Diesel Venting Devices

Frequently Asked Questions About Diesel Venting Devices

Air can enter the diesel fuel system after maintenance and repair work. Diesel bleed-off devices help to deliver fuel in a controlled manner through lines, filters, and components and to remove trapped air from the fuel system. This may be necessary, particularly after changing the fuel filter, performing repairs, or running the tank dry.

• When does a diesel fuel system need to be bled?

  Bleeding may be necessary whenever air has entered the fuel system.

  Typical situations include:

  • Replacing the fuel filter
  • Opening or replacing fuel lines
  • Replacing a fuel pump
  • Working on the low-pressure system
  • Running the vehicle until the fuel is completely depleted
  • Completely emptying the fuel tank
  • Repairs to fuel injection components

  Whether and how bleeding must be performed depends on the specific vehicle and engine manufacturer.

• Why can air in the diesel fuel system cause problems?

  The fuel system must continuously supply fuel to the high-pressure pump or fuel injection system.

  If air is present in the lines, fuel flow may be interrupted or impaired.

  Possible consequences include:

  • long start-up times
  • engine fails to start
  • rough engine operation
  • insufficient pre-feed pressure
  • insufficient rail pressure
  • repeated engine stalling

  Therefore, a diesel fuel system that has been opened or drained should be properly bled according to the manufacturer’s specifications.

• How does a diesel venting device work?

  A diesel bleed-off device specifically pumps fuel through the fuel system or, depending on the unit’s design, creates a vacuum to draw in fuel.

  This displaces or extracts any air present in lines, filter housings, and other components.

  Depending on the vehicle and test equipment, the system can be bled using:

  • an electric feed pump,
  • a vacuum pump or vacuum box,
  • or a compressed-air-powered feed/extraction unit

  The connection point and flow direction must be compatible with the respective fuel system.

• What is the difference between pressure venting and vacuum venting?

  In pressure bleeding, fuel is pumped through the system. This carries air bubbles toward the return line or the designated bleed point.
  
  In vacuum bleeding, on the other hand, fuel is drawn in by negative pressure.
  
  Which method is used depends on the vehicle, engine, and fuel system.
  
  Both methods have the same goal: to remove air from the fuel system and ensure a reliable fuel supply.

• Does a common-rail diesel system need to be bled after replacing the fuel filter?

  It depends on the vehicle.
  
  When changing the filter, air can enter the low-pressure system. If the vehicle cannot automatically remove this air, the system must be pre-filled or bled according to the manufacturer’s specifications.
  
  This prevents the high-pressure pump from receiving an insufficient supply of fuel for an extended period when attempting to start the engine.
  
  The specific procedure can vary significantly between passenger cars, commercial vehicles, and different engine systems.

• Why shouldn't you just let a diesel engine run for a long time after changing the filter?

  A prolonged start-up is not the intended venting method for every modern diesel system.
  
  If there are still large amounts of air in the fuel system, the high-pressure pump may initially not receive an adequate supply of fuel.
  
  Controlled pre-filling or bleeding may therefore be advisable or required by the manufacturer.
  
  In addition, the battery and starter are spared if unnecessarily long starting attempts do not have to be made.

• Can a diesel fuel venting device still be used after the tank has been run dry?

  Yes. A vehicle that has run out of fuel is a typical scenario.
  
  If air has entered the fuel lines, a bleed-off device can help refill the system with fuel and remove the air in a controlled manner.
  
  Before starting the engine, make sure that the bleed-off procedure specified by the vehicle manufacturer has been completed in full.

• How can you tell if there is still air in the fuel system?

  Depending on the fuel system, air bubbles can be made visible through transparent fuel lines or using appropriate testing equipment.

  Diagnostic results can also provide clues, such as:

  • significant fluctuations in pre-feed pressure
  • delayed pressure build-up
  • repeated starting problems
  • air bubbles in a visible section of the line
  • insufficient common-rail high pressure during starting

  If air continues to appear despite bleeding, the system should be inspected.

• What is the difference between a bleed-off tool and a fuel pressure tester?

  A diesel bleed-off device is primarily used to pump or draw in fuel and remove air from the system.

  A fuel pressure tester, on the other hand, measures the existing pressure or vacuum and assists in diagnosing issues with the fuel pump, filter, lines, and pressure regulators.

  These two types of devices can complement each other effectively.

  After bleeding the system, for example, you can check whether the specified pre-feed pressure is actually being achieved.

• Can a diesel bleed-off device also be used for troubleshooting?

  Partially, yes.

  If a system does not receive a stable fuel supply despite proper bleeding, or if air keeps entering the system, this may indicate another problem.

  Possible causes include, for example:

  • a leaky line
  • a damaged gasket
  • a leaky filter connection
  • a problem with the fuel pickup
  • a faulty fuel pump

  For an accurate diagnosis, appropriate pressure, vacuum, and visual inspections should then be performed.

• When is an electric diesel bleed pump a good idea?

  An electric pump is particularly well-suited for regularly scheduled maintenance and larger fuel systems, such as those in commercial vehicles.
  
  It enables a controlled fuel delivery process without having to start the engine to draw in fuel.
  
  This is especially helpful when dealing with large volumes of fuel or after extensive maintenance work.
  
  Depending on the unit, a pressure gauge and automatic pressure limit can further monitor the bleeding process.

• Which diesel venting solution is suitable for commercial vehicles?

  For commercial vehicles, a high-performance electric or air-powered solution may be appropriate.
  
  The Leitenberger DEG 01_24VDC, for example, is specifically designed for venting HDEP and MDEG diesel engines in trucks and buses.
  
  Power is supplied via the 24-V vehicle electrical system, allowing the device to be used both in a stationary setting in the workshop and for mobile service.
  
  For more general-purpose conveying, extraction, and venting tasks, the air-powered KEG 01 is also available.

• Can a fuel extraction device also be used for bleeding air from the system?

  Yes, provided the equipment is designed for that purpose.
  
  The Leitenberger KEG 01 is designed for both transferring and draining fuels, as well as for bleeding fuel systems and fuel circuits.
  
  This makes the unit suitable, for example, for major service work where fuel must first be drained or transferred and the system must then be refilled or bled.

• For which fuels is a diesel venting device suitable?

  This depends on the specific unit and its seal and pump materials.
  
  A unit may only be used with the fuels or media for which it has been expressly approved.
  
  For example, the Leitenberger DEG 01_24VDC electric vent pump is designed for standard diesel fuel in accordance with EN 590/B7.
  
  Other devices may be capable of handling additional fuels or media.
  
  The device specifications must therefore be checked before use.

• What safety rules apply when bleeding diesel fuel systems?

  Even at comparatively low fuel pressure, there are fire, environmental, and health risks.

  In particular, observe the following:

  • Do not allow fuel to come into contact with hot engine or exhaust components
  • Avoid sources of ignition
  • Collect fuel in suitable containers
  • Inspect connections and hoses before use
  • Use only approved pumps and adapters
  • Do not exceed the specified maximum pressure

  Only the testing and repair procedures specifically designed for the common-rail high-pressure system may be used on the system itself.

• What should be checked if the engine won't start even after bleeding the air?

  Once the fuel system has been properly bled, the diagnosis must continue.

  Reasonable next steps for testing might include, for example:

  • Check the fuel supply pressure
  • Check the fuel flow
  • Check the common-rail high-pressure system during startup
  • Compare the injector return flow
  • Check the fuel type and fuel quality
  • Evaluate the fault memory and sensor data

  This helps determine whether a supply problem still exists or whether the fault lies in the high-pressure or injector system.