Mercedes-Benz OM648 Engine Problems (320 CDI l6-Cylinder)

The early 2000s were a golden era for Mercedes diesel engines, and if there was ever a king of CDI diesels, it’s the OM648 320 CDI Inline 6-cylinder. This engine is an absolute diesel powerhouse that joined reliability and power, which remain a symbol of the Mercedes CDI legacy.

That’s enough praise for today. Here’s more about the actual engine.

The OM648 is a 3.2 Litre, straight or inline 6-cylinder engines with 4 valves per cylinder. It features a common rail fuel injection system. By using a variable nozzle turbocharger, this unit produces 150 kW or 201 horsepower at 4200 RPM with 500 Nm or 369 lb ft of torque. You already know how this must feel when it’s pulling, right? Diesel power indeed.

👆 Despite being an amazing engine, there are a few things you need to be aware of in terms of problems, especially with high mileage vehicles.

Common Mercedes OM648 Engine Problems

Let’s first do a favour to all of you that are in a rush. Here is a quick summary of the contents.

Common OM648 engine problems:

  • Glow plug failures and glow plug controller failure
  • Oil leaks
  • EGR valve problems
  • Oil separator blockages
  • Fuel injector washer failure and “CDI Black Death”
  • Fuel pressure control valve failure
  • Problems with the variable intake manifold flaps
  • High mileage issues

That is it, the majority of these engine problems are easily resolved or even completely prevented by adhering to the manufacturer’s maintenance plan and some additional care. We’ll tell you all about as we continue.

Pre-Glow System Problems

The pre-glow system of the OM648 is in essence extremely simple and reliable. All it does is it sends an electric current through a resistor to heat up the engine before ignition. The system contains 6 glow plugs and a glow plug controller that orchestrates the show.

Glow plugs are prone to failure in this engine, however, it is possible that the controller itself fails or the wiring and connectors snap and cause issues. Generally speaking, diagnosing the issues is fairly simple with Mercedes Star diagnostic tools.

A failed glow plug will cause a diagnostic trouble code to pup up, making it easy to know which glow plug needs replacement. Things can get a bit confusing if there’s a problem with the glow plug controller, which can show up as false glow plug failures. Testing the glow plug separately is a must.

P.S.: There are two types of glow plugs for the OM648 engine. The 4.4V pencil style glow plug, and the 12V Bosch glow plugs, which are only suitable for EU spec engines! Beware!

Oil leaks

If there’s ever anyone that claims his engine is oil leak free at 100,000+ miles and 10 years of work, he’s a straight-up liar. Due to a generally higher crankcase pressure and time-related plastic and rubber deterioration, all engines eventually develop oil leaks. Including the OM648 diesel.

These are the most common oil leaks locations of the OM648 engine:

  • Valve cover gasket
  • Oil separator oil leaks
  • EGR valve oil leaks
  • Turbo return line leaks
  • Oil filter gasket leaks

However, It’s important to note that the OM648 engine does not suffer from the same oil cooler leak like the V6 OM642 engine.

Crankcase position sensor

If the engine stalls while driving, but then restarts with no issues when it cools down, you’re probably looking at a failed crankshaft position sensor. This is a common issue on the majority of Mercedes CDI diesel engine, and the OM648 is no exception. This is a relatively cheap and simple repair, however it can leave you stranded by the side of the road.

Fuel pressure control valve failure

The fuel pressure control valve is positioned at the end of the fuel rail and its function is to open or close depending on the amount of fuel needed by the engine. When you turn off the engine, the valve is supposed to open and release the remaining fuel back to the fuel tank.

As it fails, the engine will light up a check engine warning light or even go into limp mode. Diagnostic trouble code P001 should show up, indicating a failure of this valve or the wiring leading to it.

EGR valve problems

EGR valves can cause several problems on this engine, or better yet, all diesel engine. The trusty OM648 is no different.

  1. The EGR valve can get blocked up by a mixture of carbon and oil deposits. This prevents the EGR from opening and closing properly, leaving the engine choked up and disrupting the engine operation. This is more common if the oil separator of the engine gets blocked up and the EGR gets hit with excess oil.
  2. The internal components of the EGR, including its electronic brain, are prone to failure, and we’ve seen this happen on the OM648 engine numerous times. Many owners decide to get rid of the EGR completely.

