Diesel Engine Oil Leaks – Places to Check

Part of owning a high mileage or older diesel is dealing with oil leaks.  It is said that you can tell that a diesel is running because it is leaking oil.  If you are chasing down oil leaks, here are some common sources to check.

#1 Valve Cover Gasket

If it seems that your oil leak is coming “from nowhere” you are probably suffering from a failed valve cover gasket.  The valve cover gasket is at the top of the engine, so, by the time the oil drips under the car, it has traveled around the outside of the engine and is probably dripping far away from where the leak actually is.  Check all the way around the valve cover for wet spots.

ALH Valve Cover Gasket Leak Backside Shot

Back of ALH engine with leaking valve cover.

VW TDI Hint – 4 cylinder TDIs are notorious for leaking on the backside, where it is impossible to see the leak. The oil may appear to be coming from the turbo, but, in fact is coming from the valve cover gasket. On ALH engines, where the gasket is not available separately, try using some RTV sealant instead.

#2 Oil Filter Flange Gasket

Oil leaks are especially large when the leak occurs at an area of high oil pressure. This is what makes oil filter flange leaks so problematic as oil can leak out of the engine at a very high rate. It also means that slow leaks can turn into big leaks fast! Luckily the oil filter flange is typically easy to get to and usually takes a very uniquely shaped gasket or o-ring.

OM642 Oil Filter Flange Leak Gasket

Failed Seal on Mercedes Oil Filter Flange

#3 Oil Cooler Seals

The oil cooler uses coolant to cool hot engine oil. This means that there is both hot motor oil and coolant circulating around each other, separated only by some gaskets and o-rings. This is a sure-fire recipe for leaks. These tend to be very large leaks, too. Like the oil filter flange, the oil cooler is under high oil pressure so any leaks that start will worsen quickly. Replacing oil cooler seals can vary widely in difficulty – it is under an hour job on a VW TDI but might take 2 days on some Mercedes models.  

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Very leaky OM642 oil cooler.

Mercedes Hint: The OM642 has a notoriously leaky oil cooler.  See our dedicated article about OM642 Oil Cooler Seal Leaks here>>>

#4 Rear Main Seal

Located between the engine and transmission, the rear main seal (or “mainshaft seal”) goes around the crankshaft (also called a mainshaft, hence the name). You’ll notice oil drops collecting right where the engine and transmission meet.

This seal wears over time as the crankshaft spins inside of it, so it is likely to leak when the miles pile up. Luckily, this seal is not under high oil pressure and typically is only exposed to “splash” lubrication. This means that rear main seal leaks are usually very slow leaks. Sometimes they might be slow enough to not warrant replacement for years or thousands of miles. While the other seals we mention above are replace-immediately seals, you may choose to monitor a leaking rear main seal and oil level rather than replacing it ASAP.

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Leaky rear main seal.

Important Warning: Rear main seals on newer VWs need a special tool to install or the engine may not start! This is because the reluctor wheel for the crankshaft speed sensor is located inside the seal itself.

Other leaky, less leaky locations:

  • Front Crankshaft Seals – if your timing belt is oily, your front crank seal may be on its way out. Typically requires replacing the timing belt to replace the seal. Always use a new front crankshaft bolt.
  • Camshaft Seals – similar to crankshaft seals, typically located behind the timing belt. Replace this seal whenever doing a timing belt or camshaft job to avoid problems in the future.
  • Oil Cap Gasket – cheap, easy to replace, on some models you can even try flipping the gasket.
  • Oil Filter Housing Spindle – for engines that use cartridge filters, always replace the spindle o-ring to avoid developing leaks. New o-rings are typically included in the box with high quality oil filters.

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