Mercedes-Benz CLK-Class (W209: 2002-2009)

Valeo radiators & transmission failure

The early W209 models, manufactured before 09.2003, might have been fitted with radiators that can cause expensive automatic transmission failures. The transmission oil cooler in these cars is integrated with the radiator.

The faulty radiators were made by Valeo and had a problem with an internal seal that was supposed to separate the coolant from the transmission oil. A failed seal would release the coolant into the transmission oil circuit. This oil/coolant mixture can ruin the transmission if not flushed quickly. Cars with manual transmissions are not affected.

The coolant in the transmission is not a good thing and causes severe clutch surface wear, among other issues. The fix for a transmission contaminated with coolant is to:

1. replace the transmission (expensive)

2. rebuild the transmission (also expensive)

3. if the contamination was stopped quickly enough, a transmission flush and new transmission fluid may be enough (not that expensive)

The typical symptoms of a failing radiator seal are harsh gear changes and humming/droning noise during light acceleration up to 2500 RPM. Definitely look out for this when test driving a W209 CLK-class made before 09.2003.

The majority of the CLK-Class vehicles on the road today would have had the faulty radiators replaced by now. If you do buy a car with a Valeo radiator, consider having it replaced as there is still a small risk that it could fail. It’s much cheaper to replace a radiator as a precaution than it is to replace a dead transmission.

Only cars manufactured before 09.2003 are affected and newer Valeo radiators are fine.

You can identify a Valeo radiator by the crimping method i.e. how the metal core is joined with the plastic sides. Here’s what it looks like:

Convertible roof

The soft top roof is operated by 7 hydraulic actuators, powered by a hydraulic pump. Your two biggest concerns are hydraulic system issues (mainly leaks) and weather intrusion (fancy name for a leaking roof).

The typical things to check in a Mercedes-Benz convertible are as follows:

• Check for any dampness in the footwells and carpets. Also, check in the boot underneath the spare wheel. It’s best to do it after rainfall.

• Open and close the roof a couple times to make sure it works.

• Check the condition of the roof fabric.

• Check the condition of the roof seals. Ideally, the should be lubricated regularly – ask the previous owner about it.

• Check if the level of hydraulic fluid is correct. If it isn’t, then the car may have been neglected or there is a leak somewhere.

• Look for hydraulic fluid leaks – in the boot, behind the seats and around the hydraulic rams.

• Get the car up to 60 mph with the roof closed and listen for excessive wind noise.

• Listen for rattling noises while driving with the roof closed.

• Take the car to a high pressure (touchless) car wash and see if any water gets inside the cabin or the boot.

• Accident-free history is important for a soft top convertible and very important for a hardtop convertible. If the car participated in a large crash, the roof may be out of alignment. It will be more likely to leak if the chassis of the car isn’t “straight” anymore.

The hydraulic rams last only so long before the seals deteriorate and start leaking (15 years reliably at most as I see it). The OEM rams are expensive but there are companies that sell reconditioned units. You can also buy just the seals and replace them yourself (advanced DIY).

The roof lock cylinder is usually the first one to fail. When it does, the headliner may get soaked in hydraulic oil.

Along with the hydraulic system, there is plenty of electronics to manage the roof on the W209. These cars are getting old and sometimes the roof can be a bit fussy. The roof seals, unless replaced at some point, might be tired so the roof may let some water in when it rains (adjustment and seal lubrication helps).

M271 engine – “Kompressor” lifespan

These engines are fitted with superchargers. The superchargers are reliable but have a finite lifespan. I estimate that they will need to be rebuilt after around 150k miles. The engine and other components under the bonnet will last a lot longer, so it’s very likely that the car will need to have the supercharger rebuilt or replaced at some point during its lifetime.

M271 engine – timing chain stretch

Generally, when a timing chain is used, the intention is for it to last the “lifetime” of the engine (very roughly 200k miles). Leaving it for this long in the M271 engine is very risky, in my opinion.

There have been cases of premature chain stretch in these engines. As the chain stretches, at some point it will reach the limit of what the chain tensioner can compensate for. The tensioner uses oil pressure to apply tension to the chain when the car is running. There is also a spring that applies some tensions when the engine is off and there is no oil pressure.

What happens is that when the chain is stretched too much, the spring alone cannot apply enough tension, and it becomes possible for the loose chain to jump some teeth on the timing gears. I believe this is why most of the M271 failures occur during engine start up. When the chain skips, major engine damage can occur if the valve timing goes out of sync far enough for the valves to hit the pistons.

For some reason, Mercedes-Benz decided to use a single row timing chain in the M271 engine while its predecessors, the M111 and the even older M102, had double row timing chains, which were very reliable.

The timing chain should be watched in the M271. If it starts showing any symptoms of stretch, it should be replaced. It’s difficult to estimate precisely how long the chain will last. There have been cases of chain failures at 80k miles, and there are cars that go far past 100k without any problems.

In my opinion, you should treat the chain as a 100k miles service item if you want to be really safe.

You have three options:

1. Avoid the M271 engine entirely.

2. Get one that already had the chain and chain tensioner replaced. The replacement chain should be good for another 100k miles.

3. Buy the car, and plan to get the timing chain replaced at some point. The timing sprockets may need replacing as well if they are worn.

