Mercedes-Benz S-Class (W221: 2005-2013)

Airmatic suspension failure

The Airmatic air suspension is part of standard equipment unless the car has the optional Active Body Control suspension (ABC).

The ABC is a hydro-pneumatic suspension, and you can check if any particular W221 has the ABC by decoding the VIN number. The option code for ABC is 487. Also, all V12 and AMG models have ABC as standard instead of Airmatic.

Air suspension is commonly used in trucks and buses because vehicles with air suspension can maintain correct height and level despite the load. Have you ever seen a bus tilt and soon after level out when the passengers get off at the bus stop? That’s air suspension reacting to new conditions.

The air suspension in Mercedes-Benz cars can react in the same way, and it’s capable of changing the ride height and damping stiffness. The Airmatic ride quality is indeed very good.

Instead of traditional coil springs and dampers, the Mercedes-Benz W221 is fitted with four Airmatic struts, which are air springs and dampers integrated into single units.

Here’s how the Airmatic is different from traditional spring suspension:

 

Traditional suspension

• four springs – they support the car and allow suspension movement (suspension travel)

• four shock absorbers – they dampen the spring oscillations and prevent excessive suspension movement (bouncing)

Airmatic

• air pump (generates air pressure for the entire system)

• air reservoir (stores compressed air)

• ride height sensors (they measure the ride height)

• valve body (distributes air to the air springs)

• four struts (shock absorber and air spring in one)

• air lines that connect all of the above

In my opinion, the Airmatic is not unreliable, but it is significantly more complex than traditional suspension.

Some people got burned by trying to fix their Airmatic suspensions at a Mercedes-Benz dealership. The prices at the dealership can be high when it comes to sorting out Airmatic issues, and the typical dealership approach is to keep replacing components until the problem is fixed, without actually trying to locate the exact failure point.

The parts can be expensive but most things, like struts or the air pump, are easy to replace. In my opinion, you will be happy with the Airmatic suspension provided that you learn how the system works, and how to diagnose problems yourself, so you don’t get ripped off when something starts leaking or stops working.

Also, be aware that if some critical components in the Airmatic fail, for example, the air pump, the car will become undrivable.

There are multiple options available on the market today that can help you save money servicing the Airmatic suspension:

• compressor repair kits

• aftermarket parts (Arnott has a good reputation)

• reconditioned parts

• coil spring conversion kits (not recommended)

 

When checking the Airmatic suspension, look out for the following symptoms of malfunction:

• air compressor running too loud (worn out)

• compressor turning on too often (air leak)

• Airmatic fault messages when the car is cornering or braking hard

• the car taking too long to raise from lowest position (air leak or worn out air pump)

• cracked dust covers on the struts (any damage there will massively accelerate the wear by allowing dirt and moisture in)

• oil leaks at the bottom of the struts (leaking shock absorbers)

• check that the car responds quickly to manually adjusting the height with no warning messages

• car dropping too quickly – on one corner or more than one

Check the car after it has been standing still overnight – it should not be on the ground. A little bit of pressure may escape, but the car should stay up and level.

A system in good condition should be able to hold air for a couple days easily. If it drops in a couple hours, something is worn out or there is a significant leak somewhere. If the car drops too low to drive, “Car too low” message will appear on the dashboard – it’s a bad sign.

Driving with air leaks will prematurely kill the air pump because then it has to turn on more often to replace the escaping air. If you are going to buy a car that has a leaking system, expect the air pump to be on its way out too.

ABC suspension failure

The Mercedes-Benz W221 may be equipped with hydro-pneumatic suspension called Active Body Control (ABC). The ABC is standard equipment in the S 600 L and AMG models (S 63 AMG, S 65 L AMG). It was an optional extra in other models, except cars with all-wheel-drive (4MATIC).

When it works, the ABC exceeds what standard coil spring suspension can do. It is also better than the Airmatic suspension when it comes to handling. The ABC suspension is very comfortable, and at the same time, there is very little body roll when cornering or braking (hence the word “Active” in the name).

