Mercedes M103 engine

Ultimate Mercedes-Benz M103 Engine Guide

Chandler Stark

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Chandler is an automotive expert with over a decade of experience working on and modifying cars. A couple of his favorites were his heavily modded 2016 Subaru WRX and his current 2020 VW Golf GTI. He’s also a big fan of American Muscle and automotive history. Chandler’s passion and knowledge of the automotive industry help him deliver high-quality, insightful content to TuningPro readers.

Mercedes-Benz produced the M103 E26 and E30 inline-six motors from 1985–1996, and they were extremely successful. The M103s powered everything from the high performance 300SL to the G-Wagon, and came in 2.6 or 3.0 liters in displacement. Over the years, the M103 engine proved itself to be very reliable and solid, with only a few common problems and issues. In addition, it has proven as a favorite to turbocharge, and there are many M103 turbos out there today. Let’s take a look at one of Mercedes-Benz stoutest motors, the inline-six M103 engine. 

Make sure to check out our other Mercedes-Benz content, including our C63 vs E63 AMG comparison, C43 vs C63 AMG Comparison, and our M156 V8 engine guide

Mercedes M103 engine
Mercedes M103 engine (Credit: Jo664/Wikimedia)

Mercedes-Benz 3.0 & 2.6 Engine History

Mercedes-Benz introduced the straight or inline-six M103 E26 and E30 engines in the 1985 model year. They succeeded the M123 (E26) and M110 (E30) inline-six engines, before themselves being taken over by the M104 in the mid-1990s. Both the M103s produced relatively average horsepower and torque numbers, but gained a good reputation for being easy to maintain and very reliable. 

Mercedes used the M103 engine in many different vehicles. The smaller E26 made 160-165 horsepower and 162-168 lb-ft of torque, and Mercedes put it inside the 190E 2.6, 260E, 300E 2.6, and 260SE. They used the E26 in the more luxurious models, like the 190E 2.6 Sportline.

For the larger E30, Mercedes used it in a much bigger variety of vehicles, including roadsters, coupes, and convertibles. Most prominent were the 300SL roadster and G300/300GE “G-Wagons.” The M130 E30 made a little more horsepower and torque than the E26, and Mercedes rated the E30 at 180-190 horsepower and 188-192 lb-ft of torque. 

Even though it had a relatively limited run of just 11 years, the M103 has proven a very popular engine with enthusiasts. One of the most common aftermarket setups on the M103 is to run a turbocharger and really crank up the boost. In this example from Raceosd, they swapped an M103 E30 into a Nissan Laurel, then fitted a Holset Hx52 turbocharger to it and turned it into a drift monster. There are many more examples of boosted M103 engines, and we’ll get into those further below. 

Mercedes M103 Engine Specs

EngineM103 E26M103 E30
Engine FamilyMercedes-Benz M103Mercedes-Benz M103
Model Years1985–19931985–1996
Displacement2.6 liters (2,597 cid)3.0 liters (2,960 cid)
AspirationNaturally AspiratedNaturally Aspirated
Compression Ratio9.2:19.2:1
Bore and Stroke82.9 mm x 80.2 mm 88.5 mm x 80.2 mm
Valve Train12 (2 valve/cyl)12 (2 valve/cyl)
Variable Valve TimingNoNo
Fuel SystemBosch KE-Jetronic (CIS-E)Bosch KE-Jetronic (CIS-E)
Head MaterialAluminumAluminum
Block MaterialCast IronCast Iron
Horsepower Output180-190 horsepower160-165 horsepower
Torque Output188-192 lb-ft162-168 lb-ft

Mercedes 3.0 & 2.0 Powered Vehicles

Mercedes-Benz 300SL (R107) (Credit: Granada/Wikimedia)

The Mercedes M103 E26 appeared in the following vehicles:

  • 1985–1993 Mercedes-Benz 190E 2.6 (W201)
  • 1985–1993 Mercedes-Benz 260E (W124)
  • 1986–1993 Mercedes-Benz 300E 2.6 (W124)
  • 1985–1991 Mercedes-Benz 260SE (W126)

The Mercedes M103 E30 appeared in the following vehicles:

