Announced in 2010, the M278 engine by Mercedes-Benz continued the companies’ tradition of creating V8 power houses. Mercedes was known for legendary V8 engines such as the M119 or the M113, so there’s always big expectations when a new V8 from Stuttgart roles out. Did the M278 meet the expectations? This article covers all the known M278 Mercedes engine problems!
Basic M278 Specifications
The original M278 engine featured a displacement of 4.7 L and a power output of 320-335 kW or 429-445 bhp. There was no lack of torque as well, with 700 Nm or 516 lb ft of torque at 1800-3500 RPM, you could feel this engine pull like a train.
💡 Compared to its older brother, the M273 engine, the M278 also features a redesigned timing chain system, different variable valve timing system and modified/replaced engine accessories.
To get this kind of power, Mercedes equipped the V8 with dual turbochargers provided by Garrett Motion and a direct fuel injection system with piezo electric fuel injectors for laser sharp fuel injection and a multi-spark ignition.
Common Mercedes M278 Engine Problems
People love to compare the modern V8 engines with those made in the 90s (M119), and when it comes to long term reliability, there’s no doubt that those older engines prevail. However, we think those comparisons are completely nonsensical.
Yeah, the M119 engine is going to be more reliable, it’s also much more simple, less powerful and less environmentally friendly. So naturally there are more things that can go wrong with engines like the M278, but that does not mean they’re bad, they just require more care and unfortunately financial input.
Here are all the common Mercedes M278 engine problems:
- Timing chain tensioner problems
- Oil leaks on the engine wiring harness (camshaft sensors and camshaft adjusters)
- Engine valve guide wear
- Engine cam adjuster issues
- Various oil leaks
- Fuel injector issues on early models
- Occasional long cranking issue
Now that you know what the problems are, we’re sure you’re thirsty for some more details and the sources for our claims. Let’s get into it.
M278 timing chain tensioner problems
The M278 engine features three timing chains, one of which is primary and the other two are referred to as secondary. There was a known rattling issue on cold start up until the engine built up sufficient engine oil pressure. Once the oil pressure built up, the secondary chain tensioners provided enough tension on the timing chain and the rattling stopped.
💡 You can read this technical service bulletin for more information and to see if your engine is affected. This issue was resolved in later model year engines.
Replacing the left and right secondary chain tensioner and fitting non-return check valves minimized oil drainage from tensioners. This resolved the issue for good. According to this owners’ forum thread, the cost of having this repaired at a dealership is around $1500.
Oil dripping on the engine wiring harness
Having oil dripping onto any electrical components is never a good thing, especially if we’re talking about the engine wiring harness. A quick Google search on “M278 wiring harness oil leak” will reveal this common M278 engine problem.
The source of this leak is at the camshaft position sensors and camshaft magnets, and it influences both the V8 M278 engine and the V6 M276 engine. It’s important to recognize this issues early on, before the oil manages to seep from the wiring harness into other electrical components. Here is more information on the camshaft sensor leaks.
Replacing the cam shaft sensors and magnets is fairly simple and cheap, so this isn’t a big issue.
M278 Engine valve guide wear
Early production model M278 are known for experiencing severe issues with elongated valve seats and damaged valve guides.
This issue was described in a technical service bulletin and according to that document, the typical symptoms include an illuminated check engine light, DTCs that indicate misfiring, rough engine operation, poor fuel economy and a lack of power.
Diagnosing the issue involves removing the cylinder head and visually inspecting the cylinder heads and valves. As you can imagine, this isn’t a walk in the park. The issue can be resolved by replacing the entire cylinder head, valves, camshafts, and other internal components (basically an engine rebuild procedure almost.
Mercedes remedied this serious issue by switching from a Silitec cylinder lining technology to Nanoslide coating.
M278 Camshaft adjusters issues
This issue is recognized on several Mercedes motors, including the M278 V8 unit. Hydraulic camshaft adjusters are a critical component of the engine’s variable valve timing system and unfortunately one of the common M278 engine problems.
As they become worn out (prematurely) they release a distinct engine rattle, especially on cold starts. The difference between this rattle and the one we described before (from the timing chain) is that this rattle comes from the valve train area, but it eventually leads to the engine misfiring and numerous DTCs.
Replacing them is the only option, but fortunately enough, this is not a super common issue, so you shouldn’t be so worried.
Various oil leaks
Mercedes acknowledged various oil leaks with the release of this technical service bulletin.
You can expect oil leaks in the following areas:
- Timing case cover
- Around the water pump
- Oil pan
- Coolant thermostats
- Other sealed surface locations
Fuel injector issues on early models
A number of owners of early versions of the M278 engine were met with failed fuel injectors. These piezo electrical fuel injectors were later updated so the issues were not present any more, however, you should keep this in mind if you’re in the market for an early model year engine with original fuel injectors.
💡 Read our guide on fuel injector problems to see all the symptoms of failed fuel injectors.
We also have to point out this TSB by Mercedes that addresses other reasons why the M278 engine could be misfiring.
Occasional long cranking issue
Owners of the M278 have reported random, occasional long cranking issues. This occurs once a week or less, according to an owners’ forum thread. Service technicians have been unable to diagnose the issue, so owners have had to learn to live with it.
Which Mercedes Models Have the M278 Engine?
The M278 engine was featured in the following Mercedes vehicles:
- 2012–2020 SL-Class 500/550
- 2011–2014 CL-Class 500/550
- 2013–2019 GL-Class 500/GLS 500/550 GL 550
- 2013–2014 GL-Class 450
- 2011–2018 CLS-Class 500/550
- 2012–2014 E-Class 500/550
- 2012-2014 ML-Class 500/550
- 2015-2017 S-Class 500/550 Coupé
- 2011–2017 S-Class 500/550
When it comes to modern, technologically advanced V8 engines that have to abide by the numerous CO2 emission standards, the M278 represents a technological marvel. In fact, it was the first Mercedes V8 engine that curved the US Gas Guzzler Tax.
In terms of reliability, we’re a bit disappointed to hear about the valve guide wear issues on the early model years of the engine. It is the only issue that really puts us off. Later versions of the M278 are nothing short of amazing. With most of the early issues dealt with, the M278 proved it’s worth, and we’d have no hesitation to buy one immediately.
As always, if you’re in the market for a used Mercedes with this engine, make sure there’s a clear service history and no signs of neglect. These are sensitive machines, and you don’t want to be on the receiving end of out-of-warranty repair bills.
Is the Mercedes M278 engine reliable?
Yes, we would say that the M278 is a reliable engine, especially when considering its technological complexity. We do recommend staying away from the early model years of this engine, only buy cars with engine number ending in 30 191844 or higher.