Fuel around the high-pressure pump is a problem Mercedes-Benz diesel owners should take seriously. Even a small wet patch can become a larger leak, especially when the engine is hot, the vehicle is driven regularly or the rubber hoses have started to age.
A leaking high-pressure fuel pump does not always mean that the complete pump has failed. In many cases, the real problem is a low-pressure fuel hose, a worn clamp, a damaged O-ring, a leaking flow control valve or a sealing issue around the pump housing.
This guide explains the common causes of fuel loss around the Mercedes-Benz high-pressure pump and outlines the correct inspection process. It is based on Mercedes-Benz workshop information covering several diesel engine families, including OM611, OM612, OM613, OM628, OM629, OM640, OM642, OM646, OM647 and OM648.
Safety note: Diesel fuel is flammable and harmful to skin. Fuel system repairs should be carried out by a qualified technician with the engine switched off and the vehicle safely supported.
Common Symptoms of a Mercedes High-Pressure Pump Leak
A fuel leak around the pump may appear in several different ways. Some drivers notice a strong diesel smell, while others see fuel dripping onto the engine or under the vehicle.
Typical symptoms include:
- Diesel fuel smell in the engine bay
- Wetness around the high-pressure pump
- Fuel drops on the ground after parking
- Poor engine starting
- Longer cranking before the engine fires
- Rough running after the vehicle has been parked
- Loss of fuel pressure
- Damp or swollen fuel hoses
- Fuel contamination on the serpentine belt
- Diesel-soaked coolant hoses
- An illuminated engine warning light
A leak may be easier to find during cold weather. On some Mercedes diesel engines, the connection between the low-pressure hose and its connector can leak when temperatures fall. Rubber becomes less flexible in cold conditions, and an old clamp may no longer provide enough tension.
Which Mercedes-Benz Diesel Engines Can Be Affected?
Mercedes-Benz workshop information identifies high-pressure pump fuel loss on a wide range of diesel models and engines.
The affected engine families include:
- OM611
- OM612
- OM613
- OM628
- OM629
- OM640
- OM642
- OM646
- OM647
- OM648
- OM668
These engines were installed in various Mercedes-Benz passenger cars, SUVs, vans and commercial vehicles. Applications include selected models from the 163, 164, 168, 169, 202, 203, 204, 209, 210, 211, 220, 221, 245, 251 and 463 model series.
The exact repair depends on the engine code and the point where fuel is escaping. A leak that only requires a hose replacement on one engine may require a pump replacement on another.
The Most Common Causes of Fuel Loss
1. Leak Around the Low-Pressure Fuel Lines
On OM611, OM612, OM613 and OM668 engines, leakage may occur around the low-pressure pipe area or the end cover of the high-pressure pump.
On the OM628 engine, the low-pressure pipe area can also become a source of fuel loss.
These pipes carry fuel to and from the high-pressure pump. Over time, the hoses can harden, crack or lose their ability to seal correctly. A small leak may leave only a damp mark at first, but air can also enter the fuel system when the engine is switched off.
This may lead to:
- Difficult starting
- Air bubbles in transparent fuel lines
- Fuel pressure loss
- Engine hesitation
- Intermittent stalling
If the inspection confirms that the leak is limited to the low-pressure pipe area, replacing the fuel supply and return lines is often the correct repair.
2. Old or Weak Clic Clamps
Several Mercedes-Benz diesel engines use Clic-style clamps to secure the low-pressure fuel hoses. Older clamps may not maintain sufficient clamping force, particularly on certain OM646 and OM648 applications.
A clamp can look acceptable from the outside while still allowing fuel to escape. This is common when the rubber hose has become hard or slightly deformed.
The correct repair may require replacing:
- The Clic clamp
- The fuel supply hose
- The fuel return hose
A used clamp should not be reused if it has lost its tension. Reconnecting the low-pressure hose and then removing it again can also create a sealing problem, depending on the connector design.
A fresh clamp and properly fitted hose are far more reliable than trying to tighten or reuse an old component.