  • 1985–1993 Mercedes-Benz 300E (W124)
  • 1988–1989 Mercedes-Benz 300CE (W124)
  • 1986–1993 Mercedes-Benz 300TE (W124)
  • 1985–1991 Mercedes-Benz 300SE (W126)
  • 1985–1991 Mercedes-Benz 300SEL (W126)
  • 1985–1989 Mercedes-Benz 300SL (R107/C107)
  • 1989–1993 Mercedes-Benz 300SL (R129)
  • 1990–1996 Mercedes-Benz 300GE; G300 (W463)

Mercedes M103 Engine Design Basics

Both the M103 E26 and E30 are very similar motors, most distinguishable by their displacement. The E26 displaces 2.6 liters, while the E30 displaces 3.0 liters. The M103 E30 is essentially a bored version of the E26, with a 6.6 mm larger bore and identical stroke. Mercedes used cast iron for the engine blocks and aluminum for the cylinder heads. This allows the iron block to take a ton of stress while aiding flow and shedding weight with aluminum heads. The cylinder heads use a crossflow design and the engine has a 15° right inclination. 

When Mercedes developed the M103 engine in the 1980s, emissions regulations were beginning to ramp up. As a result, they gave some versions of the engine a catalytic converter but not others. The introduction of a catalytic converter dropped performance by roughly 5-10 horsepower and 5-6 lb-ft of torque. Mercedes uses two exhaust manifolds for the M103, with each covering three cylinders before merging downstream. 

For the valve train, Mercedes used a single-overhead camshaft (SOHC) with hydraulic lifters. There are two valves per cylinder for 12-valves total, and the camshaft runs off a timing chain. Going to SOHC was somewhat of a reversion for Mercedes, as the outgoing M110 was a dual-overhead camshaft (DOHC) engine. The use of a single camshaft helped to simplify things, aiding overall reliability and decreasing maintenance. 

For fueling, Mercedes used Bosch’s KE-Jetronic mechanical fuel injection system, also known as the “Continuous Injection System – Electronic” CIS-E. This was an early version of mechanical fuel injection that used an electronic control unit to regulate the fuel delivery. While not nearly as efficient as EFI today, it was a solid improvement over carburetors and the purely mechanical K-Jetronic system. 

Mercedes M103 Engine Reliability and Problems

As I mentioned, the Mercedes M103 engine has developed a pretty good reputation for being reliable. While most of these engines are no longer on the road, as Mercedes hasn’t built them since the mid-1990s, during their time they were very durable and could last well over 150,000 miles with proper maintenance. There have been a lot of 200,000+ mile versions as well, which is pretty good for a mid-1980s designed engine. 

However, the engine is far from bulletproof and perfect. The M103 engine suffers from many small common problems, but luckily nothing extremely catastrophic. A lot of the problems had to deal with the valve train, and they were especially prevalent on earlier versions. Below, I’ll go over the four most common problems with the M103 engine (E26 & E30) that owners have experienced over the years. 

Most Common Mercedes M103 Problems

The four most common Mercedes M103 E26 and E30 engine problems are:

  • KE-Jetronic Problems and Clogged Injectors
  • Worn Camshaft Lobes
  • Oil Leaks
  • Bad Valve Stems

The first problem with the Mercedes M103 engines has to do with clogged fuel injectors. The KE-Jetronic was one of the earliest mechanical fuel injection systems to be controlled by an ECU, and it was unfortunately not the most reliable. One of the most widespread issues with the system is that the injectors would become dirty and clogged. This is not really unique to the M103, but is a problem with many KE-Jetronic systems in many vehicles. Generally, the best way to get rid of the issue is to clean the injectors.

Another big problem with the system is problems with communication between the ECU and the injectors. Without communication between the two the engine will struggle to get enough fuel and can die and struggle to run. Additionally, the idle speed-control motor can also fail. This controls the engine idle speed and can result in misfiring when it fails as well as unsteady idle.

Mercedes M103 Worn Camshaft Lobes

Next up is worn camshaft lobes. The problem has to do with Mercedes using tin-plating in them, which was supposed to help with break-in with the rocker arms. However, it often caused worn cam lobes. As a result, starting in 1989 Mercedes revised the camshaft/rocker arm design. The new design uses a chilled cast iron camshaft and rocker arms with carbide inserts.

