What are the common problems in the 1991 Mercedes Benz 300E?

This blog will explain the common problems in the 1991 Mercedes Benz 300E and also answer the following questions: what are the technical specifications of the Mercedes Benz 300E? How fast does the Mercedes Benz 300 E go? Should you buy a used 1991 Mercedes Benz 300E?

What are the common problems in the 1991 Mercedes Benz 300E?

The common problems in the 1991 Mercedes Benz 300 E are given below:

  • Overvoltage protection relay malfunction
  • Engine compartment fuse box problems
  • Door and trunk locks not working
  • Unstable engine speeds
  • Oil leaks

Let’s take a closer look at the problems and the remedies

Overvoltage relay protection malfunction

The 1991 Mercedes 300E has an overvoltage protection relay, it acts as a safety device for the engine and the ABS control unit in the vehicle. The relay has two inbuilt 10 A fuses if there is a sudden surge in the power supply of the vehicle, the fuses will blow to protect the control units.

The input signals to the relay consist of circuit 30 which is a direct power supply from the battery and circuit 15. The circuit 15 signal means ignition ON. When both the signals are present at the overvoltage protection relay activates the power supply to the control units.

When this relay malfunctions, the instrument cluster lights do not turn ON and even the ABS  will not work and the ABS light will remain ON in the instrument cluster. The cause is an open circuit in the internal circuit board of the relay. If both the fuses are okay then replacing the relay is the only remedy.

It is possible to open the relay housing and repair the open circuit but the fault can easily occur again and it is not a reliable repair. Mercedes Benz also improved the design of the relay and provided replacement parts for the OVP relay.

Engine compartment fuse box problems

The 1991 Mercedes 300E used a different type of fuse and fuse box design. Instead of interesting the fuse in the contact points, the contact points or terminals were exposed. A spring on one end of the terminal held the fuse in position.

The fuse box has a capsule fuse with a small metal plate running across the two ends of the capsule. The thickness of the metal depends on the rating of the fuse. It is very easy to spot a blown fuse and replace it.

However, this design made the fuse box prone to moisture and corrosion. Even if the fuse looks okay from the top there can be excessive resistance at the contact points. The most common problem is with the fuse of the air conditioning control unit.

The air conditioning in 1991 300 E stopped working as the power supply of the control unit is under voltage. Replacing the entire fuse box with the fuses is the only solution, cleaning the contact points and replacing the fuses can solve the problem temporarily.

Door and trunk lock not working

This problem is common in the Mercedes Benz 300E with high age and mileage. The door and trunk locks are controlled by vacuum actuators. A vacuum pump is located under the rear seats, it generates the vacuum for the actuation of the door and the trunk lock actuators.

The vacuum system is reliable but the vacuum cells on the locks get cracked and start to leak the vacuum. If there is a gross leakage then the complete central locking system will become inoperative.

Locating the vacuum leak can be a difficult task but the cost of the replacement lock actuators is not high. There are also lots of aftermarket parts available that can work as well as OEM parts. The actuators can cost between 30 -50 USD depending on the installation location.

Unstable engine speed

The 1991 Mercedes 300E is powered by a 3.0-liter inline six-cylinder engine. The engine uses a mechanical fuel injector system with a vacuum-operated shutoff valve. If the mileage of the vehicle is high, the fuel injector nozzles get clogged and do not inject a uniform flow of fuel into the cylinder.

Bad flow from the injector nozzles leads to unstable engine speed, especially at idling RPM. The injectors can be removed and cleaned but if the flow of the nozzles is okay then it must be replaced.

Another cause of fluctuations in the engine speed and unstable engine idling speed is the distributor cap. Moisture can build up in the cap and the ignition system does not work properly.

Oil leaks

There are engine oil leaks from the crankshaft oil seals and cylinder head cover of the 1991 Mercedes Benz 300E. The valve seals are also known for leaking engine oil into the combustion chamber and causing excessive oil leaks.

Oil leaks are not present on all the 1991 Mercedes 300E but if the engine is not periodically maintained then these problems can occur. The cost of repairs depends on the area of the oil leaks but on average oil leaks are expensive to repair.

If the engine oil level goes below the minimum oil level mark, catastrophic damage can occur to the moving parts of the engine. Therefore, if there is excessive oil consumption; replace the valve stem seals as soon as possible.

What are the technical specifications of the 1991 Mercedes Benz 300 E?

The technical specifications of the 1991 Mercedes Benz 300E are given in the table below:

Technical specifications1991 Mercedes 300 E
Engine3.0 L M103
transmission5 Speed Manual/4 speed Automatic
Wheelbase 280.0 cm
Width 174.0 cm 
Height 143.0 cm
Curb weight1340 Kg 
Trunk capacity520 L 

The M103 engine used in the 1991 Mercedes 300E has a lightweight cast iron engine block. There are two valves per cylinder driven by a single overhead camshaft, which not only helps to keep the production cost low but also reduces the weight of the engine. 

The cylinder head is made up of aluminum and there are hydraulic valve lifters in the valvetrain. There is no need to perform a valve clearance check as the hydraulic lifters can easily compensate for any excessive play. The M103 is a very durable engine and can last for more than 200,000 miles without any major repairs.

How fast does the 1991 Mercedes Benz 300 E go?

The top speed of the 1991 Mercedes Benz 300 E is 142 Mph and 0-62 mph is achieved in 7.9 sec. The 3.0-liter engine makes 185 hp and 260 Nm of torque, which gives the Mercedes 300E a power-to-weight ratio of 7.1 kg/ hp.


This blog explained the common problems in the 1991 Mercedes Benz 300E and listed the technical specifications. The 1991 Mercedes Benz 300 E is one of the most reliable cars and even after more than 20 years, it is sold for 15000 USD in the used car market.