Few words about your vehicle will stop you in your tracks quicker than, "you have a rear main seal leak." This has been the death knell for many a vehicle over the years, as the cost of a fix for an older car is often more than the car is worth. A replacement isn't something you're going to want to take on yourself and even a seasoned mechanic can struggle with it as it's in a painfully difficult position.
Step 1: The First Thing to Look For
Since the rear main seal is in a position that you won't be able to slide under your car and view it directly you need to go through a process of elimination. If you seem to be losing oil but you don't have a noticeable drip when the car is parked you may be in the early stages of a leak. Start the vehicle up and let it idle for 15 minutes and see if that helps you identify a leak, or at least begins to show some oil leaking in the engine.
Once you've determined you've got a dripping leak, you'll want to start from the bottom and work your way up. If you see oil dripping on the back of your oil pan or on the front of your transmission bell housing it's possible you have a rear main seal leak.
But before you panic, you'll want to check other options higher up that may be dripping down.
Step 2: Check Your Cover Gasket and Valley Pan
Get under your car, use a flash light and thoroughly inspect your cover gasket and valley pan for leaks. A missed diagnosis here could be costly. Either of these issues will be substantially less difficult to deal with since they're easier to get to. The problem with the rear main seal leak is that it requires much of your engine to be disassembled - so the labor is typically too much to do at home and the labor costs skyrocket when you take it to the shop.
Step 3: Is It Worth Fixing?
If your car's a little older it becomes a very real question of whether or not to fix a rear main seal leak. There are a few things to tackle here:
1. Ask local dealerships, body shops about the unrepaired value of the vehicle.
2. Search local websites for things like mechanic specials and see if you can find similar vehicles with the same issue.
3. If the previous two approaches aren't successful, you can look into the tax write off value of the vehicle if you donate to a charity like Cars for Veterans.
4. Once you have a value for your unrepaired car, check online sources like kbb.com or nada.com to get a number for the repaired value of your vehicle.
5. Get a couple of quotes for a repair.
6. Compare the cost of your repair to the difference between the value of your unrepaired and repaired vehicle. If the repair cost is greater than that difference it's time to sell the car as is and start shopping for something new.
Step 4: Alternative Options
If selling the vehicle or paying for a repair is the absolute last thing you want to do, you can still try a few alternative options to see if it can remedy your situation. One tool that some people have had success with is an oil additive called Blue Devil Rear Main Sealer. If you're out of options, this is a fairly low cost method that has worked for some people with a leaking rear main seal. Just add one 8 oz. bottle for every 8 quarts of engine oil and it should at least slow down the leak and may even stop it. At $15 per bottle, it's a small price to pay for a product that doesn't seem to have any other risk to it.
Another similar product that supposedly works similarly is Bar's Rear Main Seal Repair. Again, if you're in a spot where you're going to donate the car or sell it instead of repair it, these are potential low cost fixes that may be worth a shot.