How to Troubleshoot the Vacuum on a Mercedes

Written by chuck brown
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How to Troubleshoot the Vacuum on a Mercedes
Vacuum repair is a dreaded task for many technicians. (motor image by Petr Efremov from

For many, vacuum repair is a puzzlement that can frustrate and confuse even with their best efforts. With older diesel Mercedes, this is particularly true, since they have a rather sophisticated vacuum system with many components furnishing vacuum to the engine and practically every other on board feature. The vacuum system around the oil filter housing is one of the more common areas for these leaks to occur. Follow a deliberate process to check the most common vacuum leak locations.

Skill level:

Things you need

  • Vacuum leak repair kit
  • Vacuum pump
  • Vacuum gauge

Show MoreHide


  1. 1

    Check the main vacuum pump first. Crank the car, raise the bonnet and disconnect the main brake booster fitting using a 19 millimetre wrench. Hook the vacuum gauge up to get a vacuum pressure reading.

  2. 2

    Look for a vacuum pressure of around 20 hg to let you know the main vacuum pump is furnishing good vacuum pressure to the system. Check the check valve in the vacuum pump, or replace the vacuum pump if the vacuum pressure reading is low or fluctuates greatly. Move on to the next vacuum check if you get a solid vacuum pressure reading.

    How to Troubleshoot the Vacuum on a Mercedes
    Vacuum leaks can be very difficult to diagnose, especially when there are multiple leaks. (breakage image by Rina from
  3. 3

    Check the metering valves in the main supply line next. The supply line metering valve has two small, fragile connectors, which are brittle and will crack and leak. Use caution when removing them.

  4. 4

    Disconnect the supply side of the line servicing the dashpot, or transmission vacuum, next. Hook your gauge to it and check it for leaks. Replace the dashpot and connector if you see leaks or get an insufficient pressure reading, such as 10 hg or below, and the car is experiencing harsh, jerky transmission shifting.

    How to Troubleshoot the Vacuum on a Mercedes
    Finding the vacuum leak is the tricky part, but repairing it is usually simple if you are careful. (vehicle maintenance list and tools image by Warren Millar from
  5. 5

    Test the door lock check valve next. Hook up your hand vacuum pump to the supply side. Block both outlet nipples and take a reading. Replace the door lock check valve and both pieces attached to it if you detect a leak after testing it.

  6. 6

    Hook up the hand pump to both yellow lines of the door lock check valve, one at a time, with the other blocked off. Try to pull and hold vacuum on each of them. Check the door actuators for leaks, or a master vacuum switch if you cannot hold vacuum on either or both of these yellow lines.

    How to Troubleshoot the Vacuum on a Mercedes
    That unrepaired leak can leave you stranded. (young red-haired woman with her broken car. image by ta_samaya from
  7. 7

    Locate the vacuum lines coming through the firewall. Find the green line with the yellow stripe. This line is the one most apt to have more than one leak because of how it services the HVAC and all its features. Test it in the same way by pulling and holding vacuum.

  8. 8

    Be sure to check the solid black line with your hand pump. Replace it if you find a leak since it is the supply line for the vacuum reservoir. Find the vacuum reservoir either behind the driver's side front wheel well or in the boot.

    How to Troubleshoot the Vacuum on a Mercedes
    Vacuum leaks can ruin your love affair with your Mercedes. (brass car key fob image by leafy from

Tips and warnings

  • Test before and after component/connector replacements.
  • Use the hand pump on check valves and to pull vacuum on moving components.

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