The system can adjust the suspension position up to 5 times per second (5 hertz). Also, it is self-levelling, which means that it will maintain the ride height despite the load in the car.

If you think the Airmatic suspension is complex, never buy a car with the ABC suspension. The ABC is a lot more advanced than the Airmatic, let alone a standard suspension. Thus, you can expect the maintenance costs to be accordingly higher.

Here’s how the ABC is different from coil spring suspension:

 

Traditional suspension

• four springs – they support the car and allow suspension movement (suspension travel)

• four shock absorbers – they dampen the spring oscillations and prevent excessive suspension movement (bouncing)

 

Active Body Control

• hydraulic piston pump (generates pressure for the entire system)

• oil reservoir (stores hydraulic oil)

• two valve blocks – one per car axle

• four struts (a strut is coil spring, shock absorber and hydraulic actuator integrated into one unit)

• two accumulators (nitrogen spheres) – one per car axle

• pulsation damper and return accumulator (two more nitrogen spheres)

• ECU (the brain of the system)

• hydraulic hoses that connect all of the above

• over a dozen electronic sensors

 

Let me try to briefly explain how this system works:

The suspension struts contain a regular coil spring and a shock absorber – just like in a typical car. Additionally, there is a hydraulic cylinder in each strut that can change the height of the suspension (each wheel individually).

The system is powered by a hydraulic pump, monitored by sensors and controlled using solenoid valves. This allows for real-time adjustments to ride height and vehicle body lean, for example during cornering or braking.

There are four pressure accumulators that have a membrane inside and pressurized nitrogen gas behind it. The membrane is flexible. On one side of the membrane there is gas and on the other, there is hydraulic fluid. The membrane can deflect to store energy (pressure) because nitrogen gas is compressible.

There are two accumulators that store the pressure to feed the system as explained above. The other two are there to dampen pressure spikes in the system. The hydraulic pump is a piston pump so it generates pressure pulses with every stroke. The pulsation damper, which is just another nitrogen accumulator, evens out these pulses (membrane deflecting). The return accumulator does a similar job after the oil leaves the struts.

The ABC is really a conventional coil spring suspension with added hydraulic levelling. It’s the coil spring inside the struts that does the job of absorbing the shocks from the road, and the hydraulic system is there just to adjust the car body lean and height.

If you compare the ABC to other hydro-pneumatic suspension setups:

• In the Mercedes-Benz R129 SL-Class, the ADS suspension has conventional coil springs but no conventional shock absorbers. The hydraulic accumulators serve the function of the shock absorbers. The gas behind the membrane inside the nitrogen sphere is compressible and can absorb the shocks from the road (membrane deflects).

• In older Citroen cars, which were famous for their hydro-pneumatic suspensions, there is a nitrogen sphere at each wheel that serves the function of both the shock absorber and the spring. You won’t find any coil springs in Citroen cars with hydro-pneumatic suspension, and when a nitrogen sphere fails, there is virtually no damping in a Citroen car.

Similarly, the damping is lost when an accumulator fails in the R129 SL-Class with the ADS suspension. Citroen just took it one step further by getting rid of the coil springs altogether.

I hope I made this clear enough…

 

Here’s a video of an accumulator taken apart. It’s not from an ABC car, but its construction is the same.

 

Anyway, the key points are:

• The ABC is a complex, high-performance system and it will be more expensive to maintain than standard suspension or even the Airmatic.

• The ABC was improved in 2007. If the high maintenance cost doesn’t scare you, and you still want a car with the ABC suspension, look for a 2008 model or newer.

• The age of the vehicle is important – the rubber hoses and seals age, which increases the likelihood of failure. The hydraulic hoses deteriorate and have a finite lifespan (they can burst). If a hose blows, the suspension shuts down. If you don’t stop on time, the hydraulic pump may die from running dry. As you can see, it’s important to keep the hydraulic hoses in good condition.

