused by an engine that has been rebuilt without properly aligning the shafts.
8. Poor Installation:
Another common cause of rear main seal leaks is poor installation. This is often the case when the seal is not properly seated, or the wrong size seal is used.
When this happens, it can cause the rear main seal to leak. To prevent this from happening, have a professional install your rear main seal.
How to Replace a Rear Main Seal?
The rear main seal repair method would vary depending on the make and model of your car, but in general, there are five parts to replacing the rear main seal. Let’s see each of these steps in some detail.
Mind you; this is not a replacement guide per se but more of an overview of the process.
1. Transmission Removal:
The first and foremost step of replacing the rear main seal is to remove the transmission. This would require jacking up your car and removing the transmission pan. Once the transmission is out, you can access the rear main seal.
- Disconnect the negative battery cable. This will prevent any electrical issues while you are working on the car.
- Raise the car on jack stands and place them under the vehicle’s frame. Make sure that the car is level before proceeding.
- Locate the transmission dipstick and pull it out. Drain the fluid into a transmission and oil pan. You will need this fluid later, so do not throw it away.
- Find the transmission cooler lines and disconnect them from the transmission. There will be two lines, one for the input and one for the output.
- Unbolt the torque converter from the flexplate. You may need a special tool to do this, so check with your local auto parts store before proceeding.
- Remove all of the bolts that hold the transmission to the engine block. There will be several of them, so take your time and make sure that you remove them all.
- Carefully lower the transmission down and out of the car. You may need help to do this as they can be quite heavy.
2. Engine Removal:
The second step is to remove the engine from the car. This would require disconnecting a few things first.
- Drain the radiator fluid into a catch pan. You will need this later, so do not throw it away.
- Remove the radiator hoses from the engine. There are usually two of them, one for the input and one for the output.
- Unbolt the fan from the water pump and remove it from the engine bay.
- Remove all of the belts from the engine. These include the alternator belt, power steering belt, and air conditioning belt.
- Unbolt the water pump from the engine and remove it from the engine bay.
- Remove all bolts holding the engine to the car’s frame. There will be several of them, so take your time and make sure that you remove them all.
- Carefully lower the engine down and out of the car.
3. Removal of the Old Rear Main Seal:
The third step is to remove the old rear main seal from the engine block. This can be done with a few tools and a little elbow grease.
- Start by cleaning off the area around the seal. This will give you a clean surface to work with and make it easier to see what you are doing.
- Once the area is clean, use a chisel or pry bar to break the old seal loose.
- Be careful not to damage the engine block while you are doing this.
- Once the old seal is loose, use a scraper to remove any remaining debris from the engine block.
- Make sure that the surface is clean and smooth before proceeding.
4. Installation of the New Rear Main Seal:
The fourth step is to install the new rear main seal into the engine block. This can be done with a few tools and a little elbow grease.
- Start by applying a bead of silicone sealant around the inside edge of the seal. This will help to keep it in place once it is installed.
- Carefully lower the new seal into place and ensure it is seated properly.
- Use a small amount of silicone sealant to seal the outside edge of the seal. This will help to keep it in place and prevent leaks.
The fifth and final step is to reassemble the engine and transmission. This would require doing the following steps in reverse order.
- Install the transmission back into the car and bolt it into place.
- Install the engine back into the car and bolt it into place.
- Refill the radiator with antifreeze and connect the radiator hoses.
- Refill the transmission with fluid and connect the transmission cooler lines.
- Install the fan onto the water pump and bolt it into place.
- Install all of the belts onto the engine.
- Install the water pump onto the engine and bolt it into place.
- Start the engine and check for leaks.
This is how you would replace the rear main seal on a car with a manual transmission. If you have an automatic transmission, the process would be similar, but a few additional steps would be involved. Consult your service manual for more information.
Rear Main Seal Replacement Cost
Replacing the rear main seal is one of the most expensive repairs that can be performed on a vehicle. The cost to replace the rear main seal can range from $600 to $1,200.
The major reason why this repair is so expensive is because of the amount of labor involved. The rear main seal is located near the transmission at the back of the engine. The engine and transmission must be removed from the vehicle to replace the rear main seal.
