Rear Main Seal Leak: Symptoms, Causes & Repair Cost

Of all the things you could deal with, a rear main seal leak is one that can cause a lot of alarm. And I am not talking only about your vehicle, but also your wallet. This seal is typically one that you don’t expect or want to leak. When it does, it can quickly ruin the day for most car owners as the repair cost is typically high. But before you start digging into your savings, it’s always best to understand the symptoms and what’s causing the leak in the first place.

This article will help you understand what to look for, what the causes are, and how much you can expect the repair to be. At the end, we also answer some of your top questions. So, without further ado, let’s get started!

What Is a Rear Main Seal?

The rear main seal, also called the rear crankshaft seal, is a crucial component of an internal combustion engine, specifically the crankshaft assembly. It is located at the back of the engine, where the crankshaft extends out of the engine block and connects to the transmission.

While its main purpose is to prevent oil leakage from the crankshaft area, it also helps to dampen vibrations.

The seal is typically made of a durable rubber or silicone material resistant to heat, oil, and other engine fluids. However, it can wear out over time due to corrosion from road salt, rotational force from the crankshaft, or other environmental factors.

When it does, it’s usually a costly repair since it requires removing the transmission and sometimes even dismantling the engine. In addition, it’s typically hard to diagnose. The oil often drips and finds its way elsewhere, making it easy to misdiagnose for something else. This is also why seeking expertise is important if you’re unsure.

What are Symptoms of a Rear Main Seal Leak?

One of the most common symptoms is leaking oil, which may reveal itself in a pool under your vehicle. You may also notice an increase in engine noise or smell a burning odor from the engine bay. Either way, if you suspect your rear main seal leaks, it’s best to get it checked swiftly.

Listed below are the most common symptoms of a rear main seal leak:

  • Oil Leakage
  • Low Engine Oil
  • Smell of Burning Oil
  • Oil on the Undercarriage
  • Clutch Slippage (in manual transmissions)

1. Oil Leakage

Finding oil leakage is one of the first things that grabs the drivers attention, and also the reason behind all of the following symptoms below. This is often manifested by spots or a puddle of oil beneath your vehicle, particularly in the area between the engine and the transmission, and could very well be a sign of a rear main seal leak.

Over time, the persistent dripping or slow ooze can form a significant amount, which is easily visible after the car has been stationary. The only question is to figure out where the dripping is coming from.

2. Low Engine Oil

The engine relies on oil for lubrication, helping reduce the friction between moving parts and ensuring that the engine components don’t wear out prematurely. If there’s a rear main seal leak, you might find that the engine’s oil level decreases at an unusual rate, due to the consistent loss of oil.

Regularly monitoring the oil level via the dipstick can provide insight into this. A rapid decline in oil level between routine oil changes, especially if no other symptoms of engine issues are apparent, can be indicative of a leak.

Note: If the engine oil light comes on while driving, pull over and turn off the vehicle as it indicates that the oil level is dangerously low.

3. Smell of Burning Oil

The unique odor of burning oil is hard to miss and personally, I can’t stand these types of smells. Anyway, as oil from a leaking rear main seal drips onto hot engine components or the exhaust system, it burns and gives off this distinct smell.

No matter if you’re driving, or just idling, as soon as you start catching a whiff of this, it’s time to pay attention.

4. Clutch Slippage (in manual transmissions)

While not all rear main oil seal leaks result in clutch failure, oil can sometimes find its way under the bolt heads of the flywheel and contaminate the clutch. This contamination can impede the clutch’s function, making it unable to engage or disengage as efficiently as it should.

So if you’re experiencing a sudden decrease in your vehicle’s acceleration despite revving the engine, or if changing gears becomes problematic due to the clutch not engaging or disengaging smoothly, the rear main seal could be the culprit.

What Causes a Rear Main Seal Leak?

