The rear main seal is essential for keeping oil in your engine and preventing it from leaking out of the crankshaft and into the transmission.
This location typically requires removing the transmission to access it, which can lead to a costly repair bill.
If your car suffers a rear main seal failure, you might be able to avoid a full repair – depending on the damage suffered.
Read on and find out what a rear main seal is, the signs of failure, and the replacement and repair options for worn and leaking rear main seals.
What is a rear main seal?
A rear main seal is often called a rear crankshaft seal, as it is located on the rear side of the engine where the crankshaft connects to the transmission. The seal prevents engine oil from leaking out of the crankshaft.
Your car puts the crankshaft under a lot of force. It holds the pistons and is connected to the flywheel and the pulleys that power all the engine accessories.
Due to the force and stress put on the crankshaft, it has some main bearings to support it. These main bearings hold the crankshaft in place and allow it to rotate, as well as allowing oil to pass to the crankshaft.
This keeps it lubricated and avoids friction damage. The rear main seal sits outside the rear main bearing to hold the oil in. The seal is typically made of silicone or rubber and wears down from friction between the seal and the crankshaft.
If an engine runs on low oil levels, it can starve the rear main seal of oil, which will cause excess friction between the seal and the crankshaft, deteriorating the seal and resulting in an oil leak.
Where Is the Rear Main Seal Located In a Car?
The automobile engine’s rear is where you’ll find the rear main seal. They can be hidden in the bell-housing of the transmission and are driven into the back cover, making them more difficult to view.
Most of the time, it cannot be viewed without removing other important parts, sometimes even the gearbox.
Although the seal is situated at the rear of the engine, this is not always the case. Distinct engines have different faces. The front of the engine is typically facing the front of a rear-wheel or four-wheel-drive vehicle.
On some all-wheel drive and front-wheel drive cars, however, the engine is mounted transversely. The rear of this engine would be off to the other side since the front is facing either the left or right side of the car.
Normally, the rear main seal is placed in the opposite corner from where the belts and pulleys are. Typically, the front of the engine houses these components.
What does a rear main seal do?
The rear main seal is designed to prevent oil leaks between the block and crankshaft. As it is buried deep in your vehicle’s engine compartment and is hard to reach, manufacturers originally intended rear seals to last the life of a vehicle.
So that no replacement is needed, unlike many other engine components under the hood that require replacement every few years. The rear main seal is situated just outside of your rear main bearing, which supports the crankshaft for your motor in your engine.
These bearings carry the majority of the load in your engine and play a crucial role in turning the axial motion of your piston into the rotary motion of the crankshaft.
The main bearings do this by absorbing the axial load of the piston being forced down by combustion, all the while still allowing the crankshaft to spin.
To accomplish this task, these bearings have to be incredibly smooth and stay well lubricated by the oil in your engine.
In other words, the correct oil for your engine is extremely important in creating the optimal condition for the main bearings to do their job without excessive wear and tear.
This oil also has to be contained within your engine to continuously keep everything lubricated, and this is exactly where the rear main seal comes in.
The rear main seal allows the crankshaft to exit the engine case so it can bolt to the flywheel or flex plate and transfer its energy into the transmission.
Types of rear main seal
The majority of older vehicle models use the rope or wick seal, with a small number using the split seals. Meanwhile, newer models use the one-piece seal for improved performance.
Rope seals and split seals are much more universal and can work on almost any engine, while one-piece seals are less prone to leaks.
#1. Rope or Wick Seal.
Named because of its resemblance to rope and lamp wicks, this seal is designed to handle the impacts of intense crankshaft rotations and the friction caused by fluctuations in temperature.
Consequently, a rope/wick seal can fail in two ways: shrinking in cold weather or expanding in hot weather.
The seal is lubricated to avoid drying out with a small amount of oil, which also keeps it fitted in its place next to the crankshaft. In modern cars, the new design of rope seals lack the strength of the older version when it comes to preventing stray oil.
According to many professionals and specialists, such a change in design is due to stricter environmental regulations, which have restricted the use of materials like asbestos. Such regulations forced manufacturers to cut corners to lower the cost of producing the rear main seal.
As a result, most modern day rope/wick seals are vulnerable to crankshaft contact and engine speed, and therefore need to match the RPM requirements of your engine.
To avoid bad replacement rope/wick seals, only buy from reputable parts suppliers and best to check reviews on Amazon before making a purchase.
