We’ve recently dug deep into the V8 Mercedes M278 engine’s problems. Our findings? It’s a fairly reliable engine when you factor in the complex build. Honestly, we’d have no fear buying one of them.
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Today we’re looking at the M278 spiced up cousin, the M157 AMG engine. This engine is a high-performance variant of the M278, which is why they share many of the same components. The M157 is a powerhouse, featuring twin-turbos, a high-pressure direct injection system, and a host of other advanced features.
Basic Mercedes M157 Engine Specs
Mercedes-AMG’s engineering department took the already impressive M278 engine, increased the displacement from its original size to a 5.5 liter engine, and further increased the boost pressure by 15 psi.
But that’s no all, they developed a total of 6 different variants of the engine for different models:
- S-Class & CL
400 kW/536 bhp @ 5,500 rpm with 800 Nm (590 lb ft) of torque or 420 kW/563 bhp @ 5,500 rpm with 900 Nm (664 lb ft) of torque with AMG Performance Package
- 2013-2015 SL-Class
395 kW/530 bhp @ 5,500 rpm or 415 kW/564 bhp @ 5,500 rpm with AMG Performance Package
- 2012-2013 E-Class & CLS
386 kW/518 bhp @ 5,500 rpm with 700 Nm (516 lb ft) of torque or 410 kW/550 bhp @ 5,500 rpm with 800 Nm (590 lb ft) of torque with AMG Performance Package
- 2014 model years and forward
410 kW/550 bhp @ 5,500 rpm with 720 Nm (531 lb ft) of torque or 430 kW/577 bhp @ 5,500 rpm with 800 Nm (590 lb ft) of torque with AMG Performance Package
Common Mercedes M157 Engine Problems
If you’re in a rush, we prepared a short summary of all our findings, remember to check our M278 article as well which covers some basic issues as well.
- Pre-2013 Timing chain problems
- Replacing consumables
- Timing case cover leaks
- Coolant hose leaks
- Cylinder scoring
- Seized low pressure fuel pump
- Engine harness oil pollution
If you’re in the market for an M157 powered AMG, you can’t really be satisfied with this. Here’s more detail about all the these concerns.
M157 Pre-2013 Timing chain problems
Just like the M278, the early 2011 and 2012 model year engines suffer from timing chain tensioner issues. This issue occurs due to oil starvation in the cylinder heads on start-up.
This problem first shows up as 5 second metallic rattling on start-up, which indicates that the timing chain tensioners aren’t doing their job as they should. Mercedes acknowledged this issue in a technical service bulletin.
If ignored, this problem eventually leads to timing chain stretching, meaning that the timing chains themselves need replacing. However, if you catch the issue on time, simply replacing the left and right timing chain tensioner is enough. As a precautionary measure, Mercedes recommend installing non-return check valves.
Check valves inserted into the tensioner oil supply opening in the left and right cylinder head ensures reduced, minimized oil drainage from timing chain tensioners.
Timing case cover leaks
Another issue, acknowledged by Mercedes-Benz in a 2015 technical service bulletin, is oil or coolant leaking from the timing case cover.
The timing case cover features a drain hole that is prone to leaking oil or coolant when the front cover is not sealed correctly to the engine block.
This draining hole is hard to notice, so it helps to know that it’s near to the thermostat piping. These leaks usually start off slow, but they can develop into serious leaks which can starve the engine of either oil or coolant.
Resealing the timing case cover requires patience, care and experience, so it’s best left to professionals that finished the procedure multiple times before.
Coolant hose leaks
The M157 runs a bit hotter than your run-of-the-mill M278 engine. Many owners of these engines also tend to drive this engine a bit harder, which brings additional heat stress to the numerous rubber hoses and seals.
Coolant hoses on the M157 engine are prone to leaking, cracking, or even tearing off completely. As you can imagine, this can lead to rapid coolant loss and even engine damage. Inspecting these hoses, being on the lookout for any coolant leaks, overheating, and noticing suspicious coolant smell is a great way to recognize these issues early on.
Replacing coolant hoses is cheap and simple, so you shouldn’t risk damaging your engine by ignoring these issues.
M157 Cylinder scoring
A number of owners report cylinder scoring issues, just like the owners in this owners’ forum thread. According to some opinions, and the manufacturers of Nanoslide, there are certain levels of cylinder scoring that are considered within tolerances and normal at higher mileage.
A cylinder leak down test and cylinder compression testing is the only way to diagnose if cylinder scoring is in fact causing engine problems. There are rumours that these issues tend to be more exaggerated in areas with colder climates.