This post from BenzWorld shows Mercedes official literature around the replacement. It should be easy to tell whether or not an M103 engine has had the camshaft replaced or not. On engines that have had the replacement, Mercedes instructed techs to put an “S” mark on the exhaust side of the cylinder head right next to the headers. 

Mercedes 3.0 & 2.6 Other Problems

One problem that has seemingly plagued the Mercedes M103 engine is oil leaks. There are four main areas that develop oil leaks, the oil filler cap, valve cover gasket, timing chain cover, and head gasket near the rear right. The oil cap is plastic and prone to cracking which can cause oil leaking, especially on the highway at higher speeds. The valve cover gasket also cracks quite frequently and can lead to oil leaking onto the manifold. 

The timing chain cover also is a common place for leaks, as is the head gasket near the rear right. In both cases, oil is usually visible leaking out fairly quickly, and it can happen as soon as 50,000 miles.  The final issue I’ll go over are bad valve stems and guides. Both of these can wear and fail over time, leading to oil leaks. This is typically a problem past 100,000 miles, but can show up earlier on some engines. 

Overall, the M103 engine is reliable and durable, but it is definitely not without its problems. However, they are relatively minor and mainly relate to the KE-Jetronic system, valve train, and oil leaking, and are not really catastrophic. 

How to Turbo Your M103

M103 turbo
Turbocharged M103 Engine (Credit: The Benz House/YouTube)

It might sound a bit surprising at first, but the M103 is a great engine to turbocharge. The stock bottom end is stout and can support 8 PSI of boost fairly easily, and the engine bay is large enough to accommodate a turbo. 

This YouTube video from The Benz House gives a really good step-by-step breakdown about how to turbo both the 2.6 or 3.0 versions of the M103. The main things you’ll have to figure out are the turbo manifold to replace the naturally aspirated stock one, oil feed and drain lines to cool the turbo, the turbocharger itself, and an exhaust solution. 

From there, depending on your turbo and how much boost you want to run, you can add things like an intercooler, boost gauge, blow-off-valve, external wastegate, etc.. One important thing to keep in mind is that the bottom of the oil pan is too thin to be tapped. It will compromise the pan and lead to leaks, so you need to tap the side instead.

On a different build from Driftin’ Rust, they slapped a turbo onto a E26 out of a 190E sedan. The entire build looks beautiful, and they were able to extract a healthy 385 horsepower and 374 lb-ft of torque. That was good for around 320 wheel-horsepower, which really makes the 190E feel like a rocket. 

Mercedes M103 Engine Summary

Mercedes-Benz’s M103 engine is an inline-six that comes in two different flavors, the 2.6 liter E26 and 3.0 liter E30. Both of them are known for being very reliable and dependable, and they were very popular in the late-’80s and early-’90s. Its most popular applications were probably the 190E, 300E, 300SL roadster, 300GE, and G300 G-Wagons. 

Depending on the specific model, year, and whether or not Mercedes equipped it with a catalytic converter, the E26 made 160-165 horsepower and 162-168 lb-ft of torque. The larger E30 produced 180-190 horsepower and 188-192 lb-ft of torque. 

A few of the most common problems associated with the M103 engine are problems with the KE-Jetronic fuel injection system, worn camshaft lobes (1985-1988 only), various oil leaks, and bad valve stems and guides. Overall, the engine is very stout, but it is not perfect nor bulletproof. 

Mercedes M103 Engine FAQ

What engine does the 300E have?

The Mercedes-Benz 300E used both the 2.6 liter and 3.0 liter versions of the M103 engine.

Is the Mercedes M103 a good engine?

Yes, the Mercedes M103 is a solid and reliable inline-six engine produced from 1985-1996. While it did not crack 200 horsepower or torque, it was durable and very popular. For even more power, adding a turbocharger is a frequent upgrade. 

Is the Mercedes M103 turbocharged?

No, Mercedes-Benz left the M103 engine naturally aspirated from the factory. However, aftermarket M103 turbo kits are very popular, and with a built engine you make gobs of horsepower and torque.

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