Rear Main Seal Leak Symptoms: Causes and Replacement cost?

Do you see oil puddles under your vehicle, especially when the engine is running? That might be one of the most noticeable rear main seal leak symptoms. So, don’t be surprised when a mechanic tells you that the oil leaks from the rear main seal. The rear main seal is one of the essential seals in your vehicle that you don’t expect to leak unless you’re dropping your engine or transmission because replacing this seal requires dissembling the engine or transmission before you can reach it.

The primary cause of the rear main seal failure is a rupture inside the engine or on a component connected to the engine. For many car owners, such news could cause a bad day because the rear main seal replacement is one of the most expensive, labor-intensive, and dreaded repair jobs to do on your vehicle. It can cost you $600 to $900  or more at a dealership. Do not panic – there’s a shortcut to it. However, it is not a permanent fix.  So, in this article, we’ll discuss at length the signs of a rear main seal leak, what causes a rear main seal leak, and how to replace it.

how to fix rear main seal leak

What is a rear main seal?

What is a rear main seal, and how does it work? The rear main seal is found at the back of the engine between the engine and the transmission, which seals the crankshaft’s back end from leaking. It is an expensive fix because, most times, it requires removing the transmission or dismantling the engine.

On front-wheel cars, the engine is typically mounted diagonally, so the back of the engine is the side opposite the one with idler pulleys, tensioners, and the serpentine belt. Engine oil leak from the rear crankshaft seal is hard to diagnose, except if you’re a gearhead like me because the oil usually drips from where the transmission connects to the engine and drips elsewhere, such as the crankcase.

The rear crankshaft seal is usually made of silicone or rubber. The seal wears out over time as a result of corrosion from road salt, crankshaft rotational forces, and other environmental factors. Driving with a rear main seal leak can be very dangerous and can lead to total engine failure.

Rear Main Seal Leak Symptoms

The location of this seal makes detecting and fixing an oil leak challenging and complicated. Just like every other mechanical component, some signs and symptoms will pop up to notify the driver of a defective or leaking rear crankshaft seal.

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 that is burning excess oil and not a rear crankshaft seal leak.

Accumulation of dirt and debris:  Oil leaks act as an adhesive. 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. Basically, 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.

Quick Oil Reduction: If your vehicle has suddenly started losing significant amounts of oil without any apparent leaks or damage to the engine, then it is likely that the root cause is related to your rear main seal. This type of leak can be caused by several factors, including wear and tear on the seals themselves.

In your routine checkups, you can see the dipstick oil level comes down to an unexpectedly low level. When the car suffers from faster engine oil loss, you will hear noise while moving, and the engine oil light will keep blinking. The best way out, in this case, is to check your car oil every morning before going outside.

What causes the rear main seal leak?

Many factors can cause the rear main seal to leak. Let’s briefly look at the factors that can deform or deteriorate the rear crankshaft seal.

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.

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.

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.

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.

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.

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.

rear main seal leak repair cost

How to Replace the Rear Main Seal

A rear main seal leak can be catastrophic to your vehicle. If you’re losing oil in that location, you have to act fast to fix that leak before it can cause significant damage to your engine. You don’t have to spend a lot of your time if the seal is shrunk or slightly deteriorates, or deformed due to irregular oil change. If this is the case, the seal doesn’t have to be replaced; it only needs to be reconditioned.

It is no news that engine seals deteriorate over a long period of use due to oil acidity, heat, and several other related issues, usually due to lack of maintenance. In such a case, the rear main seal stop leak is needed to recondition the seal and work for a few thousand miles.

To recondition the rear main seal, get a blue devil rear main sealer. This product is specially formulated to stop the rear main seal leak. It does this by restoring rubber gaskets and seals, it is also an excellent repair for timing cover leaks, and it’s safe to be used for both gas and diesel engines. It’s also compatible with both synthetic and conventional oils. Keep in mind that this will not work if you have a chunk, tear, or hole on the seal. No product can repair a big chunk or hole on a rubber seal. So, if you have a giant hole in the rear crankshaft seal, follow the below steps to fix it.

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.

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.

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.

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.

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.

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.

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.

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.

Frequently Asked Questions – FAQs

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.

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.

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.

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.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

2 thoughts on “Rear Main Seal Leak Symptoms: Causes and Replacement cost?

  1. Hey Randall,

    Sorry, you had to experience that. However, you don’t have to blame yourself for anything. Cars break down even when you give them the necessary maintenance at the right interval.

    However, maintaining good oil conditions and replacing clogged PCV valves early could be all you need to prevent the issue.

    In terms of replacing it yourself, it’s a simple repair guide any experienced DIYer can do. But keep in mind, you have to disassemble the transmission from the engine and remove the clutch assembly before you can get to the rear main seal.

    You can go ahead if this looks like what you can do. But if not, have an experienced mechanic do the job.

    Best of luck, Randall.

  2. i appreciate the attention to detail in both the overall repair procedure and the causes. I keep racking my brain rehearsing why did this happen and how much of that was my fault and should I attempt to mess with the repair.

    Thank you

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