Generally speaking, cylinder scoring does tend to appear when running a cold engine under heavy loads, which is why it’s so important to heat up the engine before having a go on the gas pedal.
At the same time, it’s not just Mercedes that experiences problems with cylinder scoring, other car manufacturers like Toyota and Subaru are fighting the same issues with their high boost performance engines.
To make sure these problems don’t cause serious engine damage, it’s critical to warm up the engine before hard use, use high quality engine oil and the highest quality fuel available. Tuned up engines are even more prone to these issues, so be careful when buying one or tuning up your engine!
Seized low pressure fuel pump
A number of owners report failures of the low pressure fuel pump. While this is a nuisance, it’s a fairly simple repair, so it’s not a deal braking issue. If you want to learn more, we advise you watch this rather long but highly informative video.
Engine harness oil pollution
Oil leaking onto electrical components is never ideal, especially when it comes to the engine wiring harness. A Google search for “M157 wiring harness oil leak” reveals this common M157 engine problem which can cause significant damage if left unchecked.
This type of leak is often caused by the camshaft position sensors and camshaft magnets. This issue does not only plague the M157 engine, it’s known to occur on the M278 V8 and the M276 V6 engine as well.
In order to prevent any further damage, it is important to identify this issue as soon as possible, before the oil seeps from the wiring harness into other electrical components. Not only can it seep into other electrical components, the hot dripping oil will eventually eat away at the wire coatings, leaving live wires exposed and short-circuiting. If you are unsure of the source of the leak, it is recommended to consult an experienced mechanic or technician.
Additionally, more information can be found in various owners forum threads, which provides in-depth information on camshaft sensor leaks and how to identify and address them. Therefore, it is essential to be aware of this issue and take the necessary steps to remedy the cause before major electrical issues develop.
Owners of these vehicles report that in comparison to the M278 engine, the M157 engine tends to be a lot harsher on various engine consumables. This includes more frequent spark plug replacements and engine mount replacements.
To avoid problems such as misfiring, rough idling, poor fuel economy and reduced power, owners recommend replacing M157 spark plugs every 50,000 miles (ca. 80,467 km) or sooner if the engine is modified.
Once these cars reach the 80,000 miles (ca. 128,748 km) mark, the majority of them need replacement engine mounts. Many owners tend to ignore the symptoms and just go along with the reduced ride quality. Check this owner’s forum thread to learn more.
Rare M157 Oil Pump Failures
Before we end this list, we want to shine light on an issue that affects not just the M157 engine, but other Mercedes engines without an oil pressure sensor as well. Despite being a rare occurrence, there are instances of oil pumps failing on the M157 engines.
Without an oil pressure gauge to alert the driver to a decrease in engine oil pressure, the engine can fail suddenly and severely before the driver is aware of any issue with the oil pump. This led many drivers to install aftermarket oil pressure sensors as a preventive measure.
Which Mercedes Models Have the M157 Engine?
The following Mercedes vehicles all feature the M157 engine:
- 2012–2016 W212 E 63 AMG
- 2011–2013 W221 S 63 AMG
- 2011–2014 C216 CL 63 AMG
- 2012–2018 C218 CLS 63 AMG
- 2015–2019 W166 AMG GLE 63 (facelift with the GLE designation)
- 2013-2017 W222 S 63 AMG
- 2013–2018 G 63 AMG
- 2012–2015 W166 ML 63 AMG
- 2016–2019 X166 AMG GLS 63 (facelift with the GLS designation)
- 2012–2016 X166 GL 63 AMG
- 2012–2019 R231 SL 63 AMG
To conclude, the Mercedes M157 engine is a powerful, reliable engine when properly maintained. While some engine issues exist, they can be prevented and easily fixed by regular maintenance and inspections. Knowing the common problems associated with the M157 engine and being on the lookout for them will make sure your engine runs smoothly for many miles.
Is Mercedes M157 Engine Reliable?
Yes, considering the complexity of its build, we can safely say that the M157 engine is reliable and without major issues that would cause premature engine failure.
All the common M157 engine problems listed in this article can be easily resolved with regular maintenance and care.
What is the difference between M177 vs M157 engine?
The newer M177 engine and the M157 engine are completely different engines. Based on our research the M177 seems to be significantly more complex and harder to work on as the M157 engine. The M177 features a smaller 4.0 L displacement compared to the 5.5 L displacement of the M157 engine.
Generally speaking, there is more known problems with the M177 engine, which is why the M157 is often reffered to as the more sensible option if you’re buying used. Keep in mind that there isn’t much of a power difference between the two engines, however, the M157 engine still brings up different emotions with its big displacement V8.