#2. Neoprene or Split Seal.
Made of rubbery material, the split seal has a lip shape and is fitted in one direction to prevent the passage of oil. Though effective for this purpose, neoprene seals are equally vulnerable to leaks as rope/wick seals.
#3. One-Piece Seal.
One-piece seals are the hardest seals to service and lack the universality of rope and split seals. With its unique round design, the one–piece seal only works on specially-made crankshafts.
Therefore, you cannot install one–piece seals in older engines without a crankshaft replacement.
Symptoms of Rear Main Seal Leak
A rear main seal leak can be identified in a number of ways, some of which prove far more evident than others. Being mindful of such symptoms will assist you in coming to an early diagnosis, thereby expediting the repair process.
The following are the most common signs of a rear main seal leak.
#1. Puddling of Oil in Driveway.
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.
This, in turn, leads to the puddling of oil beneath a vehicle’s engine, leaving behind ample evidence of such issues.
Therefore, the sudden appearance of an oil stain within the driveway is cause for concern and should be carefully studied to determine its point of origin. It is also possible for this pooling of oil to worsen with time.
#2. Need For Frequent Top-Offs.
Rear main seal leaks are known for their tendency to leak an excessive amount of oil in short order. In severe cases, a compromised rear main seal can leak as much as a quart of oil in a week’s time.
This rapid loss of oil will necessitate frequent top-offs, to avoid an engine’s oil from dropping to dangerously low levels.
If a weekly inspection of our vehicle’s engine reveals unanticipated oil loss, or if you suddenly find yourself adding more oil to your engine than normal, a worn rear main seal could be to blame.
Therefore, additional time should be taken to assess the cause of this expedited oil usage.
#3. Oil Saturation of Vehicle Underbody.
Since a leaking rear main seal tends to cause pooling of oil within an engine/transmission’s bell housing, it is not uncommon for blowback to cause extensive oil saturation of a vehicle’s underbody.
As a vehicle is operated, leaking oil is swept rearward, coating all that lies behind the rear of a vehicle’s engine.
This places additional importance upon periodically inspecting your vehicle. Simply glancing beneath your vehicle will reveal such tell-tale signs of rear main seal failure, in the vast majority of cases. This allows you to plan for repair before the offending leak grows in severity.
#4. Illumination of Oil Light.
If you are driving along only to notice the sudden illumination of your vehicle’s low oil light, there are two main things that you should do.
The first of which is to pull to the nearest shoulder of the roadway and shut off your engine as soon as possible. The second task of essence is to figure out where your engine’s oil has disappeared. In a case of this nature, an engine’s rear main seal is suspect.
A severe rear main seal leak can cause rapid oil loss, especially when an engine is warmed to operating temperature, and the affected vehicle is in motion.
If oil loss exceeds a particular point predetermined by a vehicle’s manufacturer, a low oil light will illuminate upon the dash. Likewise, some vehicles will display a low oil pressure light or message in a similar manner.
causes of rear main seal leak
Here is some causes of rear main seal leak:
#1. 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 a low oil level. Most engine oils have chemical additives that might affect the seals in the engine. An irregular oil change will deteriorate the seals.
Over time, the seals will deplete, along with buffers in the oil. The inner lip that drives on the crankshaft will dry off and become stiff and unable to seal the crankshaft.
#2. Worn main bearing.
If your engine main bearing is bad or worn out, the crankshaft will dangle inside the bearing leading to stretching and moving of 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’s engine and replacing many other faulty parts along with the rear crankshaft seal itself.
#3. Crankshaft condition.
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. The surface can be restored by installing a sleeve kit on it.
#4. Clogged or defective PVC system.
As 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.
What could cause too much pressure in the crankcase? A clogged or defective positive crankcase ventilation (PCV) system will increase the crankcase pressure and may result in pushing out the seal.
Also, 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.
#5. Misalignment issues.
Whether you own a manual or automatic transmission, problems with the input shaft of the transmission or a damaged flex plate can stress the rear crankshaft seal.
Ensure you check the flexplate for damage on an automatic transmission. On manual transmissions, check the input shaft.
#6. Seal coating.
Some rear crankshaft seals has a polytetrafluoroethylene coating that is 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 pretty much better than Viton or silicone materials. If the seal is coated with oil during installation, it will cause leaks in a short period.
