Why Do I have a Leaking Rear Main Seal?


leaking rear main sealHaving a leaking rear main seal can be difficult to diagnose and even more difficult to fix.  If you’ve ever had a rear main seal leak on a car, then you know it can be one of those repairs where it’s easier to just keep adding oil to your engine rather than actually getting the leak fixed.  Unfortunately, regularly adding oil to your engine is not the proper way to maintain your vehicle and can get costly for you as the average quart of oil now costs $4.50!

Before we get into why you have a leaking rear main seal, it’s important to be sure that is actually what your problem is.  A leaking oil pan gasket or even leaking valve cover gaskets can easily be mistaken for a leaking rear main seal. To be sure you’ve got a leaking rear main seal and not some other problem, make sure you read our article on how to identify a rear main seal leak.

Once you’ve positively identified your problem as a rear main seal leak, it’s important to understand how you got here.

Causes of a leaking rear main seal:

  • Low miles on an older car
  • Infrequent oil changes
  • Low-quality engine oil
  • Worn engine components

Low Miles on an Older Car

The most common killer of rear main seals is simply not driving the car.  Cars love to be driven! If they sit for extended periods of time (over 1 month) you will often start to get leaks.  In the case of the rear main seal, the crankshaft will stretch the seal into an oval shape as it sits. Next time you start the car the oval-shaped seal will not form a perfect seal around the perfectly round crankshaft and you will have a leak.

Infrequent Oil Changes

If you are like most Americans your oil changes intervals have stretched from 3000 to 5000 miles and if we’re being honest, sometimes a lot more than 5000 miles.  Depending on the grade of oil and type of filter you use these long oil change intervals may be damaging to the seals in your engine. As engine oil wears out it will lose its ability to properly lubricate the rear main seal as the crankshaft spins through it causing wear and a leak.  Similarly, old oil will carry wear products in it like dirt and tiny metal shavings from engine wear which can also accelerate wear of your rear main seal causing a leak.

Low-quality engine oil

Similarly, low-quality engine oil can cause improper lubrication of the crankshaft where it meets the rear main seal and cause wear.  Also, higher quality oils often have additives that help keep seals soft which helps combat age hardening of your rear main seal. Using a higher quality engine oil will help prevent rear main seal leaks in your vehicle.

Worn engine components

If your engine has worn bearings, either from infrequent oil changes, low-quality oil, or simply high mileage, the worn bearing will allow the crankshaft to wobble ever so slightly as it spins.  This wobbling with stretch the rear main seal opening again causing a leak.

If you’ve got a rear main seal leak for any of these reasons, it’s important to take your vehicle to a mechanic rather than continuing to add engine oil every week.  If replacing the rear main seal on your vehicle is too costly or doesn’t make sense for your car, consider using BlueDevil Rear Main Sealer to stop the leak!

You can find BlueDevil Rear Main Sealer at any of our partnering local auto parts stores like:

  • AutoZone
  • Advance Auto Parts
  • Bennett Auto Supply
  • CarQuest Auto Parts
  • NAPA Auto Parts
  • O’Reilly Auto Parts
  • Pep Boys
  • Fast Track
  • Bumper to Bumper Auto Parts Specialists
  • S&E Quick Lube Distributor
  • DYK Automotive
  • Fisher Auto Parts stores
  • Auto Plus Auto Parts stores
  • Hovis Auto & Truck Supply stores
  • Salvo Auto Parts
  • Advantage Auto Stores
  • Genuine Auto Parts stores
  • Bond Auto Parts stores
  • Tidewater Fleet Supply
  • Bumper to Bumper Auto Parts
  • Any Part Auto Parts
  • Consumer Auto Parts

Picture Provided By:
leaking_rear_main_seal.jpg – By LoveTheWind – Licensed by Getty Images – Original Link

4 responses to "Why Do I have a Leaking Rear Main Seal?"


  1. Reed Litcher on May 29, 2018 at 9:29 pm

    I need a product that Will help a rear main on an older, (1968) Pontiac 400 with low miles, but very infrequent usage. It’s a rope seal, is your product effective on rope and not a seal that can expandm

    • BlueDevil Pro on May 30, 2018 at 8:35 am


      Thank you for asking about your Pontiac 400. Unfortunately, we do not manufacture a product intended to stop leaks from rope seals. The BlueDevil Rear Main Sealer is only intended for rubber seals. A hard part repair may be your best option.

      Thank you!


  2. John Barbano on January 4, 2020 at 5:36 pm

    Two different BMW’s both have leaks
    1) Transmission leak
    2) Differential leak

    What Blue Devil Products should I be using ?

    • BlueDevil Pro on January 6, 2020 at 8:53 am


      BlueDevil Oil Stop Leak is more concentrated and is the universal application that will work for various leaks such as oil pan leaks, rear main leaks, valve cover gasket leaks, transmission leaks, power steering leaks etc. BlueDevil Oil Stop Leak would treat both of the leaks you’ve described, whereas the BlueDevil Transmission Sealer would only be treating transmission fluid leaks.

      Hope this helps!


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