A rear main seal leak shows up as oil dripping from where the engine meets the transmission, usually a puddle under the middle of the car, oil on the bottom of the bell housing, and an oil level that drops over time. The rear main seal sits around the back of the crankshaft, so replacing it usually means removing the transmission, which makes it a labor-heavy repair even though the seal itself is inexpensive.
Rear main seal leak symptoms
- Oil drips or a puddle under the middle of the vehicle, below the engine-to-transmission joint
- Oil on the bottom of the bell housing or the inspection cover
- Dropping oil level between changes
- On manual cars, a slipping or grabbing clutch if oil reaches it; see clutch slipping
Rear main seal or something else?
| Source | How to tell |
|---|---|
| Rear main seal | Oil at the bell housing seam, fresh oil from the gap between engine and transmission |
| Oil pan gasket | Oil along the pan edges; see oil pan gasket leak |
| Valve cover gasket | Oil starting at the top and running down; see valve cover gasket leak |
| Transmission front seal | Fluid is red or brown and smells of transmission fluid; see transmission fluid leak |
| Oil filter or sender | Oil starting around the filter or oil pressure sender |
Why rear main seals leak
Age and heat harden the seal, high mileage wears the crankshaft surface, and high crankcase pressure from a clogged PCV system pushes oil past it; see PCV valve. On some engines, the seal housing gasket leaks rather than the seal itself.
Fixing it
The transmission, and on manual cars the clutch and flywheel, must come off. Because of that labor, many owners replace the clutch at the same time on manual cars. Seal-conditioning oil additives can slow a minor leak but rarely fix it.
Checking the PCV system first
Before replacing a rear main seal, check that the PCV valve and hoses are clear. Excess crankcase pressure can push oil out of every seal, and a new rear main seal can start leaking again if the cause is not fixed.
Is it urgent?
A slow seep can be monitored by keeping the oil topped up. A fast leak risks running the engine low on oil; see low oil pressure. If the engine also needs a rebuild or replacement, the seal is replaced as part of that job; see engine replacement cost.
Related
See oil leak and, for the 5.3 and 6.0 GM engines where owners often search for this, 5.3 Vortec problems and 6.0 Vortec problems.
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