Rear Main Seal Leaking After Replacement – What Are Causes?

When your car’s rear main leak, it could be a sign of a lot of safety-related mechanical problems. Among these leaks, the rear main seal leaking after replacement is a typical case.

Let’s learn more about its causes and how to fix them in this article!

Rear Main Seal Leak Signals

rear main seal leaking after replacement
Rear Main Seal Leaking

There are a lot of signals of rear main seal leak, including oil dripping, reduced oil supply, warning light, and trademark of oil. Let’s scroll down to learn more about these symptoms.

Oil Dripping

When the rear main oil seal at the back of the engine fails, seal leakage is inevitable. It’s possible, however, that the leak will develop more severe once the engine has warmed up.

That’s why the oil leak isn’t usually readily apparent.

Inspecting the ground in the driveway and garage after parking the car may reveal the most instances.

For example, an oil mark that wasn’t there previously on the driveway is a cause for investigation.

Reduced Oil Supply

When the rear main seal fails, it may leak a lot of oil quickly. This might mean you must continue adding oil to maintain the current levels. You may need to add a quart more per week.

While occasional oil top-offs are common, you shouldn’t have to do it too often. Instead, take the seal in for an inspection before the situation worsens.

Warning Light

The oil light is sometimes the first indicator that anything is wrong. If this warning light turns on, it’s because the oil pressure and volume are dangerously low.

To stop, just turn off the car’s ignition. You might attempt to drive home if there is enough oil inside the system, but you should do so carefully.

Rapid oil loss due to a leaking rear main seal may cause an engine to fail and become damaged beyond repair.

Trademarks Of Oil At The Bottom of Your Car

When the engine rear main seal fails, the engine oil will seep into the gearbox and engine’s bell housing. Likewise, when blowback occurs, the oil may soak through to the underneath of the car.

A simple visual check should reveal an engine oil leak throughout the entire area at the rear of the engine. When this occurs, it’s imperative to have the car checked out right away.

Why Is Your Rear Main Seal Leaking After Replacement? 

rear main seal leak
What The Causes

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.

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.

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.

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.

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.

Misapplication of Oil

engine rear main seal
Using The Incorrect Engine 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.


There are many causes of the rear main seal leaking after replacement, but the most common is due to defective components of the main seal.

If you notice any of the symptoms listed above, you should contact a mechanic to have them repaired to ensure your safety.

Rear main seal replacement typically costs between $800 and $1,500. The seal, on the other hand, may only set you back $50.

The remaining sum is due to the massive quantity of effort needed for this task.

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