• The nitrogen spheres last around 60k miles or 10 years, whichever comes first. They are service items and will need to be replaced regularly. Even if you don’t drive the car, the nitrogen gas will eventually escape, just like air escapes from a seemingly airtight balloon.

• The hydraulic pump will need replacing at some point (very expensive). The pump has two sections – one that powers the suspension, the other runs the power steering.

• There are some options available on the market today that can help you save money servicing the ABC suspension: repair kits, aftermarket parts (Arnott has a good reputation) and reconditioned parts. Please be aware that it’s still going to be expensive…

 

If you’re still convinced that you want an S-Class car with the ABC suspension, here’s what to look out for:

• Car dropping to quickly (on one corner or more than one) – visually check the car after it has been standing still for a couple days – the chassis should not be on the ground. According to Mercedes-Benz, a system in good working order should be able to hold the oil pressure at least for one week. If the car drops earlier, something is wrong with the valve block or there is a leak somewhere (probably a strut).

• Any ABC warning messages, appearing for a couple seconds when cornering, braking or after hitting a pothole, are bad signs. Before buying a car with the ABC, I recommend taking it to a professional to check the diagnostics log for any stored errors.

• Check the hydraulic fluid – it should be green. If it’s dark brown, black or the fluid level is too low, it means the car has been neglected or maintained poorly. Do not buy such a vehicle. Running the hydraulic pump without oil will kill it very quickly. The dark colour of the fluid means that it is dirty and full of abrasive particles – the entire hydraulic system could be worn out.

• Also, do not buy a car without maintenance history (including filter and hydraulic fluid replacements).

• Check the struts and the underside of the car for any oil leaks. Watch out for cracked dust covers on the struts (any damage there will accelerate the wear by allowing dirt and moisture in).

• Check that the car responds correctly, with no warning messages, to manually adjusting the ride height.

• Humming or howling noise between idle and 2000 RPM means that the pulsation damper has failed.

• The ride should be smooth and at the same time, there should be very little body lean when cornering or braking. If the car feels bouncy or hard, something is wrong.

M272 V6 & M273 V8 engine – soft balance shaft gears

The M272 V6 engines are equipped with a balance shaft. A balance shaft is an eccentric weighted shaft, which is used to eliminate engine vibration. It is driven by the timing chain as it needs to in sync with the engine pistons. A balance shaft is needed to make a V6 engine smooth because they are inherently unbalanced.

The M272 engines manufactured between 2004 and 2008 were fitted with balance shaft gears and idler gears that can wear out prematurely. The M273 engines don’t have balance shafts but still have the defective idler gears.

As these gears wear, the engine timing is altered due to increased slack in the timing chain. The first indication of a problem is the “Check Engine” light and P0017 or P0016 error codes. As the sprockets continue to wear, the engine will develop a rattle from the loose timing chain. All the while, more and more metal is being ground away from the gears.

If not fixed, one of the gears may fail eventually and take the engine out (valves hitting the pistons).

It is a problem that you should not ignore. The fix is to replace the gears with updated ones, which don’t wear out. It is very labour intensive, which makes it a very expensive repair.

Not all M272 and M273 engines made before 2009 are affected. Engine serial numbers below have updated gears, which are free from the problem:

• M272 engines with serial numbers higher than 2729 . . 30 468993

• M273 engines with serial numbers higher than 2739 . . 30 088611

Some engines seem to last despite having the unlucky gears, but I would not take the risk. In my opinion, affected engines are fine only as long as there is proof that the faulty gears (along with a number of other parts) have been replaced, and the engine is running well after the repair.

M272 and M273 engine – variable intake manifold

The intake manifold in these engines is very complex. It’s called a Variable (Length) Intake Manifold (VIM or VLIM), and it consists of three vacuum actuators, multiple levers, tumble flaps and valves that adjust the length of the intake tract.

It’s not a very reliable design with lots of plastic parts that can fail. When there’s a problem with the intake manifold, the engine performance is reduced (increased fuel consumption, loss of power, poor idle), and the “Check Engine” light may come on.