3. Leaking Low-Pressure Connector
On OM646.9, OM647, OM648, OM640, OM642 and OM629 engines, fuel may leak at the connection between the hose and the low-pressure connector.
This problem can be more noticeable during cold weather. The connection may appear dry after the engine warms up, making the leak difficult to reproduce during a quick inspection.
If the connection is confirmed as the source, the usual repair is to replace the affected Clic clamps and the supply or return hose.
On the OM648 engine, the hexagonal low-pressure connector may also be replaceable. However, if the pump uses a polygonal-style low-pressure connector, that connector should not be replaced separately. In that situation, the high-pressure pump may need to be replaced.
This distinction matters because replacing the wrong part can waste time and still leave the vehicle with a fuel leak.
4. Leak Between the Cylinder Head and Pump Housing
On certain OM646.8 engines, fuel may leak in the area between the cylinder head and the steel pump housing. Leakage may also develop between the aluminium flange and the steel housing.
This type of leak is more serious than a simple hose problem because it may indicate a sealing issue within the pump assembly or its mounting area.
The pump should be cleaned thoroughly before making a final diagnosis. If the leak is confirmed at the pump housing, replacement may be necessary.
5. Internal High-Pressure Pump Leakage
Sometimes the high-pressure pump itself is leaking. This can happen at a sealing surface, the pump body, the end cover or another internal area.
A complete pump replacement may be required when:
- Fuel continues to escape after hose and clamp replacement
- The pump body is visibly wet
- The leak comes from a non-serviceable housing area
- A polygonal low-pressure connector cannot be replaced separately
- The pump has internal damage
- The repair procedure identifies the pump as the failed component
However, it is important not to replace the pump before confirming the leak location. A leaking hose near the pump can make the pump appear faulty, even when the pump itself is still serviceable.
Flow Control Valve Leaks
The fuel quantity or flow control valve is another possible source of leakage.
If the valve is leaking, the technician should inspect its O-ring. A damaged O-ring may be replaced separately without replacing the entire high-pressure pump.
If the flow control valve itself is defective, the valve can often be replaced as an individual component.
This is one of the most important diagnostic points because replacing a complete high-pressure pump for a simple O-ring or control-valve fault can result in a much higher repair bill.
The relevant damage code for a flow control valve is:
0727404 – Flow control valve
How to Locate the Exact Leak
Finding the precise source of a fuel leak requires more than looking at the wettest area. Diesel fuel can travel along the pump, hoses and engine surfaces before dripping down.
A reliable inspection normally follows these steps.
Step 1: Clean the High-Pressure Pump
The pump and surrounding area should be cleaned so that old fuel residue does not confuse the diagnosis.
The technician should also inspect nearby components, including:
- Fuel supply lines
- Fuel return lines
- Low-pressure connectors
- Clamps
- Flow control valve
- Pump end cover
- Pump mounting surfaces
- Nearby coolant hoses
- Serpentine belt and pulleys
Step 2: Apply Contrast Spray
Mercedes-Benz workshop procedures recommend cleaning the pump and applying a contrast spray designed to make fresh fuel movement easier to see.
The vehicle can then be inspected again after starting the engine and allowing the fuel system to operate. The first point where fresh fuel appears should be marked.
Step 3: Classify the Leak
The leak should be assessed using a simple severity scale:
- Severe leak – obvious fuel loss or active dripping
- Drop leak – visible individual drops forming
- Seepage – light wetness or a slowly spreading film
This classification helps document the condition and supports the correct repair decision.
Step 4: Recheck After Repair
After the repair, the engine should be cleaned and the area inspected again. The vehicle should be checked both during operation and after it has been parked.
A short road test followed by another inspection is often useful because some leaks only appear under engine vibration or changing temperature.
Inspect the Serpentine Belt and Coolant Hoses
Fuel leaking from the high-pressure pump can spread to surrounding components. Two parts deserve special attention:
- Coolant hoses
- The serpentine or V-ribbed belt
If either component has been soaked with diesel fuel, replacement is recommended.
