My 2004 Volvo XC90 T6 2.9 has a Vacuum assist pump for the brake booster that should only come on briefly when the engine is cold and then not come on anymore. Mine comes on periodically all the time, mostly when I either step on the brake or take my foot off the brake and even when I don't touch the brake. I have replaced the power booster, master cylinder, valve switch, throttle body gasket and intake gasket. I don't see any cracks in the hose and I replaced all the clamps at the hose ends. The only other symptom I have which might not even be related is I have been getting a weird smell in the cabin that makes my nose tingle. I'm at my wits end but not giving up and I am hoping someone on here can assist me with identifying the problem. Thanks in advance for any feedback.

I took the one way valve that splits between the intake and cooling pipe for the turbo. The end that splits to the turbo cooling pipe does not allow any air in either direction. I'm assuming the connection to the turbo cooling pipe assists with the vacuum?

Vacuum assist

I replaced the booster vacuum ejector today and the problem with the booster pump kicking in every time I touch the brake continues. If I turn the key to the 2nd position without starting the car and touch the brake the pump kicks in, does that suggest that the problem exists between the peddle and the master cylinder? Could it be that I did not seat the O-Ring properly where the master cylinder meets the firewall?

  • Can you provide pictures of how the hoses are laid out? Is this the setup where the switch and pump are located by the air filter? Are there check valves inline with the hoses? I'm not familiar with Volvo's but I assume the vacuum switch is either installed on the booster or it's inline with a vacuum hose. I'd check the wiring, make sure it's not an intermittent short to power after the switch. If that checks out put a vacuum gauge on the switch supply and check for leak down and check the source vacuum as well.
    – Ben
    Sep 23, 2017 at 13:25
  • That looks like a check valve on the outlet to the switch or some kind of spring setup. Put a vacuum gauge inline on that switch outlet and see if it holds vacuum after the pump runs. If it holds I'd say put the gauge inline on the other end and idle the car make sure it pulls good vacuum for your altitude.
    – Ben
    Sep 23, 2017 at 22:48
  • I believe pressing the brake KOEO and the pump coming on is normal. If you are at full vacuum and KOEO the pump comes on by it self I'd say there's still a leak somewhere.
    – Ben
    Sep 25, 2017 at 22:13

1 Answer 1


You are losing vacuum somewhere in the induction system and the brake booster assist is making up for the loss. The first thing to check is the PCV valve and its tubing, if your car has it. Check the EGR valve for vac. leak. You can also spray raw, unlit propane from a hand-held torch around the intake manifold gaskets, induction system and vacuum lines to help find the leak (note when RPM changes); also spray carburetor cleaner around intake gasket to find leaks.

The tingly smell is likely sulphurous, unburned fuel caused by the computer pumping more gas into the engine to compensate for the extra, unmetered air intake. You can really damage an engine with air leaks. Head temperatures will become too hot, as will exhaust gases that damage the catalytic converter(s).

  • I think it would have to be a massive vacuum leak for it to trip the vacuum pump switch, but I don't have access to specifics... so maybe. I would think the MIL would be on or you'd see something in fuel trim. OP should probably measure engine vacuum at idle.
    – Ben
    Sep 23, 2017 at 13:32

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