Engine diagnostics and troubleshooting (Mercedes-Benz E-Class W124)

Contents: The engine crankshaft does not…↡ The engine crankshaft rotates but…↡ Difficulty starting a cold engine ↡ Difficulty starting a hot engine ↡ Loud noise when starting the starter ↡ The engine starts but stops…↡ Unstable engine operation at idle…↡ Misfires at idle speed ↡ Misfires at all engine speeds ↡ Engine acceleration occurs with a…↡ The engine stops ↡ Reduced engine power ↡ Oil pressure warning light comes on…↡ The engine continues to run after it…↡ Extraneous noises in the engine ↡

The engine crankshaft does not rotate or rotates very slowly when the engine is started with the starter

Poor battery terminal connection.
The battery is discharged or faulty.
The wiring in the starter power supply circuit is poorly connected or faulty.
The traction relay or starter switch contacts are faulty.
Starter damage.
The starter gear does not engage with the flywheel ring gear.
The engine ground bus is damaged or not securely connected.

The engine crankshaft rotates but the engine does not start

No fuel.
The battery is discharged. At the same time, the engine crankshaft rotates very slowly.
The battery terminals are not securely connected.
On models with petrol engines, the ignition system components are damaged or contain moisture vapor.
On models with gasoline engines, the wiring in the ignition system is faulty.
On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with diesel engines, the pre-heating system is faulty.
On models with petrol engines with a carburetor, the throttle mechanism is faulty or incorrectly adjusted, or the fuel shut-off valve flap is faulty.
On models with diesel engines, the engine shut-off valve is faulty.
On models with diesel engines there is air in the fuel system.
Serious mechanical defect of the engine.

Difficulty starting a cold engine

The battery is discharged.
Poor connection or oxidation of battery terminals.
On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with diesel engines, the pre-heating system is faulty.
On models with petrol engines with a carburetor, the throttle mechanism is faulty or incorrectly adjusted, or the fuel shut-off valve flap is faulty.
On models with petrol engines, the fuel injection system is faulty.
On models with gasoline engines, the wiring in the ignition system is faulty.
On models with diesel engines, the high idle speed valve is incorrectly adjusted.
Low compression in engine cylinders.

Difficulty starting a hot engine

The air filter element is dirty.
On models with petrol engines with a carburetor, the throttle mechanism is faulty or incorrectly adjusted, or the fuel shut-off valve flap is faulty.
On models with petrol engines, the fuel injection system is faulty.
Low compression in engine cylinders.

Loud noise when starting the starter

The starter gear teeth or flywheel ring gear teeth are worn or broken.
Starter mounting bolts are loose or missing.
The internal parts of the starter are worn or damaged.

The engine starts but stops immediately

On models with petrol engines, the contacts in the electrical connectors are faulty or loose.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.

Unstable engine operation at idle speed

The air filter element is dirty.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
Low compression in engine cylinders.
The camshaft cams are worn out.
The drive chain is not installed correctly.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.
On models with diesel engines, the fuel injectors are faulty.

Misfires at idle speed

On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with petrol engines, the high-voltage wires are faulty.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.
On models with diesel engines, the fuel injectors are faulty.
On models with petrol engines there are cracks on the distributor cap.
Uneven or low compression in engine cylinders.
Crankcase ventilation hoses are not securely connected or are damaged.

Misfires at all engine speeds

Fuel filter contamination.
The fuel pump is faulty or delivers fuel under low pressure.
Contamination of the fuel tank or fuel lines.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with petrol engines, the high-voltage wires are faulty.
On models with diesel engines, the fuel injectors are faulty.
On models with petrol engines, the ignition coil is faulty.
On models with petrol engines there are cracks on the distributor cap.
Uneven or low compression in engine cylinders.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.

Engine acceleration occurs with a delay

On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.
On models with diesel engines, the fuel injectors are faulty.

The engine stops

On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
Fuel filter contamination.
The fuel pump is faulty or delivers fuel under low pressure.
Contamination of the fuel tank or fuel lines.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.
On models with diesel engines, the fuel injectors are faulty.

Reduced engine power

The drive chain is not installed correctly.
Fuel filter contamination.
The fuel pump is faulty or delivers fuel under low pressure.
On models with petrol engines, the spark plugs are faulty or the spark plug gap is not adjusted.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.
On models with diesel engines, the fuel injectors are faulty.
On models with diesel engines, the fuel injection timing is incorrectly adjusted.
Braking of wheels while the vehicle is moving.
Incomplete clutch engagement (the clutch is slipping).

Oil pressure warning light comes on when engine is running

Low engine oil level or incorrect type of oil has been added.
The oil pressure sensor is faulty.
The crankshaft main bearings and/or oil pump are worn.
Engine overheating.
The oil pump safety valve is faulty.
Contamination of the mesh filter of the oil pump oil inlet pipe.

The engine continues to run after it is turned off

Thick layer of carbon deposits in the combustion chambers.
Engine overheating.
On models with petrol engines with a carburetor, the winding of the fuel cut-off electromagnetic valve is faulty.
On models with petrol engines with fuel injection system, damage to the fuel injection system.
On models with diesel engines, the engine stop solenoid valve is faulty.

Extraneous noises in the engine

Detonation or knocking when accelerating or when the vehicle is fully loaded

On models with petrol engines, the ignition timing is incorrectly adjusted.
On models with gasoline engines, the spark plugs are not the correct type.
Low octane fuel is used.
On models with petrol engines, there is an additional air intake in the carburetor body or intake manifold.
Thick layer of carbon deposits in the combustion chambers.
On models with gasoline engines with a carburetor, the jets or internal channels of the carburetor are clogged.
On models with gasoline engines with fuel injection, the fuel injection system or injector is damaged or clogged.

Whistling or wheezing noises

On models with gasoline engines, the intake manifold gasket or carburetor/throttle body gasket is leaking.
The exhaust manifold gasket or the connection between the exhaust manifold and the exhaust pipe is leaking.
Damage to the brake booster vacuum hose.
Burnt cylinder head gasket.

[Original can be found on the website: MercedesMAN.ru]

Minor knocking or rattling noise

Wear of the valve drive mechanism or camshaft.
Wear or damage to engine accessories (water pump, generator, etc).

Loud or dull knocking sound

The crankshaft connecting rod bearings are worn out (a constant loud knocking noise that decreases with increasing load).
The crankshaft main bearings are worn out (a loud knocking noise that increases with increasing load).
Piston damage (especially noticeable when the engine is cold).