Mercedes-Benz SLK-Class (R171: 2004-2011)

Complex hardtop roof

The hardtop roof on the R171 is actually pretty good. Few companies succeeded in making a reliable hardtop convertible that doesn’t leak and isn’t as complicated as an open heart surgery.

The hardtop “Vario” roof is operated by five 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).

Things to check regarding the Vario roof:

• 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 that it works. The roof should take approximately 22 seconds to open or close. If it takes much longer than that, the pump may be on its way out.

• Check the condition of the roof seals. Ideally, they 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, on the headliner 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 very important for any hardtop convertible. If the car participated in a large crash, the roof may be out of alignment, which can be a nightmare to waterproof 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 roof lock cylinder is usually the first one to fail. When it does, the headliner may get soaked in hydraulic oil.

The OEM rams are expensive but there are companies that sell reconditioned units. Also, you can buy just the seals and replace them yourself (advanced DIY).

Along with the hydraulic system, there is plenty of electronics to manage the roof on the R171. Some of these cars are getting old and sometimes the roof can be a bit fussy (usually small issues though).

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. Follow this link to learn more about timing chains and how to check them.

M271 – 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, 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.

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 150,000 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.

M272 – 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. 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 this a very expensive repair.

Engines with serial numbers higher than 2729 . . 30 468993 have the updated gears and are free from this problem. 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 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 on virtually all road cars.

Summary of problems & additional information

• Most Mercedes-Benz cars, R171 included, made in 2003 and onwards have improved corrosion protection (galvanized body). Mercedes-Benz finally got it right.

• If you’re not into DIY and you’re not willing to understand how the roof mechanism operates, don’t get an older hardtop convertible like the R171. It may be reliable, but the roof is a complex piece of machinery and many small things may at some point require fixing (like one of the roof microswitches) or adjusting (like the roof catches). If you can’t diagnose or repair these small things yourself, trying to get the roof sorted out at a car repair shop can get expensive quickly.

• Watch out for balance shaft gear issues in the M272 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 an R171 with the M272 engine, be very careful not to buy one with worn gears. Look for one that had them already replaced or one with updated gears – check the engine serial number.

• Pick your poison: potential timing chain issues in the M271 engines or balance shaft gear issues in the M272 engines. Luckily, the V8 M113 engine doesn’t seem to have any major flaws (AMG cars). Also, the M272 engines have improved balance shaft gears since 2008.

• The M271 engines without VVT cable extensions may suffer from oil entering the wiring loom. I recommend that you find a car that has the pigtails fitted, or fit them yourself. It’s cheap insurance against a pretty ridiculous problem.

• All R171 engines are fitted with timing chains, which normally don’t have a specified replacement interval. However, they will not last forever. Click here to learn more about timing chains and belts.

• 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.

• The R171 SLK-Class looks like a little brother of Mercedes-Benz SLR McLaren.

Petrol engines – specs & performance figures

Petrol engines – technical details