- The M278 V8's magnesium front covers commonly start leaking after 4 to 5 years.
- The leaking oil soaks the alternator below it. A new one runs $1,500–$2,500.
- The turbo coolant lines have to come off for this job and almost always break, so they should be replaced.
- Doing the full repair in one visit means you pay for the teardown only once.
- Over 100,000 miles? Replace the water pump at the same time and save about 8 hours of labor.
Which cars have the M278?
The M278 is Mercedes-Benz's 4.6-liter twin-turbo V8, used across the “550” models for most of the 2010s, including versions of the S550, E550, CLS550, CL550, SL550, ML550, GL550, GLE and GLS 550, and the G550. Not sure what's under your hood? Call us with your year and model and we'll tell you.
Why it leaks
The front of the engine is sealed by cylinder covers made of magnesium. Magnesium is light, but the factory sealant doesn't bond well to it over time. Heat cycles slowly break the seal down, and most of these engines start leaking after 4 to 5 years.
It often starts as a little oil on the front of the engine, a burning-oil smell after a drive, or drips under the front of the car. It doesn't fix itself, and it gets worse.
Why you shouldn't ignore it: the alternator
The alternator sits right below the front covers. As oil seeps out, it soaks the front of the engine and the alternator. Oil inside an alternator shortens its life, and replacing one runs $1,500–$2,500.
The turbo coolant lines
To reach the front covers, the turbo coolant lines have to come off. These plastic lines get brittle with heat and age. They break almost every time they're removed, and they only last about 60,000 miles anyway.
That's why we replace them with new factory Mercedes-Benz lines as part of the job. Reusing brittle lines is how a “fixed” car comes back with a coolant leak a month later.
Fix it once: why the full repair saves money
Most of the cost of this repair is labor to take the front of the engine apart. If the covers, lines, belt and pulleys are done on separate visits, you pay for that teardown over and over. We do it all at once, so you pay for it once.
What our M278 repair includes
- Front covers resealed, with the surfaces sanded for a lasting bond
- New drive belt and all pulleys
- New turbo coolant lines (factory Mercedes-Benz parts)
- Mobil 1 oil change and filter
- Engine coolant flush
- Full engine, engine bay and lower panel cleaning
- 3-year / 36,000-mile warranty
- Done in 2–3 days on most vehicles
Common questions
How do I know if my M278 front covers are leaking?
Common signs are oil on the front of the engine, a burning-oil smell after driving, and drips under the front of the car. We can confirm it during an inspection or your yearly service.
Can a front cover oil leak damage the alternator?
Yes. The alternator sits below the front covers, and leaking oil soaks it. Replacing an alternator typically costs $1,500 to $2,500, so fixing the leak early can save money.
Why replace the turbo coolant lines during a front cover reseal?
They have to come off for the job, they get brittle with age and break almost every time, and they only last about 60,000 miles. Replacing them with factory Mercedes-Benz lines prevents a coolant leak later.
How long does the M278 front cover repair take?
2 to 3 days on most vehicles.
Questions about your Mercedes?
Talk to a Mercedes technician directly, or book online in about a minute.
This guide is general information from our technicians' experience and isn't a diagnosis of your vehicle. Prices are averages, include tax where noted, and are confirmed in writing before any work begins. Elite Benz is an independent Mercedes-Benz specialist, not affiliated with or endorsed by Mercedes-Benz USA.