You can imagine how hard it can be to remove the entire transmission or even the engine just to get to the rear main seal. Even if you have all the necessary tools, this repair can still be very time-consuming and expensive.
The good news is that there are ways to avoid this expensive repair. One way is to make sure you change your engine oil regularly. Dirty oil can cause the rear main seal to leak. By changing your oil regularly, you can help prevent this problem.
Secondly, as a temporary fix, you can try using a rear main seal stop leak product. These products can temporarily stop the leak until you have the time and money to do the full repair.
Repairing yourself is also an option, but it is not recommended unless you are experienced with car repairs. Even then, it is still a very difficult and expensive repair.
If you have a rear main seal leak, getting it fixed as soon as possible is important. This type of leak can cause serious damage to your engine if left untreated.
While the repair can be expensive, there are ways to avoid the cost by changing your oil regularly and using a rear main seal stop leak product.
If you need to replace the rear main seal, it is best to leave it to a professional. Trying to repair yourself can be difficult and expensive.
We hope this article has helped you learn everything you need to know about rear main seal leaks. Thanks for reading!
Frequently Asked Questions
Is it worth fixing a rear main seal leak?
It would entirely depend upon the extent of the leak. If it is a small leak and you have a relatively new car, you might want to spend your time and money on fixing it. In this case, you have a real chance of solving the problem without any lasting effects.
However, if your rear main seal is leaking substantially, or if you have an older car, it might not be worth fixing the leak. The cost and time required to fix a substantial leak may outweigh the benefits of keeping your vehicle on the road. In this case, you might be better off selling your car and using the money to buy a new one.
How long does it take to replace the rear main seal leak?
The length of time required to replace the rear main seal will vary depending on the make and model of your car. On average, it takes about 6-8 hours to complete this repair. The main reason why this repair takes so long is the fact that the engine and transmission must be removed to access the rear main seal.
If you are experienced with car repairs, you might be able to do this repair in less time. However, it is still a very difficult and expensive repair.
Is it ok to drive with a rear main seal leak?
The answer would depend on the severity of the leak. A small leak may not pose a problem, but a large one could result in low oil levels and cause engine damage.
Generally, a rear main seal leak is not something you want to ignore. A leaking rear main seal can lead to a dry-running engine. And you don’t even want to know what can happen when an engine runs dry.
So it’s best not to drive with a rear main seal leak. You should get it fixed as soon as possible.
Can you replace a rear main seal without removing the transmission?
The answer is no. You cannot replace the rear main seal without removing the transmission or the engine. This is because the rear main seal is located between the engine and transmission.
So if you want to replace the rear main seal, you’ll need to remove the engine and transmission. No matter how many web pages keep telling you that you can do it without removing the transmission, please don’t believe them. They’re wrong.
And by the way, it would be a bad idea even if you could replace the rear main seal without removing the transmission.
The reason is that if the rear main seal is leaking, chances are there’s some other part in the transmission or engine that has caused it to leak.
So if you’re going to replace the rear main seal, you might as well replace the faulty bearing or a bent shaft transmission while you’re at it.
What are the consequences of engine oil leaks from the rear main seal?
Consequences of engine oil leaks from the rear main seal include low oil levels, which can lead to engine damage, and potential fires if the oil leaks onto hot engine parts.
In addition, rear main seal leaks can cause drivability issues, such as transmission slipping or hard starting. If you suspect losing oil, it is important to have your vehicle checked by a qualified mechanic as soon as possible.
What is an oil stop leak? Should you buy it?
An oil stop leak is a product that claims to seal leaks in your engine. There are many different brands and formulas of oil stop leaks, but they all work by essentially the same principle.
The vast majority of oil stop leaks on the market are nothing more than snake oil. They might temporarily seal a small leak, but they will do nothing to fix the underlying problem.
In addition, most oil stop leaks contain harmful chemicals that can damage your engine. For these reasons, we do not recommend using a stop oil leak kit.
What is the average lifespan of the rear main seal?
The rear main seal is designed to last the lifetime of the engine. The main reason behind this idea is that the rear crankshaft seal is buried so deep within the engine that it is very difficult to replace.
That’s why auto companies try their best regarding material selection and design of the rear main seal. The average lifespan of the rear main seal is quite long. It is not uncommon for them to last over 200,000 miles.