What Causes A Crankshaft Seal To Leak?

Is the crankshaft seal in your car leaking? Here's exactly why the crankshaft seal is leaking as well as how you can fix it with ease.

A crankshaft seal stops oil from escaping the engine at the crankshaft pulley. It also keeps air out.

When the crankshaft seal leaks, it’s no longer performing these tasks efficiently.

In this guide, I’ll guide you through the front crankshaft seal and explain why it leaks.

Key Takeaways

  • This guide covers the front crankshaft seal (the one you probably mean).
  • Crankshaft seals leak due to age (wear and tear) or improper installation of some kind.
  • Check for a leaking crankshaft seal by looking for oil around the crankshaft pulley.
  • A new crankshaft seal should cost around $400 to $500, including labor.
  • Don’t attempt a DIY repair unless you’re confident in what you’re doing.

Table of ContentsShow

What Is A Front Main Crankshaft Seal?


Most people refer to the front main crankshaft seal when they say ‘crankshaft seal’ (as I will on this page). However, you might be referring to the rear seal, also known as the rear main seal.

A crankshaft seal is a small circular rubber gasket.

It sits in a metal shell on the end of the crankshaft, just behind the harmonic damper and crankshaft pulley. This pulley is responsible for driving the serpentine drive belt – the visible belt when you open the hood.

Its job is to seal the oil in the engine and the timing cover, preventing it from escaping. From the other perspective, it also stops air from getting in.

Crankshaft seals are a vital part of the engine oil system. They also help regulate the crankcase pressure.

Causes Of A Leaking Crankshaft Seal

As the title of this guide suggests, I’ll dive straight into what causes a leaking crankshaft seal.

Age (Wear And Tear)

Old Camshaft Seal

In most cases of a leaking camshaft seal, the cause is simply age. There’s nothing you can do about it.

Just like any machine parts, they deteriorate as time passes. The rubber material becomes fragile and will eventually break or distort.

The rubber seal is still technically a service component (although it should only need replacing at around 100,000 miles).

Mechanics usually replace the crankshaft seal at the same time as the timing belt or chain.

Wrong Part Installed

You wouldn’t have thought a relatively simple piece of rubber would be much of a problem. And it’s not – provided precisely the correct part is installed!

I’ve come across many cases of a failed crankshaft seal stemming from this issue. During a previous repair, the technician or DIY enthusiast has installed the wrong part.

These ‘wrong’ seals are non-OEM, aftermarket versions.

It’s particularly confusing because they often fit the dimensions exactly. Parts stores recommend them for your car for this reason.

However, they may be missing structural elements, such as a reinforced outer edge. Given a bit of time, this new crankshaft will leak because it doesn’t do the job as well as the original.

Improper Installation

Man Calling Mechanic Service After Vehicle Problem

Any kind of work this deep into the engine requires precision.

In short, getting something wrong means an awkward conversation with your mechanic and insurance provider.

It could be as simple as not replacing the metal housing properly or inserting the seal far enough. Perhaps the rubber tore when it was put in.

Anything like this will mean you need a new crankshaft seal.

Symptoms Of A Leaking Crankshaft Seal

Here’s what you’d expect to notice when the crankshaft seal leaks.

The only way you can tell for sure is to take a look underneath your engine.

Oil Leak (Low Oil Levels)

Car with oil and hand break icon. Warning, maintenance and servi

I’ve grouped all the obviously-related oil problems in this category here.

When the crankshaft seal fails, oil passes by and sprays out into the front of your engine bay. It’ll coat the area in a light film (or heavy puddle!).

Of course, it’s most noticeable when you get underneath the car and take a look for yourself. You’ll see oil covering the surfaces behind and around the crankshaft pulley.

Because this oil is seeping out of your engine, the oil levels will drop. Verify this using the dipstick, as you typically would during monthly maintenance checks.

You might also see warning lights such as the low oil pressure icon or Check Engine light.

If this problem persists or the seal fails quite spectacularly, the motor oil might drop dangerously low.

At this point, you’ll first notice the engine coolant overheating (on your dashboard gauge).

When this overheating gets really bad, your engine will seize. And that will be its last day.

So, in short – don’t put this repair off!

Squeaking Serpentine Belt

serpentine belt

Lots of cars have squeaky serpentine drive belts. So far as I know, this is just one of those things.

However, if it’s a pronounced sound and seems to have started suddenly, check the crankshaft seal.

Oil might be spraying out past it onto the belt. This affects its tension and grip on the pulleys.

The squeaking or squealing sound you’re hearing is the belt slipping around them instead of turning them.

Under-Hood Accessory Problems

opening the hood of a broken car

This issue follows from the one I mentioned above. If oil coats the belt, it’ll affect the components driven by it.

This list comprises the subsequent items:

  • Alternator
  • Water pump (for the coolant)
  • Power steering pump (if hydraulic)
  • Air conditioning compressor (if applicable)
  • Supercharger (if applicable)

The belt turning these accessories slips around the pulleys, representing an efficiency drop. Thus, they simply don’t get as much power as they need.

You may observe electrical troubles, problems with power steering, lack of cold air from the A/C, and so on.


The crankshaft seal could be intact. If any of the components propelled by the crankshaft pulley fail, they’ll put drag on the system, leading to the same symptoms.

Leaking Oil Crankshaft Seal? Here’s What To Do

car technician holding the wrench

Once you’ve identified the crankshaft seal as the problem, go to a mechanic.

Yes, I know YouTube is full of explainer videos about changing your seal. But please don’t. It’s one of those ‘only if you know what you’re doing!’ things.

These DIY explainer videos are often filled with ‘hacks’ and ‘easy ways to do it without special tools’. Some people can get these to work.

In reality, though, if you don’t know what you’re doing, you’re going to do more harm than good. And then you’ll need to go to a local shop anyway.

You need specialist tools (seal extractors and toolkits) to avoid damage. And having a two-post automotive ramp makes all the difference on jobs like these.

It’s best to brace yourself and head down to the garage. A mechanic will replace it for you in a few hours.

How Much Is A New Front Crankshaft Seal?

Handing Over Money Cost Price

Like many automotive parts, it’s almost impossible to give you an accurate estimate. Your local shops will take a wide variety of factors into account, such as:

  • Seal cost
  • Seal availability
  • Labor rate
  • Car make and model
  • How easy the seal is to access
  • Other damage caused by the leaking seal
  • Previously-attempted ‘repairs’

Overall, a new front crankshaft seal – including labor – is likely to be around $400 to $500 (plus or minus quite a lot).

The seal itself is usually between $30 and $100, depending on your car. The rest is labor.

Can You Restore A Leaking Crankshaft Seal?

Fix or Repair

No. Sorry.

Once the rubber goes brittle, the seal has failed. By definition, it no longer works, and needs to be replaced with a new one.

Again, you might find online examples talking about restored crankshaft seals. I can assure you this isn’t a long-term solution and will almost certainly lead to more (worse) problems.


A crankshaft seal is vital to your car (even if you’ve never thought about it before).

If you notice signs of yours leaking, it’s time to go to a trusted mechanic.

You’ll soon be back on the road with your car running much better!

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!