Listed below are the most common causes of rear main seal leaks:

  • Engine Oil Condition
  • Worn Main Bearing
  • Bad Crankshaft Condition
  • Clogged or Defective PVC System
  • Faulty Seal Coating

Engine Oil Condition

Different engines require different types of oil, and using the wrong type can cause damage to engine components, including the rear main seal. For example, some oil additives contain chemicals that can degrade rubber seals.

Worn Main Bearing

The main bearing is a crucial component supporting the engine block’s crankshaft to rotate smoothly. When it wears down, it can lead to excessive vibration and movement of the crankshaft, putting additional stress on the rear main seal. Over time, the seal will deteriorate and develop leaks.

Bad Crankshaft Condition

If the surface of the crankshaft that the rear main seal rides on is damaged, pitted, or rusted, it can cause the seal to fail prematurely. This can, however, often be restored by installing a sleeve kit on it.

Clogged or Defective PVC System

When the PCV (Positive Crankcase Ventilation) valve is clogged or defective, it can cause pressure to build up in the crankcase. This excessive pressure can eventually push the oil past the seal or cause it to swell, resulting in a leak.

Faulty Seal Coating

Sometimes, the seal coating on the rear main seal is made of polytetrafluoroethylene (PTFE) and is designed to be installed completely dry to create a proper seal. If oil is applied to the seal during installation, it can cause the seal to leak shortly after installation.

How Serious Is a Rear Main Seal Leak?

A rear main seal leak is a serious concern but should, in most cases, not cause short-term damage to your engine as long as the leak is small. But it all depends on your situation. In some cases, as the force from the rotating crankshaft keeps tearing it down, it can rapidly lose oil to dangerous levels. It’s, therefore, always best to address the issue promptly.

Additionally, and as mentioned, if oil finds its way to the clutch, it can result in clutch slippage. In the worst-case scenario, oil leakage can also be a fire hazard when enough comes in contact with too much heat.

How Much Does It Cost to Replace a Rear Main Seal?

The average cost of replacing a rear main seal is around $1,100. While the seal usually costs $5-$110, it’s a labor-intensive repair that often takes 6 to 10 hours or more since it requires removing the transmission and/or even dismantling the engine. The labor itself can cost anywhere from $350-$2,000.

While the cost for the seal can vary, the largest factor is labor cost. And unfortunately, engines are built differently. For example, the engine is typically positioned diagonally in front-wheel vehicles, making it much harder to access the seal and is a lot more time-consuming.

In addition, while you have your transmission out, it’s sometimes a good idea to have other parts replaced, such as the clutch. So please keep in mind that the actual cost can vary depending on your make and model.

Source: Thecostguys


Can you drive with a rear main seal leak?

Yes, you can drive with a rear main seal leak, but it is not recommended. Driving with a small rear main seal leak may not cause immediate issues, but if left unchecked, the leak could worsen and lead to more serious damage to your car’s engine.

How long can you drive with a rear main seal leak?

The length of time you can drive with a rear main seal leak depends on the severity of the leak. In some cases, the leak may be minor and you can drive for a while without immediate issues. However, a significant leak can quickly cause further damage. So it’s important to have the seal repaired as soon as possible.

Can you replace a rear main seal without removing the transmission?

In some cases, it is possible to replace a rear main seal without removing the transmission. However, this varies depending on the specific vehicle and its design. It is recommended to consult a professional mechanic or refer to the vehicle’s service manual for guidance on replacing the rear main seal without removing the transmission.

Is it worth fixing a rear main seal leak?

Fixing a rear main seal leak is generally worth it, especially if you plan on keeping your car for an extended period. Ignoring a rear main seal leak can lead to further damage to your engine and potentially costly repairs down the line.

Can a rear main seal stop leaking on its own?

In most cases, a rear main seal will not stop leaking on its own. Once a rear main seal starts leaking, it is unlikely to fix itself without intervention. It is best to address the issue and have the seal repaired or replaced to prevent further leaks


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Robin Söder

Robin has always been passionate about cars ever since young age. Over the years, Robin has worked on thousands of cars and gained invaluable experience in the process. He is considered an expert in his field and is often consulted by other mechanics for advice.

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