The typical cause of failure is increased friction in the mechanism from the accumulation of oil and carbon deposits in the intake manifold. Additionally, over the years the plastic parts in the mechanism become weaker and more brittle.

A couple of years ago you had to buy an entire manifold assembly to fix this problem (£800 at the dealership). Luckily, there are repair kits available on the market these days, which makes this a much smaller issue.

If you are curious how carbon deposits and oil appear in the engine intake manifold – they come from the Exhaust Recirculation Valve (EGR) and Crankcase Ventilation System (CVS). These are standard systems used in virtually all road cars.

M156 engine – head bolt corrosion (S 63 AMG)

There have been cases of head bolt failures due to corrosion in the M156 engines. The head bolts are the fasteners that hold the cylinder heads in place. They are in constant tension to create a seal between the cylinder head and the engine block (engine block – head gasket – cylinder head).

When the bolts start corroding, the clamping force may be reduced, which could allow coolant to leak into the combustion chamber or mix with the engine oil. A typical symptom is dropping coolant level while the oil level is rising. If left untreated, the oil/coolant mix will eventually turn to sludge.

Coolant entering the combustion chamber can also cause increased smoking and a rough idle.

If any of the bolts holding the cylinder heads break off, a severe head gasket leak may appear. If the leak is large enough, the coolant entering the combustion chamber has the potential to hydrolock the engine.

Hydrolock is a situation when the piston tries to compress the incompressible liquid. The liquid is not going to give in but something in the engine will – usually the connecting rods. To illustrate, here’s a photograph of a bent connecting rod after hydrolock (not from a Mercedes-Benz):

 

 

Mechanical engine damage is the worst case scenario, usually, the cars start losing coolant before failing like this.

The head bolts were eventually updated by AMG around 06.2010. All engines made before this date are affected – engines with serial numbers beyond 569xx 60 060658 have the updated bolts. All cars produced before 2010/2011 may develop this problem. I believe that 2011 cars may be affected too.

The M156 engine was replaced with the M157 unit in 2011. Therefore, I recommend looking for an S 63 AMG with the newer M157 engine.

Alternatively, you can buy the older S63 with the M156 as long as you can get a warranty that covers potential engine meltdown, or replace the head bolts preemptively. The percentage of engines that fail is small, as I see it, but it’s not worth the risk to me.

M156 engine – camshaft wear (S 63 AMG)

Another weak point of the M156 engines are the camshafts, which are susceptible to premature wear. A typical symptom of a worn camshaft is ticking noise, initially only when the car is cold started. Over time, it becomes more persistent as the wear progresses.

I believe that this is caused by a combination of too soft camshaft lobes and too hard cam followers. The camshaft lobes are quite sharp (small nose radius), which may contribute to the problem. It’s not only the camshafts that can wear. Once a camshaft is scored and loses its profile, the cam followers can wear too – in severe cases to the point of breaking a hole in the centre.

If you decide to get one of these cars, listen for a ticking noise for the first couple seconds after a cold start. It’s the same procedure as checking for timing chain issues, so look out for any chain rattle too. If you hear a ticking noise, it’s likely the camshaft. If you can hear a rattle which disappears after a couple seconds, it’s most likely the timing chain. Both types of noises are a bad sign.

Also, regular oil changes are very important for camshaft life. You should avoid cars with incomplete service history.

Here’s what a worn-out camshaft in the M156 engine looks like:

M275 engine maintenance costs

The M275 is based on the M137 – a naturally aspirated V12 engine that had some issues. For example, replacing a leaking oil cooler in the M137 required the cylinder heads to be removed, which made it a very expensive job.

The M275 is better in this regard, but it’s still a huge engine that fills out the engine bay completely, so replacing parts can be a nightmare. Something as simple as changing the motor mounts may require the engine to be removed from the car. It is possible to do it with the engine in the car if you get creative, but my point is that even small repairs can be pretty expensive.

There are 24 spark plugs and two ignition coil packs in this engine – one coil pack per cylinder bank. Each coil pack is an integrated unit that provides ignition for 6 cylinders. Imagine that you’re trying to fix a rough idle or misfire. Replacing the spark plugs alone will cost £500 or more because it’s not as straightforward as in an average engine and because there is 24 of them.

The coil packs fail occasionally and sometimes when there’s a misfire, replacing the spark plugs alone might not be enough. So you’ve already spent a lot of money on the spark plugs, and now you’ll try replacing the coil packs…

… a single coil pack is around £800.

If you compare this to the V6 M272 engine – a Delphi (OEM) ignition coil for the V6 is £40. There is one coil pack per cylinder and you can replace them individually, so it’s unlikely that you’ll need all six at the same time. I hope this gives you an idea what you’re getting yourself into when you buy a second-hand V12 Mercedes-Benz W221.

M276, M278 and M157 engines – timing chain rattle

When these engines first came out, some of them developed a timing chain rattle during a cold start. The rattling noise is a result of the chain tensioners not applying enough tension until oil pressure builds up (a couple seconds after cold start).

Mercedes-Benz was aware of the problem and came up with a fix documented in the Technical Bulletin LI05.10-P-056435. The repair involves fitting non-return valves (check valves) into the oil passages that supply oil to the two affected timing chain tensioners.

The check valves prevent the engine oil from draining from the chain tensioners when the engine is off.

The second step of the repair is to fit updated timing chain tensioners. This step is dependent on whether the affected car has the original tensioner design or already the updated one. The chain tensioners had been updated before check-valves came into play, so not all rattling engines need new tensioners.

The engine serial numbers below are affected (no check-valves or older timing chain tensioners used):

• M276.8 engines up to number 276 8xx 30 001281

• M276.9 engines up to number 276 9xx 30 406603 (February 2013)

• M278 engines up to number 278 9xx 30 103675 (February 2013)

• M157 engines up to number 157 9xx 60 022333 (December 2012)

Unfortunately, the production of the Mercedes-Benz W221 stopped soon after the check valves were introduced. This means that nearly all (if not all) Mercedes-Benz W221 models with the M276, M278 and M157 engines are affected.

If you are in the market to buy a Mercedes-Benz W221 with one of these direct-injection engines, definitely check if the engine doesn’t rattle during a cold start. I would even go a step further and call the dealership to find out if the particular car you are looking to buy has had the check valves fitted.

Here’s what the timing chain rattle sounds like:

 

I don’t recommend buying the car if it hasn’t got the check valves and updated timing chain tensioners or if you encounter noise during a cold start. It would be quite expensive to pay for this update out of your own pocket.

OM651 engine – premature timing chain wear

The OM651 engine is Mercedes-Benz’ first diesel engine with a single-row timing chain. Up until this point, all M-B diesel engines had dual-row timing chains that were superior in terms of durability and longevity.

There have been reports of timing chains wearing out as early as 100,000 miles in the OM651-powered cars. To makes matters worse, the OM651 is the first Mercedes-Benz motor to have the timing gears in the back of the engine where access is restricted. This makes replacing the timing chain more expensive.

I’m not trying to scare you. Failures aren’t that common. However, if you decide to get one of these cars, listen for any timing chain noises during the first couple seconds after a cold start. Follow this link to read more about timing chains and how to check them.

OM651 engine – faulty piezoelectric injectors (S 250 CDI)

The 125kW and 150kW OM651 engines were released with short-lived piezoelectric injectors made by Delphi. The most problematic cars were the ones manufactured in 2009 and 2010. However, newer cars also experienced problems.

In response to the failing OM651-powered cars, Mercedes-Benz initially decided to replace the faulty injectors with new ones, but eventually, scrapped the piezoelectric injectors completely. The final solution to the injector problems was to recall the cars to fit solenoid injectors, which is an older but proven technology.

It wasn’t an official recall, but in reality, it kind of was. By now, most if not all faulty injectors should have been replaced. The original ones had been failing at relatively low mileage (even below 50,000 miles). From mid-2012, the OM651-powered cars were fitted with solenoid injectors at the factory.