Will Thicker Oil Help a Rear Main Seal Leak?
In certain cases, running engine oil of a thicker viscosity can slow rear main seal leakage. This is due to the fact that oil thins as it warms during normal engine operation.
This, in turn, allows oil to escape more easily through aging, worn, or damaged seals. This is one of the reasons that a leaking rear main seal appears to worsen when an engine is at operating temperature.
Keeping this in mind, it is reasonable to assume that oil of thicker viscosity would be less likely to seep from such a seal, even when warm.
In fact, almost everyone knows of somebody who has sworn that switching from 10w30 to 10w40 has slowed their vehicle’s oil leak.
Likewise, it is even common to hear of heavier diesel motor oils, such as 15W40, being used in gasoline engines to slow a leak.
However, one must also take into consideration the possible ramifications that could arise from deviating away from their vehicle manufacturer’s specified oil viscosity.
Manufacturers specify the use of a certain viscosity of oil based upon several factors, including the measure of an engine’s bearing tolerances.
Therefore, though rare, making such changes in oil selection can lead to possible oil delivery-related issues.
As such, one should proceed with caution, when determining whether or not to use engine oil with a viscosity other than that recommended by their vehicle’s manufacturer.
How to Diagnose a Rear Main Seal Leak
Few words about your vehicle will stop you in your tracks quicker than, “you have a rear main seal leak.” This has been the death knell for many a vehicle over the years, as the cost of a fix for an older car is often more than the car is worth.
A replacement isn’t something you’re going to want to take on yourself and even a seasoned mechanic can struggle with it as it’s in a painfully difficult position.
Step 1: The First Thing to Look For
Since the rear main seal is in a position that you won’t be able to slide under your car and view it directly you need to go through a process of elimination.
If you seem to be losing oil but you don’t have a noticeable drip when the car is parked you may be in the early stages of a leak.
Start the vehicle up and let it idle for 15 minutes and see if that helps you identify a leak, or at least begins to show some oil leaking in the engine.
Once you’ve determined you’ve got a dripping leak, you’ll want to start from the bottom and work your way up. If you see oil dripping on the back of your oil pan or on the front of your transmission bell housing it’s possible you have a rear main seal leak.
But before you panic, you’ll want to check other options higher up that may be dripping down.
Step 2: Check Your Cover Gasket and Valley Pan
Get under your car, use a flash light and thoroughly inspect your cover gasket and valley pan for leaks.
A missed diagnosis here could be costly. Either of these issues will be substantially less difficult to deal with since they’re easier to get to.
The problem with the rear main seal leak is that it requires much of your engine to be disassembled – so the labor is typically too much to do at home and the labor costs skyrocket when you take it to the shop.
Step 3: Is It Worth Fixing?
If your car’s a little older it becomes a very real question of whether or not to fix a rear main seal leak. There are a few things to tackle here:
1. Ask local dealerships, body shops about the unrepaired value of the vehicle.
2. Search local websites for things like mechanic specials and see if you can find similar vehicles with the same issue.
3. If the previous two approaches aren’t successful, you can look into the tax write off value of the vehicle if you donate to a charity like Cars for Veterans.
4. Get a couple of quotes for a repair.
5. Compare the cost of your repair to the difference between the value of your unrepaired and repaired vehicle. If the repair cost is greater than that difference it’s time to sell the car as is and start shopping for something new.
How to Replace the Rear Main Seal
#1. Disconnect the battery.
You have to disconnect the negative battery terminal because you’ll need to remove the starter. To avoid an electrical jolt or short circuit, you need to disconnect the battery terminal.
#2. Remove the transmission.
some vehicles require removing some vital components, such as the exhaust system and driveshaft, prior to removing the transmission. Remove whatever is hindering you from removing the transmission.
If you’re working on an automatic vehicle, you will need to disconnect the torque converter from the flex plate. Lose the torque converter bolts one at a time with the correct socket size and ratchet handle.
After losing the first bolt, turn the engine clockwise until the next bolt appears. Lose the next bolt and repeat this process to lose the remaining bolts.
#3. Remove the flywheel.
If you own a manual transmission vehicle, you will have to remove the clutch and the flywheel. On the other hand, if you own automatic transmission, you will need to remove the flex plate.
#4. Remove the rear main seal bolts.
After removing the transmission, the clutch, and the flexplate, the next thing is to remove the rear main seal. You may see crankcase bolts that link the crankcase to the rear crankshaft seal housing, break free and take off these bolts.
