A bit of the background: I have a Mercedes that has a serpentine belt-driven air pump to help reduce smog in the exhaust. I heard this whole pump can be removed without causing any trouble, so I decided to go ahead to reduce repair problems.

Then I realised there is a shutoff valve connected between pump and exhaust manifold. And this valve seems vacuum-actuated as there is a thin clear line that goes into it, which I assume is a vacuum line.

If I removed this air pump and shut off valve, the vacuum line would be open, which I think may cause vacuum leak. Are there any ways to address this vacuum line issue? Thanks.

  • 1
    The purpose of the smog pump was to give enough oxygen to the catalytic converter to help it heat quicker. Without the smog pump, the cat(s) will clog over time. If you have removed the cats, this will not be an issue. If you haven't, be prepared. Commented Aug 3, 2015 at 11:32
  • 2
    I have decided to leave it on until it stops working then I will try to bypass it. It is more complex to remove it properly than I first thought.
    – David Xu
    Commented Aug 3, 2015 at 13:10
  • With small vacuum lines, I often just use a bolt of the appropriate diameter. If it's loose enough that I don't have to screw it in, I also put a piece of duct tape over it.
    – TMN
    Commented Aug 25, 2015 at 17:45

2 Answers 2


You can put a plug in the vacuum line or put a cap over the port at the other end of the line. Ask for Vacuum Caps.

Note: The air injection system on my car is much more than just the smog pump. I think you should try to understand the operation of the system before you try to remove.

  • In some countries there are no regulations regarding smog test. The pump can be a liability in an old car.
    – David Xu
    Commented Aug 3, 2015 at 6:52

A few comments in addition to David's answer:

  • You can be creative with how the vacuum hose is closed off

    Something as simple as a bulldog clip will do - the purpose is to seal off one end. Another more permanent solution would be to remove the hose and weld the nipple shut

  • Don't forget the other holes resulting from a smog pump delete

    There are other holes that will need to be plugged, the line going from the pump to the exhaust side in particular. If this hole isn't plugged up, it can and will throw off the fuel-injection management; resulting in poor fuel economy and/or lumpy throttle at best.

Hope this helps.

  • Hi Zaid, the route goes like this: air pump->rubber hose->shutoff valve->rubber hose->check valve->metal pipe->exhaust manifild. I'm speculating the check valve is there to stop the hot exhaust from backflowing to pump, hence the metal pipe after check valve and rubber hose before check valve. Do you think I could like leave the check valve there to act as plug? Thanks. I suppose the exhaust gas from exhaust manifold would be very hot that can melt platics?
    – David Xu
    Commented Aug 3, 2015 at 9:23
  • 1
    @DavidXu : I don't know about your Merc, but when folks do the smog pump delete on the M5 they take it all out. Check valves can leak, so I'd strongly recommend welding the open end of the metal pipe or exhaust manifold shut.
    – Zaid
    Commented Aug 3, 2015 at 9:38

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .