Car Repair and Maintenance

Rear Main Seal Leak Symptoms – Causes and Replacement Cost

Rear main seal leak symptoms

Beneath your car, if you see any oil stains or a leaking engine block, you want to look for rear main seal leak symptoms.

A rear main seal leak is one of the worst things that can happen to any car since it involves extensive engine disassembly and is extremely difficult to access. Most often occurring in older vehicles, the breakdown of the seal on most contemporary engines is brought on by an issue within the engine or a component related to the engine’s rear.

Since rear main seal replacement is often more expensive than the value of cars old enough to have such leaks, the discovery of such a problem signals to many drivers that it may be time to buy a new car.

Next to changing the head gaskets or rebuilding your gearbox, replacing a rear main seal is unquestionably one of the most costly, labor-intensive, and hated tasks you can do on your car.

If the seal has worn off, there are also no fast solutions or workarounds. A replacement isn’t something you should do on your own since it’s in a challenging position, and even an experienced technician may find it challenging.

Having a professional confirm if the source of your present oil leak is a rear main seal leak can save you a lot of money after a diagnosis.

Rear main seal leak symptoms

What does a rear main seal do?

Oil leaks between the block and crankshaft are avoided by the rear main seal. Manufacturers initially designed rear seals to last the life of a vehicle so that there is no replacement required, unlike many other engine components under the hood that need replacement every few years since they are located deep in your car’s engine compartment and are challenging to access.

The rear main seal, which supports the crankshaft for your engine’s motor, is where the rear main seal is located. The bulk of your engine’s load is supported by these bearings, which are also essential for converting your piston’s axial motion into the crankshaft’s rotating motion.

By absorbing the axial stress of the piston that is being pressed downward by combustion and yet enabling the crankshaft to rotate, the main bearings accomplish this. These bearings must be very smooth and consistently well-lubricated by the engine oil in order to do this duty.

In other words, choosing the right oil for your engine is crucial to ensure that the major bearings operate at their best, minimizing wear and tear.

To keep everything constantly lubricated, this oil must also be confined within your engine, and this is where the rear main seal enters the picture. The crankshaft may attach to the flywheel or flex plate and send its energy into the gearbox by escaping the engine case via the rear main seal.

The rear main seal also retains engine oil, keeping it within the engine where it can grease the main bearings.

Rear main seal leak symptoms

There are many techniques to spot a rear main seal leak, some of which are far more obvious than others. Being aware of these indicators can help you make an early diagnosis and speed up the repair procedure.

The most typical signs of rear main seal leak symptoms:

Oil spills in the driveway

When an engine has warmed up to its normal working condition, rear main seal leaks can become worse. This is why a leaky rear main seal is often most noticeable minutes to hours after a car has been parked for the day.

This then results in oil spilling underneath an engine, giving plenty of proof of such problems. As a result, the unexpected presence of an oil stain in the driveway should be investigated carefully to see where it came from. Additionally, this oil pooling might become worse with time.

Requirement for regular top-offs

It is well known that rear main seal breaches have the propensity to release an excessive quantity of oil quickly. In extreme circumstances, a damaged rear main seal might leak up to a quart of oil in a week. It will be necessary to top up engines’ oil often as a result of the quick oil loss in order to prevent the oil from becoming dangerously low.

A worn rear main seal is damaged if a weekly check of your car’s engine shows unexpected oil loss or if you suddenly find yourself pouring more oil than usual. Therefore, more effort should be spent determining the reason for this increased oil use.

Dirt and particle buildup

Oil leaks function as an adhesive. Driveway dirt and debris attach to the areas where the oil has leaked from the rear crankshaft seal. The buildup of dirt and debris is a precursor to a component that is leaking. Basically, the buildup of dirt and debris between the gearbox and the engine is a strong sign that the rear crankshaft seal is leaking. To stop the leak at this stage, you need a rear main seal leak sealer.

Underbody of vehicle saturated with oil

Since a leaky rear main seal often results in oil collecting within an engine or transmission’s bell housing, blowback frequently results in substantial underbody oil saturation.

Leaking oil is swept backward when a car is driven, covering everything behind the engine’s rear.

This emphasizes how crucial it is to regularly examine your car. In the great majority of situations, a glance under your car will show these telltale indicators of a failed rear main seal. This enables you to prepare a repair strategy before the leak causes further damage.

Oil light illumination

There are two basic actions you should do if the low oil light on your car suddenly illuminates when you are driving.

The first is to stop your motor as soon as you can and go to the next available shoulder of the road. Locating the location of the missing engine oil is the second essential step. The rear main seal of an engine is questionable in a situation like this.

Particularly when an engine is warmed to operating temperature and the afflicted vehicle is moving, a serious rear main seal leak may result in a fast loss of oil.

A low oil light will come on on the dash if oil loss reaches a certain threshold set by the vehicle’s manufacturer. A low oil pressure indicator or warning can also appear on certain cars in a similar way.

What causes the leak in the rear main seal?

The rear main seal might leak for a variety of reasons. Here are the causes:

a. Worn main bearing

If your engine’s main bearing is damaged or worn out, the crankshaft will hang within the bearing while the engine is operating, extending and shifting the rear crankshaft seal. This often indicates a worn main bearing in your engine.

If this is the case, you will need to have your vehicle’s engine rebuilt and numerous other defective components replaced in addition to the rear crankshaft seal itself.

b. Crankshaft condition

The rear crankshaft seal lip rides directly on the crankshaft, which is the state of the crankshaft. The inner seal’s driving surface on the crankshaft is thus crucial. A leak may occur as a consequence of crankshaft wear or flaws. A sleeve kit may be put on the surface to restore it.

c. Engine oil condition

Using the incorrect kind of engine oil or having low oil pressure are two issues that might lead to the rear crankshaft seal leaking. Chemical additives included in most engine oils have the potential to harm the engine’s seals.

The seals will degrade from erratic oil changes. The buffers in the oil and the seals will both wear down over time. The inner lip that engages with the crankshaft will dry up, get rigid, and eventually cease to be able to seal the crankshaft.

d. Clogged or flawed PVC system

When the crankcase exerts too much pressure, the rear crankshaft seal’s inner lip rides on the shaft, dragging the seal onto the crankshaft and ultimately causing it to bulge and produce an oil leak. What may increase the crankcase’s pressure? The crankcase pressure will rise as a consequence of a blocked or malfunctioning positive crankcase ventilation (PCV) system, which might force out the seal.

Additionally, excessive engine blow-by caused by faulty or worn-out piston rings may raise the crankcase pressure, damaging the seals and causing oil leaks if you have a turbocharged or supercharged engine.

e. Misalignment problems

Whether you have an automatic or manual gearbox, faults with the input shaft or a broken flex plate can place extra strain on the rear crankshaft seal. Make careful you inspect the flexplate on an automatic gearbox for damage. Check the input shaft on manual gearboxes.

How to replace the rear main seal

Your car might suffer severe damage from a rear main seal leak. You must act quickly to stop the leak if you notice that you are losing oil there before it may seriously harm your engine. If the seal has shrunk, has somewhat deteriorated, or has warped as a result of irregular oil changes, you don’t need to spend a lot of time on it. If this is the case, the seal merely has to be reconditioned rather than replaced.

Here are the things you can do:

  • Remove the battery. Since the starter must be removed, you must disengage the negative battery connection. You must unplug the battery terminal to prevent an electrical shock or short circuit.
  • Remove the flywheel from your car if it has a manual gearbox. You must also remove the clutch. On the other hand, you must remove the flex plate if you have an automatic gearbox.
  • Remove the gearbox/transmission. On some cars, you must first remove some crucial parts, such as the driveshaft and exhaust system, before you can remove the transmission. Get rid of anything that is preventing you from removing the transmission. You must take the torque converter away from the flex plate while working on an automatic car. Use the appropriate socket size and ratchet handle to remove the torque converter bolts one at a time. Once the first bolt is lost, rotate the engine counterclockwise until the second bolt surfaces. To lose the remaining bolts, lose the next bolt and then continue the procedure.
  • Disconnect the rear main seal bolts. After removing the gearbox, clutch, and flexplate, remove the bolts holding the rear main seal in place. The next step is to remove the rear main seal. Occasionally, the crankcase bolts holding the crankcase to the housing for the rear crankshaft seal may come loose. Remove these bolts. Some 10mm or 12mm bolts are used to secure the rear crankshaft seal housing to the engine block. The seal housing may be removed by removing these bolts. Use a Flathead screwdriver to pry the housing off if it seems resistant to coming off.
  • Compare the new seal. Put the two seals side by side and evaluate them. Make that the inner and outer diameters of the new seal are the same as the old one.
  • Remove the rear crankshaft seal. The rear main seal must be carefully pried off the housing and removed. It may be difficult to come off the housing. Clean the seal housing well before installing the replacement rear main seal. To clean the housing’s old seal debris, use a gasket scraper. Then uniformly hammer the seal into the housing.