If you are planning to buy an S 250 CDI BlueEFFICIENCY, get one that already has the solenoid injectors. Even if some cars managed to last this long with the piezo injectors, I’m not sure about the availability of spare injectors.

If a piezo injector fails, you may be forced to have your car fitted with solenoid injectors. It would be very expensive to do so because it’s not a simple plug-and-play injector replacement (missing fuel return lines, new ECU required, four new injectors needed).

To check if a particular car has the solenoid or the piezoelectric injectors, you need to remove the plastic engine cover.

Here’s what to look for:

• Piezoelectric injectors don’t have fuel return lines. The fuel return lines are rubber tubes that are attached to the injectors along with the metal fuel supply lines. If the car has fuel return lines, it has solenoid injectors.

• The sticker on the ECU should have the word “solenoid” on it in a car with solenoid injectors. The ECU is the shiny metal box that sits on top of the air filter housing. You will definitely see it once you remove the plastic engine cover.

The sales brochures and old car reviews still say that the 150kW OM651 engine is equipped with state-of-the-art piezoelectric injectors. Well, now you know that it’s not the case anymore.

OM642 engine – oil cooler leaks

The pre-2010 OM642 engines were notorious for leaking oil from the oil cooler seals. The original seals (orange colour) weren’t able to withstand the heat and developed leaks. Updated seals (purple colour) were introduced in 2010.

Ideally, get a post-2010 model with the updated seals. In the case of older cars, there was a recall for this issue. Hopefully, most vehicles would have had the seals replaced by now. Before buying one, check if the seals have indeed been replaced and inspect the car for oil leaks.

The seals are cheap. However, their replacement is expensive as the fuel injection system and intake manifold need to be disassembled to get to the oil cooler. The oil cooler sits on top of the engine, between the cylinder banks, underneath the intake manifold. The symptoms of a leaking oil cooler are dropping oil level and oil underneath the vehicle after a longer stand still.

OM642 engine – swirl flap motor failure

There have been cases of swirl flap motor failures in the OM642 engines due to contamination with engine oil. The swirl flap motor is located below the turbocharger and the turbo intake tends to develop leaks in this engine. The oil leaking out from the air intake may get onto the swirl flap motor and cause it to fail.

When the swirl flap motor fails, it puts the car in limp home mode along with turning the “Check Engine” light on. The air vented from the crankcase through the PCV system contains oil vapours. The PCV breather pipe is connected to the air intake before the turbocharger – that’s how engine oil gets into the intake manifold.

Replacing the swirl flap motor is not as painful as replacing the oil cooler seals, but there is still a lot of parts in the way, and the motor itself is not that cheap.

For this reason, people came up with a way to disable the swirl flap motor using a simple resistor to trick the car’s ECU into thinking that the motor is working. This solution is cheaper than replacing the faulty motor at the cost of more pollution.

The swirl flaps are not essential for the engine to run – they are there to improve emissions and disabling them has a minimal impact on engine performance. Keep in mind that disabling emissions controlling equipment is considered illegal in most countries.

OM642 engine – Black Death

The first two generations of CDI diesel engines may develop leaks from the fuel injector seals (copper washers). When a leak occurs, the gases and the diesel fuel from the combustion chamber can escape and cover the area around the leaking injector with burnt, hard, tar-like substance.

“Black death” is a dramatic name for something relatively inexpensive to fix, provided that you catch it early. Any leaks should be visible once the plastic engine cover is removed. 

If there was a leak, you will see a black mess on top of the engine. You may also smell diesel fuel inside the car when the engine is running. If the leak is large enough, the engine may sound like a steam locomotive due to gases escaping from the combustion chamber.

If left untreated, it can become very expensive to fix – injectors seized in the cylinder head, damaged injectors seats and massive carbon build-up to clean.

