Mercedes-Benz W124 (1984-1997)

Corrosion (water-based paint)

In my opinion, rust is by far the number one problem with Mercedes-Benz vehicles produced between 1993 and 2004. This is because in 1993, Mercedes-Benz implemented a more environmentally friendly, water-based paint. The post-facelift W124, called the E-Class, is affected by this change. Unfortunately, the new paint offered far less protection from rust.

The pre-facelift W124 models had reasonable corrosion resistance, but because of their age, you should still pay a lot of attention to any rust spots and the condition of the floorpan. Corrosion is your number one problem with any older vehicle. Suspension and engine parts can be replaced, but a rusted body can be the death of a car.

Biodegradable wiring harness

In the early-nineties, Mercedes-Benz decided to use biodegradable wiring harnesses in their vehicles. Unfortunately, it turned out that the biodegradable wiring insulation made from soybeans degrades far too quickly.

It becomes brittle and falls apart over time. The wires become exposed, short-circuit and cause the electrical systems to malfunction. The cracking insulation also makes the wires susceptible to corrosion.

It’s the heat of the engine that accelerates the process of degradation. Therefore, the cables close to the engine are ones mostly affected.

The wiring harness problem affects Mercedes-Benz cars from 1992 until 1996. Both petrol and diesel-powered W124 cars are affected. For most models, the problem started in 1993, however, it was earlier in the case of the M119 engine. Also, cars with the KE-Jetronic are unaffected.

The components that could potentially malfunction because of a damaged wiring harness are fuel injection, starter, alternator, throttle actuator and the ECU. A faulty wiring harness can disable the car, and it is a potential fire risk.

The typical symptoms of a failing harness are electrical errors, problems with starting the engine, poor running and rough idle.

Before buying a 1992-1996 Mercedes-Benz W124, check if the harness has already been replaced. Otherwise, you will have to replace it yourself.

The wiring harness is complex, and that makes it expensive if you buy a new one from the dealership. However, you may be able to find used ones in good condition on eBay. Just make sure that you get one with the updated insulation that doesn’t disintegrate when you touch it.

M102 engine – single-row timing chain

The early M102 engines were originally fitted with single row timing chains, and there have been cases of premature failures.

In my opinion, any M102 engine with a single row timing chain should have the chain and tensioner replaced every 100,000 miles or earlier if the chain becomes noisy. Neglecting to replace these parts can result in serious engine damage (valves hitting the pistons).

Mercedes-Benz updated the M102 engine to receive a double row timing chain in 1988. These chains are a lot tougher, and under normal conditions, should last the “lifetime” of the engine. “Lifetime” can be very roughly defined as 200,000 miles.

M102, M103, M104 and M111 – head gasket failures

The head gasket is a weak point in these engines. Considering head gasket replacement is inevitable at some point, often in the 100k – 150k miles range. Before the gasket fails completely, these engines often develop an external oil leak from the head gasket at the back of the engine. Make sure to inspect that area when buying one of these cars.

Once the head gasket fails and is replaced, the new one should be good for another 100k miles. This issue mainly affects the 6-cylinder engines because of their long cylinder heads. The 4-cylinder engines are also affected but to a smaller degree.

If you’re curious why the head gaskets fail more often in the inline-6 engines – it’s because of the difference in thermal expansion between the cast iron engine block (11 m/m.K x 10^-6) and aluminium alloy cylinder head (22 m/m.K x 10^-6).

As you can see, the expansion coefficient is two times higher for aluminium. This means that when the parts get hot, the aluminium cylinder head expands twice as much as the iron block. The longer the engine, the more pronounced this effect is. The head gasket, which sits between the engine block and the cylinder head, has to deal with these expanding parts.

This was a relatively new engineering problem to solve at the time as former Mercedes-Benz engines had cast iron cylinder heads. Aluminium alloy started being commonly used in the 1980s to make lightweight cylinder heads. These days, the engine blocks are also made out of aluminium alloys.

M102 & M103 engines – KE-Jetronic fuel injection

The M103 and later M102 engines were equipped with a mechanical, Continuous Injection System (CIS) called KE-Jetronic, which was designed by Bosch. The system itself is well-engineered and reliable.

The problem is that most repair shops these days are clueless as to how this system works. Unless you can find a specialist or you understand it yourself, trying to get a KE-Jetronic problem solved might not be easy.

If you find a car that runs well, it will most likely keep going as the injection system doesn’t require any special maintenance. If it does break down though, it will be trickier to fix than a more modern electronic fuel injection system.

If you already have a car with KE-Jetronic and you are experiencing problems, often they can be fixed by replacing the vacuum pipes as the system is sensitive to vacuum leaks.

Make sure you check the following points before purchasing a car with KE-Jetronic:

• Idle quality – The idle should be even when cold and warm. The engine should not die or hesitate when coming to a stop (out of gear in a manual transmission car).

• Power delivery – it should be smooth throughout the rev range. There should be no hesitation when pressing on the accelerator pedal.

• Cold starts – see how well the car starts when the engine is cold.

• Hot starts – when warmed up – shut the engine down for 10 minutes and see how well it starts then. This step is important, repeat it twice.

It everything works well, you should be OK.