Don’t forget to apply oil on the seal. Apply a thin layer of sealant to the surface after applying the seal. Install the rear main seal housing by mounting it and tightening the seal housing’s bolts as well as the crankcase bolts. Reinstall whatever you previously deleted by using the opposite procedure.

Rear main seal replacement cost

There is no getting around the reality that repairs for a rear main seal leak will be quite expensive. Even more annoying is the fact that the majority of rear main seals can be purchased for not too much money. However, the labor expenses for such repairs are often high.

These personnel expenditures are, of course, readily justifiable. It’s not simple to replace the rear main seal on an engine, and it takes a lot of time.

This kind of repair necessitates the removal of the vehicle’s gearbox since there is no other way to get access to the engine’s rear main seal. In the case of four-wheel drive cars, the transfer case must often be removed as well.

The precise cost of replacing the rear main seal varies from vehicle to vehicle and is determined by the manufacturer’s stated flag time for such repairs. The drive layout of a certain vehicle affects the cost of replacing the rear main seal. Rear main seal replacement costs normally vary from $650 to $1,800, however, the bulk of these repairs cost between $800 and $1,200.

When should a rear main seal be replaced?

Rear crankshaft seal replacement takes a lot of time and work. The removal of the gearbox is the first step in replacing the rear main seal, as we have mostly detailed in this post. Some cars need the complete engine removed. rear main seal, the design of the seal dictates the installation process—Fel-Pro.

Imagine the time and effort required to just remove a “seal,” much alone the diagnostic work required by mechanics to identify the source of an oil leak. The seal often doesn’t cost anything, but the removal procedure does. And it explains why having a qualified technician repair it costs so much. The majority of vehicle repair companies and dealerships will demand a hefty fee to replace the seal.


Is a rear main seal leak serious?

The crankshaft spin will negatively affect the seal if it has even a little puncture, causing it to tear more, leak a lot of engine oil, and significantly lower the oil level to unsafe levels. Additionally, dangerously low oil levels might result in engine knocking by causing wear and friction within the engine.

Does the rear main seal stop leak work?

Yes, it still functions flawlessly for a shrinking or failing seal. It is designed specifically to combat rear crankshaft seal leaks, but it also effectively stops leaks from other seals, such as O-rings, camshaft seals, timing cover seals, and other seals.

What is the price of replacing the rear main seal?

The seal is essential regardless of the substance that was utilized to make it. It is intended to maintain the oil in its proper location and often has to be replaced when it shrinks or degrades. Typically, a replacement of the rear main seal will cost between $600-$900, with a service fee of between $550 and $820.

Final words

In summary, you should always be on the lookout for rear main seal leak symptoms and take action as soon as you identify one. Replacing a rear main seal might be challenging if you’re not a gearhead.

Despite how challenging the procedure is, it is still achievable. You can accomplish it on your own if you put a lot of time and effort into it. Always prioritize safety above cost-cutting while doing repairs.

Latest posts by Bernard Juchli (see all)

Bernard Juchli

Bernard Juchli is an experienced racer, mechanic and team owner who trusts Avon Tyres.Bernard is the lead driver and force behind his Big Dog Garage Race Team. He is the General Manager and Chief Mechanic of Jay Leno’s Garage. Bernard and his crew of seven are responsible for all repairs, restoration and fabrication of Jay’s incredible automobile and motorcycle collection.

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