Fuel contamination can soften rubber, reduce belt grip and damage the material over time. A belt that looks fine may still slip later, especially when the engine is under load.
A contaminated belt can produce:
- Squealing noises
- Charging problems
- Loss of coolant-pump drive
- Belt damage
- Unexpected roadside failure
Replacing the original leaking component without checking the surrounding parts is an incomplete repair.
Repair Options by Engine Family
OM611, OM612, OM613 and OM668
If the leak is located in the low-pressure pipe area, replace the affected fuel supply or return pipe.
If the end cover of the high-pressure pump is leaking, repair the pump according to the applicable Mercedes-Benz repair instructions. The complete pump does not automatically need to be replaced.
OM628
Inspect the low-pressure pipe area carefully. If the supply or return line is the source, replace the affected line.
If the pump itself is leaking, replacement may be required after the leak has been documented.
OM646.9, OM647, OM648, OM640, OM642 and OM629
For a low-pressure connection leak, replace the Clic clamps and the relevant fuel hoses.
On OM648 engines, inspect the hexagonal low-pressure connector. If it is damaged and replaceable, install a new connector. A polygonal connector should not be replaced separately; pump replacement may be required.
OM646.8
Inspect the area around the cylinder head, steel pump housing and aluminium flange. If the pump assembly is confirmed as the source, replace the pump or follow the specified repair procedure for that engine version.
When Should the High-Pressure Pump Be Replaced?
The pump should be replaced only after the leak has been correctly identified.
Pump replacement is generally appropriate when:
- The pump housing is leaking
- A non-replaceable connector is damaged
- The pump has internal leakage
- The flow control valve is not the cause
- New hoses and clamps do not correct the problem
- The pump cannot be repaired according to the applicable procedure
Replacing the pump should not be the first reaction to every fuel leak. A careful diagnosis can often identify a much less expensive repair.
Mercedes-Benz Damage Codes
The following damage codes may be used when documenting the repair:
- 0730704 – Fuel supply hose
- 0732404 – Fuel return hose
- 0727404 – Flow control valve
- 0720685 – High-pressure fuel pump
The related workshop operation is:
- 07-5712 – Remove and install high-pressure fuel pump
Workshop documentation may also refer to repair instruction AR07.02-P-1022A, which covers high-pressure pump repair procedures.
Important Documentation Requirements
When a high-pressure pump is replaced under a warranty, technical support or parts-return process, the leak location should be clearly recorded.
The technician may need to provide:
- A marked image showing the leak location
- The leak severity classification
- Details of the replaced components
- The removed pump or old parts
- Evidence of the contrast-spray inspection
- The correct damage code
Missing parts or incomplete leak documentation may result in the claim or replacement request being rejected.
Good documentation is not just paperwork. It helps prove whether the pump, hose, clamp or flow control valve was actually responsible.
Fuel loss around a Mercedes-Benz high-pressure pump should be repaired promptly. Driving with an active diesel leak can create a fire risk, damage rubber components and lead to hard starting or fuel-system problems.
The most effective approach is simple:
- Clean the pump and surrounding area.
- Inspect every hose, clamp and connector.
- Use contrast spray to locate fresh fuel movement.
- Identify whether the leak is severe, dripping or seeping.
- Replace the damaged hose, clamp, O-ring, valve or pump.
- Replace any fuel-soaked coolant hose or serpentine belt.
- Recheck the area after the repair.
A high-pressure pump leak does not always mean the pump is beyond repair. With careful inspection, many problems can be traced to an ageing fuel hose, a weak clamp or a damaged seal.
For technicians working on Mercedes-Benz diesel systems, accurate diagnosis is the key to saving time, avoiding unnecessary pump replacement and returning the vehicle to the road with a reliable fuel system.
For additional ECU, DME and ECM repair manuals, wiring information and troubleshooting resources, visit AutoCodeWorksECURepairManuals.