Summary of problems & additional information

• In traditional Mercedes-Benz fashion, the W221 S-Class was available in short and long wheelbase versions in most markets. The S 600 L and the S 65 AMG were manufactured only in a long-wheelbase version. The letter “L” indicates the long wheelbase.

• Corrosion was a big problem with the W220 S-Class model. Luckily for us, Mercedes-Benz got it right with the W221 S-Class. From the start of production, the W221 has a galvanized body and much better corrosion protection than earlier Mercedes-Benz cars.

• The Airmatic air suspension was standard equipment in the Mercedes-Benz W221 S-Class, except for the S 600 L and AMG models, which have the ABC hydro-pneumatic suspension. In other models, the ABC was an optional extra (option code 487). Both types of suspension are expensive to maintain, with the ABC being the more expensive of the two.

• Watch out for balance shaft and idler gear issues in the M272 and M273 engines. The balance shaft horror story and the overly complex variable intake manifold mechanisms are the only real issues with these engines. Once fixed, they are not bad units. Unfortunately, Mercedes-Benz improved the balance shaft gears only in 2008. If you are going to buy a W211 with the M272 or M273 engines, be very careful not to buy one with worn gears. Look for a car that had them already replaced or one with updated gears – check engine serial number.

• The M156 was the first engine designed fully by AMG and not based on a Mercedes-Benz engine. Despite having excellent performance and winning multiple awards, this engine has its problems. I recommend avoiding the S 63 AMG unless you are planning to do something about the head bolts that may corrode. Also, watch out for worn camshafts. A better choice would be to get the newer S 63 AMG with the M157 engine (from 2011).

• The direct injection petrol engines (M276, M278 and M157) may develop a timing chain rattle during a cold start. The fix to this problem is to fit check-valves and updated timing chain tensioners as per Technical Bulletin LI05.10-P-056435. Before buying a car with one of these engines, make sure there is no timing chain noise during a cold start and that the check-valves have been fitted (contact a Mercedes-Benz dealership).

• Also, be aware that carbon build-up on the intake valves is a possibility in the M276, M278 and M157 engines because of the direct fuel injection.

• Click here for an article that might help you decide if a modern diesel engine, like the Mercedes-Benz CDI, is the right choice for you. All diesel engines in the Mercedes-Benz W221 S-Class have diesel particulate filters (DPF).

• The pre-2010 OM642 engines may suffer from oil cooler leaks if the seals haven’t been replaced yet. Also, the swirl flap motor design is unfortunate in this engine. However, this is a much smaller problem than leaking oil cooler seals.

• I can’t recommend the OM651 engine (S 250 CDI) because of potential problems with the timing chain, which is at the back of the engine. Also, these engines experienced problems with the original piezoelectric fuel injectors. By now, most of the OM651 engines have the retrograde solenoid injectors, which turned out to be more reliable. I think that Mercedes-Benz tried to stay too close to the cutting-edge when designing the OM651 engine.

• All Mercedes-Benz W221 engines are fitted with timing chains, which normally don’t have a specified replacement interval. Unfortunately, some of the engines in the W221 may develop issues (OM651, M272, M273, M276, M278 and M157). Read more about timing chains and how to check them before buying this car.

• Please be aware that Mercedes-Benz is a manufacturer of high-performance, luxury cars. High performance usually goes hand in hand with increased complexity. This is particularly true for the S-Class, which has been Mercedes-Benz’ testbed for the latest technologies. The features you see in an S-class car today may be popular in more ordinary cars in a few years. When things go wrong, you can expect the servicing costs to be above average.

• The V12 W221 models are very expensive to maintain. Pick your model wisely – there will be a big difference in maintenance costs between an S 350 and an S 600 (more parts, ABC suspension, restricted access in the engine bay).

• The S 400 HYBRID was the first Mercedes-Benz hybrid car and the first hybrid car that used lithium-ion batteries. Well, this is partially correct because the first Mercedes-Benz hybrid was a bus:

Petrol engines – specs & performance figures

Diesel engines – specs & performance figures

Petrol engines – technical details

Diesel engines – technical details