Fuel injector washer problems and “Black Death”

Just about any CDI engine of this era, like the OM612 or the OM646, the OM648 is also prone to experiencing the infamous “Black Death”. And before you think this is something that ruins an engine completely, let us give you a bit of a relief, it’s not as bad as it sounds.

Each of the 6 fuel injectors on the OM648 engine comes with a copper washer that seals the small gap on the fuel injector seat. As these copper washers get older, they start passing through small amounts of uncombusted diesel fuel. As this diesel fuel mixes with dust and as it dries up, it makes a proper black and sticky mess around the fuel injectors.

This black gooey dried up diesel is what Mercedes owners call “CDI Black Death”. Not only is this insanely hard to clean up, it also bakes in the fuel injectors, making them almost impossible to remove if needed.

Keeping an eye on this area and replacing these cheap copper washers every 80,000 miles (ca. 128,748 km) is the only solution. Simple!

Problems with the variable intake manifold flaps

Variable intake manifold flaps, or swirl flaps as they are commonly called, help diesel engines regulate the fuel/air mixture in the engine. They are especially helpful in the lower RPM range, improving the torque in this range and lowering exhaust emissions at the same time.

While they may sound good, they’re almost always perceived as a nuisance. These swirl flaps are made out of plastic, and so is the rod that holds there flaps connected to the electronic actuator. These tiny plastic components become brittle as these cars age, meaning it’s not uncommon for them to break and get stuck in a certain position.

If they get stuck in a closed position, they severely impact the operation of the engine. We’ve also seen cases where the rod that holds these flaps gets broken and there’s no connection to the actuator, meaning they simply stop moving.

Another problem is carbon deposits. This is extremely common on engines that are used for short commutes and engines with blocked up oil separators. Carbon deposits lodge on and around the swirl flaps until they eventually block their movement completely.

Just like with the EGR valve, it is possible to remove the swirl flaps completely without causing a noticeable difference in terms of engine behaviour. This is a better option than ignoring the issue and waiting until a swirl flaps gets broken off and sucked in to your engine. You don’t want to experience that, trust us.

High mileage OM648 engine problems

Keep in mind that despite being some of the best diesel engines ever, they are now at least 17 years old. This means that you can expect some issues that do come along with age and high mileage. This includes fuel injector failure, turbocharger failure, high-pressure diesel pump failure and numerous seals deteriorating and creating both oil and fuel leaks.


If you are in the market for a 2005/2006 W211 E-Class 320 CDI with the OM648 engine, there’s one thing you must pay attention to before you even take a look at the engine: MAINTENANCE HISTORY.

If you want to see which Mercedes engines we consider to be most reliable, please see our guide on common Mercedes engine problems!

Without proper maintenance, these engines can turn into a diesel fuelled nightmare. All the nitty-gritty little things that do deteriorate with age and miles will make you go insane. Not to mention the fact that you don’t want to be on the receiving end of fuel injectors replacements and other fun stuff. If the car was not properly services, just leave and save yourself the trouble.

The OM648 engine was present in the following cars:

- 2005 and 2006 W211 E-Class 320 CDI (201 hp)

- 2005 and 2006 W211 E-Class 280 CDI (177 hp)

- 2003 – 2005 W220 S-Class 320 CDI (Facelift, Mopf)


What is the weight of the OM648 engine?

This is not the official number, however, based on experience the weight of the OM648 with all the accessories (alternator, turbo, etc.) is around 550 lbs or 250 kilograms.

Is the Mercedes OM648 engine reliable?

Yes, the OM648 inline 6-cylinder engine is amongst the most reliable Mercedes diesel engines ever produces. These engines will easily cover 200,000 miles (ca. 321,869 km) with just regular maintenance and care.

Which is better, the newer V6 OM642 engine or the older l6 OM648 engine?

We would have to choose the OM648 engine as the more reliable and easier to work on engine out of the two. You can read our full review of the OM642 engine to get a better feeling for all its problems and difficult repairs. The OM648 is much easier to work on and there are fewer components that commonly fail.

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