Monday 14 July 2014

E46: Vacuum-pump severe failure...

With a new servo and master-cylinder, but still no brake assistance, I decided the vacuum-pump must be leaking air. I whipped it off, only 3 bolts and no need to remove rocker-cover etc., and opened it up. This is what I found...

This chamber should be full of air, not oil. The pump bolts on to the back end of one camshaft, which turns a spinning-boss. A plastic-bar slides up and down in the spinning-boss and creates a constant vacuum into the servo to help 'suck' the brake-pedal down when you brake. My plastic bar is missing. Where is it? Well, it's smashed into pieces lying in the oil.
After a clean up it was a decent pump-body and would work fine with a new O-ring and plastic-bar. They sell O-ring kits for the N42 vac-pump separately, but plastic-bars they do not, so I bought the pump from the same donor car as the servo + M/C. All pieces seem to have sheared off with a clean break, the bar was literally blown apart, except for one of the round end-caps (the tiny piece in the middle of the pile in the photo). That had been spread into thin strips as it broke off, so it looks like that end contacted the side of the case, spread into fronds and jammed the plastic-bar, which promptly blew up. How though? Did the timing slip unbalance the camshaft enough to wreck the pump? Or did the plastic-bar break apart and jam the pump, causing the camshaft to stop briefly and the timing to slip in the first place? Hmm...
Another side effect of the vac-pump failing was a terrific oil leak. The O-ring seal had gone completely and the spinning-boss was acting like a mini oil-pump, sending a constant stream down the back of the head onto the hot exhaust [bottom of the pic]. The pressure also made the vacuum-hose connector, normally only filled with air, to leak at the jubilee clip and pour oil onto the gearbox-housing [middle of pic]. After removing the pump a huge third pool of oil arrived. This was what had run out of the vacuum-hose to the brake-servo after disconnecting.


  1. Its happening nowadays common, and i was looking for a guide how to prevent it. If its a roots vacuum pump obviously, failures happens lesser, if its has been taken from the best manufacturers, i would like recommend - keep approaching the best manufacturers for the same.

  2. 320i, N46B20 engine
    My wife had sudden loss of brakes. Hard pedal, she couldn't stop the car. Luckely nothing happened and she brought it home in one piece.
    Today i replaced the rubber hose coming from the vacuum pump, the check valve, the o-ring, the brake booster itself, and bled all the brakes, only to find the problem still exists. Hard brake pedal, no brakes. When i start the car the pedal does not sink at all. When I disconnect the check valve hose, i feel some suction, not much. If i wait long enough (10 minutes) i might get one normal pedal press. Car is undrivable/dangerous.

    The vacuum pump was leaking 2 years ago, so i replaced the O-rings. But there was oil in the first 10cm of the hose going over the vacuum pump (also after i replaced it so it was not residual.). Pump should function with leaking O-rings though?

    When your pump broke, did you experience total lack of vacuum, or was there still some suction felt?

    1. Total lack of vacuum, pedal solid as if no brake boost at all. We could get some brakes but only by standing on the pedal. I would check inside the vac pump to see if the plastic vain has broken, or take the hose off the booster and see if there is any vacuum being created.

  3. Hello. Thanks for the information. I started having hard brakes and no stopping power in my e46 318i. Changed the servo/booster. Still no change. Then changed the check valve and master cylinder and annoyingly no change . I even changed the abs pump. I am at white end and am hoping the vacuum pump could be faulty. I will start with this first thing in the morning.