The rear crankshaft seal housing has some 10mm or 12mm bolts holding it on the back of the engine block. Take off these bolts and remove the seal housing. The housing might prove stubborn to come off; use a Flathead screwdriver to pry it off.
#5. Remove the rear crankshaft seal.
The rear main seal itself may also prove stubborn to come off the housing – you have to pry it off and remove it from the housing gently.
#6. Compare the new seal.
Now, place the old and new seals together and compare them. Ensure the new seal’s inner and outer diameter match the old one.
#7. Install the new rear main seal.
Carefully clean the seal housing. Use a gasket scraper to remove old seal debris on the housing. Place the seal into the housing and evenly hammer it in. Do not forget to coat the seal with oil. After installing the seal, apply light sealant on the surface.
#8. Install the rear main seal housing.
Mount the seal housing and tighten the bolts and the crankcase bolts as well. Now, use the reverse process to reinstall everything you have removed early.
Why Is Your Rear Main Seal Leaking After Replacement?
#A. Crankshaft End Play.
A leaky crankshaft end play is the first thing that springs to mind. For example, the pressure on the thrust bearing upon that crankshaft while driving a manual transmission vehicle might increase when the clutches become engaged.
Limiting the crank’s motion in both directions and this bearing acts as a pivot point. Excessive movement may break a rubber seal and prevent a new one from forming.
Furthermore, the rear crankshaft seal wraps around the actual crankshaft, so a proper seal cannot be achieved if the crankshaft surface is damaged or worn.
The seal surfaces must be similar if you want a perfect, snug fit. Repairs should be made to the crankshaft surface if the assembly does not fit snugly.
#B. Wrong Size Of Aftermarket Oil Pan.
After more checking, it was found that the aftermarket oil pan was already too small, which is why the crank was touching it.
We checked out a wide selection of new oil pans in some cases, but they all suffered from the same problems. To resolve this, buy a suitable oil pan and install it on your system.
#C. Seal Problem.
It’s not clear if the silicone was put in the right places around the black seal. It’s possible that the illustration in the manual was misread.
Below, we’ve circled in red where you should put silicone to “stop” any oil leaks. Leaks cannot be stopped by installing the “dry” seal.
It’s also conceivable that the “half moon” side of the rubber seal is being fitted backward, as it’s very difficult to get the other half of such a seal incorrect because of the way it’s created.
In addition, a couple of the earlier one-piece seals on smaller blocks had been improperly replaced, and we’ve seen the same thing happen with the new “replacement” seals.
The seals were manufactured with too much tolerance. Therefore, they started leaking after approximately a week of use.
Have the mechanic give it a shot with the present black seal after thoroughly cleaning the region with cleanser and then dusting it with baby powder or something similar.
Turn the key and see the powder’s reaction to the oil’s darkening. If it appears at a joint, it may be due to improper sealant application or improper seal sitting.
On the other hand, if it appears at the seal lip where it sits on the crankshaft, the seal isn’t properly installed or the crankshaft has indeed been ground, rendering the seal ineffective.
#D. Main Bearings Problem.
When the engine’s main bearing fails, the crankshaft may swing freely within, straining the oil seal as the engine spins. If you strain and tug on the seal, it will break since it wasn’t designed to do so.
You will, however, be given enough notice that perhaps the main bearing is failing. Your automobile should make a tremendous amount of noise when this happens, and there’s no use in continuing to drive it until the issue is fixed.
#E. Clogged PVC System.
The oil seal for the crankshaft’s back end is a free ride. However, the crankcase pressure might force the leaky seal to fail if it is under too much stress.
This heightened pressure may result from a blocked positive crankcase ventilation system. Without intervention, it will pressure the seal, resulting in a leak.
#F. Misapplication of Oil.
Using the incorrect engine oil may cause the engine to leak. In addition, modern engine oils include chemical compounds that may deteriorate seals.
Utilizing an oil not designed for your engine might quickly degrade the seals. Oils may also dry out the seal, making it rigid and perhaps allowing leaks to form.
While changing the oil, follow the manual’s specific instructions. Overfilling the engine’s oil capacity is another potential culprit.
What are the Benefits of Replacing a Defective Rear Main Seal?
To reduce the expense of repairs on other components of your car, you must replace your rear main seal for a variety of reasons.
