What to consider before replacing a rear main seal

By: Leon Rhodes

The benefits of replacing a damaged rear main seal are improved performance and restored efficiency, but are they worth the time and cost required? To answer that question consider the age, mileage and overall condition of your vehicle.

The value of replacing a rear main seal

As a general rule of thumb it's not worth the cost and effort of replacing a rear main seal on a high mileage vehicle because other parts are still more likely to completely fail first. There are exceptions, lets discuss them.

A vehicle with a rear main seal leak can be driven but it will no longer perform optimally. You should expect fuel mileage to drop as friction increases over time due to lack of lubrication. The most urgent concern friction causes is overheating.

As with any component that is overheating, care must be taken to limit how hot components can get. In the case of a rear main seal you can minimize this concern by driving the vehicle more sparingly and limiting the distance of trips you drive it on.

Driving a vehicle with a damaged rear main seal at high speeds or for a prolonged period can lead to catastrophic failure and is never recommended.

It takes between 3 and 7 hours on average to replace a rear main seal.

Rear main seal function

The primary function of a rear main seal is to prevent oil from escaping from the back side of a vehicle's engine. Rear main seal leaks tend to start small and worsen slowly but can be significant early on as well.

The easiest way to diagnose a rear main seal leak is to observe the presence of oil on the ground underneath the vehicle when it has been parked for a time. Another clear sign of a rear main seal leak is a visible oily film over the bottom of the transmission and undercarriage. You may also notice a burning smell if oil comes into contact with the exhaust system while it's hot.

Rear main seal replacement cost

You should expect to pay between $650 and $750 for a rear main seal replacement with the majority of this cost being for labor. The seal itself is relatively inexpensive, roughly $35 on average for most vehicles, but to replace it the transmission must be removed.

The exhaust system might also need to be removed so the time factor is significant. Taxes, fees and other mechanic charges may significantly raise the price.

Hidden costs of not replacing the seal

Deciding not to replace a leaking main seal carries some costs as well. You will need to monitor the oil level closely in order to top up the engine when oil levels get low and oil isn't free. There is also an environmental cost, oil leaks cause environmental damage.

Transmission removal(to access the seal)

On most vehicles it will be necessary to remove the transmission to gain access to the rear main seal. On some vehicles it might be possible to unbolt the transmission from the engine and slide it slightly out of the way but because of the drive shaft(S) this is not always feasible.

Rear main seal failure

The most common causes of a rear main seal failure are excessive heat, old age, high mileage, infrequent use, and worn bearings. In almost all cases these problems are initiated by the engine not having enough oil in the system.

Frequent oil level checks are always a good idea and a failed main seal is a classic example why.

Using conditioners and sealers to fix a leaky seal

Treating your oil with a conditioner or sealer is a common first attempt at fixing a rear main seal. Products such as Bar's main seal repair, Hapco's pro-seal, blue devil rear main sealer, Lucas Oil 10278 Engine Oil Stop Leak and other products all promise to help recondition a seal without removing it.

Considering failure has already happened there is little to lose in trying one or all of these products, just don't expect wonders. At best you will buy a little more time which may be all you need if you plan to replace the vehicle.

Engine damage from sealing products

Sealers are designed to re-condition rubber by making it expand slightly. In theory rubber seals will then better prevent leaks however results vary greatly.

When used as instructed for your application there is little risk of further engine damage. Be careful when measuring sealer amounts because adding too much can cause a foaming effect inside the engine which will rob the oil of it's viscous properties.

Sealers and conditioners typically take 2-3 days to reach their peak repair value and require 50 miles or more of driving to properly circulate and coat all rubber surfaces. Engine sealers can work but there are no guarantees.

Alternatives to replacing a rear main seal

If the cost of fixing your leaking seal is prohibitive you have several options:

  1. Continue driving the vehicle until it stops working
  2. Put the repair costs towards a newer vehicle instead
  3. Get a dealer trade-in value towards a new vehicle
  4. Sell the vehicle yourself to get maximum value
  5. "Part out" the vehicle if it has other good parts
  6. Get an instant offer from a vehicle salvage company

Each option has it's pros and cons and the best option for you will depend on your specific needs, there is no "best" solution, only choices to make. Good luck!

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