Engines with KE-Jetronic should not be run on LPG. It is possible to convert it to LPG, using a second generation system, but it is not a good idea. In fact, I recommend that you never buy a car with KE-Jetronic that has ever had an LPG installation fitted for two reasons:

1. The fuel injection system has a mechanical fuel distributor, which is lubricated by petrol. When you switch to LPG, the distributor will be running dry and start wearing out.

2. With a simple second generation conversion, there is a risk of gas explosion in the intake manifold, which has a large volume. If this happens, you would be left stranded on the roadside with half the vacuum pipes blown off the engine. Even when you fix the car, it will likely never run the same due to the damage that was done (bent air flow sensor plate, damaged vacuum pipes and likely air leaks).

M119 engine – plastic chain guides & oil tubes

The M119 engines are reliable apart from a couple issues related to the plastic parts inside the engine.

The first problem is that over time, the plastic upper chain guides in these engines may develop cracks and pieces of the guides can break off. If they do, the timing chain loses tension and is at risk of skipping, which makes the camshafts go out of sync. It is rare, but it can cause some serious engine damage if it happens (valves hitting the pistons).

In my opinion, the guides should be replaced as part of preventative maintenance at around 100,000 miles.

Another issue, which could potentially damage the engine if left untreated, is the plastic oil tubes – 16 of them in total. These little pipes become brittle and may crack from the heat of the engine and the exposure to oil. When they break or lose their end caps, the hydraulic lifters do not get enough oil, and the engine develops a ticking sound.

The oil tubes used to be metal, and they were changed to plastic in 1993. The cars with the metal tubes are problem free. Fortunately, replacing the plastic oil tubes is a relatively small job.

OM603 engine – trap oxidizer and cylinder head issues

In 1986 and 1987, the OM603 engines for the US market were fitted with a diesel particulate filter, also called the trap oxidizer. These traps would get very hot and there were cases of trap oxidizers causing issues with the aluminium cylinder heads. There were also occurrences of turbochargers getting damaged by the debris from the trap oxidizers.

This was a big issue for Mercedes-Benz, so a campaign was launched to have these filters removed free of charge. If you are in the US, check if the car you are planning to buy still has the trap oxidizer (unlikely). Trap oxidizers were never installed in European cars.

Additionally, the weakness of the early OM603 engines were the head gaskets and the aluminium alloy cylinder heads. There were cases of cracking cylinder heads and head gasket failures in these long inline-six engines. The engines that were fitted with trap oxidizers were especially vulnerable.

Mercedes-Benz redesigned the casting moulds a couple times to combat this issue. I believe that the trap oxidizers were a big contributor to the problem of cracking cylinder heads because the occurrence of these failures seems to be much higher in the US than in Europe. The length of the straight-6 engine may also be a contributing factor.

The original #14 mould cylinder heads are considered to be the weakest, while the best ones are #18 and #22. You can check the mould number by looking at the cylinder head from the injection pump side. The number should be above the second injector. It will look like this: 603 016 14 01.

Anyway, head gasket failures in European OM603 engines are not as common, in my opinion. If you are planning to buy a car with one of these engines, regardless of where you are, remember to inspect it for any head gasket issues.

Another option is to consider the smaller brothers of the OM603 engine – the 5-cylinder OM602 or the 4-cylinder OM601, both are less likely to develop head gasket problems. The OM601 in the large W124 is painfully slow though.

Summary of problems & additional information

• Corrosion is your number one problem with any older vehicle, including this Benz. Suspension, interior and engine parts can be replaced, but a rusted body can be the death of a car. The pre-facelift W124 has better corrosion protection than the following generation of Mercedes-Benz cars, which developed a reputation of rusting easily due to the introduction of water-based paint. The W124 models made after the 1993 facelift are more susceptible to corrosion.

• The W124 chassis shares many parts with the smaller Mercedes-Benz 190 (W201). Both models were designed with longevity in mind. A diesel W124 could potentially be a million-mile car when looked after. Petrol engines are long-lived too.

• The indirect injection system in the diesel cars is very reliable and relatively insensitive to fuel quality. This is because the inline injection pump is lubricated by engine oil, unlike in modern cars where diesel fuel itself is the lubricant. Many people choose these vehicles for vegetable oil conversions (in countries where it is legal).

• Apart from the occasional head gasket replacement (petrol engines and potentially the OM603 diesel engine), all engines are reliable. With proper maintenance, they can last a very long time. In case of the V8 M119 engine, also watch out for cracking chain guides and oil tubes.

• The KE-Jetronic is an interesting piece of engineering and a part of automotive history, but it may be difficult to service. If you plan to use the car as a daily driver, it’s better to look for the M111 or M104 engines with electronic fuel injection, which all car mechanics are familiar with. These engines are also happy to run on LPG if that’s your thing.

• All W124 engines are fitted with timing chains, which normally don’t have a specified replacement interval. The timing chains in the W124 are generally reliable, except for early M102 engines, but they will not last forever. Click here to learn more about timing chains.

• Before buying a Mercedes-Benz W124 from 1992-1996, check if the biodegradable wiring harness has already been replaced. Replacing some wires may sound like a trivial job, but a new wiring harness isn’t cheap. For most models, the problem started in 1993, however, it was earlier in case of the M119 engine.

Petrol engines – specs & performance figures

Diesel engines – specs & performance figures

Petrol engines – technical details

Diesel engines – technical details