Even if it isn’t urgent, the sooner you replace the rear main seal, the sooner you will have a well-functioning car that won’t create any further issues.
You will be able to prevent overuse from wearing down your engine, gears, and other components. If the leak is not fixed, the components, such as the camshaft, will begin to wear down too soon.
Additionally, you will safeguard your car against oil leaks, which can harm and corrode internal components.
The expense of replacing the rear main seal is more and is more important to preserving the longevity of your car since oil leaks might worsen engine problems.
And last, you won’t need to pay close attention to your oil levels. You won’t be wondering when to top up your automobile while driving if you extend the life of the rear main seal and have it changed.
Can I prevent rear main seal leaks?
While it is not always possible to completely prevent rear main seal leaks, there are preventive maintenance measures that can help reduce the risk.
Regular oil changes, using high-quality lubricants, and following the manufacturer’s recommended maintenance schedule for your vehicle can help prolong the life of the rear main seal.
Additionally, avoiding excessive heat, maintaining proper engine temperature, and keeping the engine clean can also contribute to minimizing the chances of a rear main seal leak.
How much does it cost to replace the rear main seal?
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.
The rear main seal usually costs between $600-$900, with a service fee of around $550 to $820 out of the overall replacement charge.
FAQs.
Q: Is a rear main seal leak serious?
If the seal has a slight puncture, the crankshaft rotation will have a negative impact on it and cause it to tear more and leak a large amount of engine oil and drastically reducing the oil to a dangerous level.
And a dangerously low-level oil could cause friction and wears inside the engine, resulting in engine Knocking.
Q: How long does it take to replace a rear main seal?
Replacing a rear crankshaft seal requires long hours and effort. As we have primarily explained in this article, the first step in replacing the rear main seal is removing the transmission. Some vehicles require taking off the entire engine.
You need to imagine how much time and effort you need to invest just to remove a ‘seal.’ Not just that, mechanics also have to diagnose the vehicle to find out the culprit of the oil leak. The seal usually costs nothing, but the process of removing it does.
And that spells why it is very pricey for a professional mechanic to fix it. Most dealerships and auto garage shops will charge heavily to replace the seal.
q: Is a Rear Main Seal Leak Serious?
A rear main seal leak is generally regarded as being quite serious in nature, due primarily to the fact that such leaks only tend to worsen with time.
In severe cases, a rear main seal can leak to the point of creating difficulties when attempting to keep up with the resulting oil loss. Simply put, adding enough oil to maintain proper lube supply becomes troublesome.
However, this is still a far better scenario than what will likely occur should an engine become starved for oil, at the hands of a rear main seal leak.
Any substantial loss of lubricating oil poses the risk of causing bearing damage, including spun bearings, which can ultimately result in untimely engine failure.
At the very least, running an engine low in oil can, and will, cause premature internal wear. In any event, a rear main seal leak should be addressed at the first available opportunity.
While repairing a leak of this nature can get rather involved, it is still nowhere near as troublesome as being forced to replace an engine that has suffered the prolonged effects of sustained oil loss.
If you are not up to the task of completing such repairs yourself (most will not be), make an appointment with a trusted service center as soon as possible.
Q: How Much Will it Cost to Replace a Rear Main Seal in Canada?
In Canada, replacing a rear main seal typically costs between $400 and $1200. While the seal itself is relatively inexpensive (usually $30 to $80), the labour is where the cost adds up.
The complexity of the repair requires several hours of work, often 4 to 6 hours, due to the need to remove either the engine or transmission to access the seal.
Q: How Long Does a Rear Main Seal Last?
A well-maintained rear main seal can last upwards of 150,000 kilometers, though this can be significantly less in harsh driving conditions or with irregular maintenance.
Q: Does the rear main seal stop the leak work?
Yes, it works perfectly fine for a shrunk or deteriorated seal. Even though it is specially formulated to stop rear crankshaft seal leaks, it also works perfectly on other engine oil leaks, including the O-rings, camshaft seals, timing cover seals, and other seals.
Final thought
In light of all this, you should always watch out for rear main seal leak symptoms and act fast as soon as you notice a leak from that location. Unless you’re a gearhead, replacing a rear main seal may be a difficult task.
As much as the process is difficult, it does not make it impossible. With a significant investment of time and effort, you can do it on your own. In quest of saving repair costs, always put safety first.