Tips and Tricks For Mercedes-Benz Oil Leaks
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Mercedes-Benz Oil Leaks

There are a few common issues that you should be aware of whenever working on a Mercedes-Benz vehicle. Mercedes makes a fine product, but certain repairs will emerge as trends over time.

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One of the most common customer concerns would be engine oil leaks. This may manifest itself as drops of oil on their garage floor, or you may notice oil stains underneath the vehicle during service. As technicians, sometimes it can be difficult to pinpoint the exact source of the leak. We’ll start off by sharing a few tips and tricks to help you in your search, then we’ll look at a few potential leak sources to be on the lookout for when servicing a Mercedes-Benz engine.

Tips & Tricks:

Whenever you’re trying to narrow down the source of a leak under the hood, you’ll oftentimes find a trail of fluid coating a number of components. The key is to follow that trail to the farthest forward and highest up point that you can find. As the fluid leaks out it will be pulled down by gravity, but it will also be pushed rearward by the air passing through the engine bay as the vehicle is driving.

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This means that just because oil is dripping down in the area of the rear main seal, it doesn’t necessarily mean that this is the leak source. Oil could be leaking down from one of the valve covers, then dripping down from the bottom of the engine.

This is what makes it so difficult to pinpoint the actual leak source. Especially these days when it seems like engineers are trying to cram as many components into an engine bay as possible. There are so many obstructions, it can be difficult to see where the leak is really coming from.

Be creative in your search. Use flashlights, mirrors, even the camera on your cell phone to gain access to those hard-to-reach areas around the engine. If you’re really having trouble, try adding a fluorescent dye to the oil and use a UV light to pinpoint the leak source.

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Potential Source #1: Valve Cover Gaskets

Valve cover gaskets are a very likely culprit for oil leaks. The rubber gaskets that seal the valve cover to the cylinder head will break down over time and become hard. Once this happens, they won’t be able to seal as effectively anymore and oil will then leak out of the valve cover (Figure 1). The oil may drip down onto the exhaust manifolds, and the customer may notice smoke coming from their engine bay as the oil burns off.

Figure 1

Always check the OE service information – you may be required to add RTV where the cylinder head meets the timing cover, or other key areas where the rubber gasket may not be able to completely seal by itself. Some engines may only use RTV to seal the valve cover to the cylinder head, no rubber gasket required. Whenever working on this type of engine, be sure to thoroughly clean the gasket surfaces, and completely remove all of the RTV before applying the new sealant.

Potential Source #2: The PCV System

The PCV system is an emissions control system that prevents crankcase gases from being emitted into the atmosphere. These crankcase gases pass through an oil separator that separates the liquid engine oil and allows it to drain back down to the oil sump. Then the gases alone will continue on through the rest of the PCV system and into the intake system to be burned.

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This system is vulnerable to a number of potential issues. The rubber hoses that carry the crankcase gases can degrade over time, becoming hard or even brittle. If one of these hoses begins to leak (Figure 2), the customer may notice a rough idle, hard start or an illuminated check engine light. A system scan might reveal fuel trim or mass air flow codes, this is because the DME may flag codes related to air metering.

Figure 2

It’s important to inspect the rubber hoses for any signs of degradation or cracking. Check where the breather covers seal up against the valve covers, these connections are sealed with RTV alone and are known to leak. It’s also a good idea to inspect the area around the oil fill cap. Oil residue in this area could be an indicator of excess crankcase pressure caused by a faulty PCV system.

Other Potential Sources:

There are a few components on Mercedes-Benz that can appear to be oil leaks but are in fact indicators of other issues. For example, a number of suspension bushings may be filled with a hydraulic fluid. This is typically a glycol mixture, but when the fluid leaks out it typically leaves brown streaks that look like engine oil. This is an indication of a faulty bushing and it must be replaced.

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There are also a number of engine mounts used in modern Mercedes-Benz vehicles that are also filled with hydraulic fluid. These mounts will also leave brown streaks when they start to leak, and it could trick you into looking for engine oil leaks up higher in the engine bay. A failing engine mount will typically exhibit increased noise, vibration and harshness (NVH) inside the cabin and/or excessive engine movement.

Don’t forget about the engine oil cooler and the oil lines that supply it. The connections in this system are typically only sealed with O-rings, which will leak over time. Oil cooler lines utilize crimp fittings where the rubber hoses meet the metal lines, oil may begin to seep from these crimps as the vehicle ages. The seals between the oil filter housing and the engine oil cooler have been known to leak once they harden. These seals are very inexpensive, but usually require extensive disassembly in order to replace them.

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