A typical symptom of a stretched timing chain is chain rattle that lasts a couple of seconds after a cold start. In severe cases, the engine may be difficult to start as well. Usually, the “Check Engine” light won’t come on. Click here to learn more about timing chains and how to check them.

M271 engine – capillary action (oil in wiring loom)

The M271 engine is equipped with Variable Valve Timing (VVT). Mercedes-Benz employed VVT in most petrol engines from that time. The VVT system is operated using two camshaft solenoids attached to the front of the engine.

The solenoids occasionally develop minor leaks, which would normally be completely harmless. However, in the M271 the electrical plugs that connect the solenoids are pointing down…

What happens is that oil, leaking out of the camshaft solenoids, can enter the wiring harness and embark on a great journey through the wiring to the oxygen sensor, mass airflow sensor (MAF) and finally the Electronic Control Unit (ECU).

A pretty ridiculous problem, don’t you think?

This is possible because of capillary action – the ability of a liquid (engine oil in this case) to flow through narrow spaces (wire strands) without the assistance of any external forces. If this sounds astonishing, think about how difficult it is to predict such behaviour. Don’t be too hard on Mercedes-Benz engineers.

If oil gets into the wiring harness, most of it will end up at the oxygen sensor, which usually fails first. If the oil isn’t stopped, it may reach the MAF, and lastly the ECU.

The solution is simple – fix any leaks from the camshaft solenoids, and fit short extension cables (pigtails) so that the capillary action is interrupted at the pigtail. With the pigtails fitted, the oil cannot travel further to the actual wiring loom, which is plugged into the pigtail.

If you’re going to buy a car with the M271 engine, I advise you to check the plugs on the cam solenoids for any oil. They are on the front of the engine and easy to get to. If the oil has already contaminated the wiring loom, then it will need to be cleaned or replaced.

Once you buy the car, fitting the extensions is cheap insurance – much cheaper than replacing the oxygen sensor, MAF, or the ECU. Please note that many cars will have the extensions already fitted – there was a service campaign regarding this issue.

M272 & M273 engines – 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, that were manufactured between 2004 and 2008, are fitted with balance shaft gears and idler gears that can wear out prematurely. The M273 engines don’t have balance shafts, but they still have idler gears where the balance shafts would be.

These gears may be defective too. 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 & M273 engines – 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 on virtually all road cars.

M156 engine – head bolt corrosion (CLK 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.

Unfortunately, production of the CLK 63 AMG stopped before AMG updated the head bolts. Therefore, I recommend avoiding the CLK 63 AMG unless 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 (CLK 63 AMG)

As I see it, 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 looks like:

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.

There was a recall for this issue, so 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.

“Black Death” – affects diesel engines

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

• While corrosion was quite a big problem with the W208 CLK-Class, Mercedes-Benz finally got it right with the W209 CLK-Class. From mid-2003, the W209 had a galvanized body and much better corrosion protection than earlier Mercedes-Benz cars.

• [CONVERTIBLE]: The soft top roof on the CLK is reliable, but it is still relatively complex. Some small things may at some point require fixing or adjusting. If you can’t diagnose or repair these little things yourself, trying to get the roof sorted out at a car repair shop can get expensive. The W209 CLK is already quite old, so the hydraulic seals are probably on their way out if they haven’t been replaced yet. Look out for any hydraulic leaks.

• All W209 engines are fitted with timing chains, which normally don’t have a specified replacement interval. Click here to learn more about timing chains and belts. Watch out for timing chain stretch in the M271 engines. A typical symptom is a chain rattle after a cold start.

• The V6 M112  and V8 M113 engines – Mercedes-Benz got both of them right. They are closely related to each other and both are reliable.

• 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 W209 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.

• Because of the potential timing chain issues in the M271, the best petrol engines in the W209 are the V6 and V8 units. The M112 V6 and M113 V8 are reliable but only available before the 2006 facelift. The best production years for these V6 & V8 cars are 2004, 2005 and 2006. Cars from these years will have all the teething issues worked out, updated radiators and a galvanized body. If you’d prefer a newer car, the M272 V6 and M273 V8 are fine too, provided the balance shaft timing gears are okay – best years for these are late 2008 and 2009.

• 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 recommended avoiding the CLK 63 AMG unless you are planning to do something about the head bolts that may corrode.

• As for potential camshaft wear in the M156 engines, the safest thing to do would be to remove the valve covers and inspect the camshaft for wear before buying a second hand CLK 63 AMG. Many of these engines can reach a relatively high mileage without problems. If you are certain that the camshafts are in good shape in the car you are planning to buy, you should not have issues for quite a while, provided that you maintain the engine well.

• 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.

• The OM642 V6 diesel engine may suffer from oil cooler leaks if the seals haven’t been replaced yet. All diesel engine in the W209 may suffer from injector seal leaks (“Black Death“). Other than this, the things that could go wrong with the OM642 and other CDI engines are typical for most Common Rail diesel engines.

• The diesel engines in the W209 may have a diesel particulate filter (DPF). It depends on the country and year of production. Before buying a particular car, you can check if it has a DPF by decoding the Vehicle Identification Number (VIN). The DPF is marked as option 474.

• Please be aware that Mercedes-Benz is a manufacturer of high-performance luxury cars. High performance usually goes hand in hand with increased complexity. When things go wrong, you can expect the servicing costs to be above average.

Petrol engines – specs & performance figures

Diesel engines – specs & performance figures

Petrol engines – technical details

Diesel engines – technical details