E87 120d - hissing sound from dashboard

[MitchT]

A week ago my 120d (E87, 103k miles, M47 engine) started making a hissing sound, not unlike the noise you get if you start letting air out of a tyre valve, but not as loud. It comes form the dashboard area. I initially thought it was air coming through the vents but, after closing them all and then turning the system off altogether, it continued. It starts when I start the engine and continues until I turn it off. When I turn the engine off the hissing doesn't stop dead, it fades away over a couple of seconds. The sound is constant regardless of revs, speed, gear, etc. Any ideas?

[rottenegg]

Does the hissing noise stop when pressing the brake pedal?

[LastPoster]

Does it have AC. If it does than shut it off completely

[MitchT]

rottenegg said:

Does the hissing noise stop when pressing the brake pedal?

No. It’s there from engine on to engine off.

[MitchT]

LastPoster said:

Does it have AC. If it does than shut it off completely

Yes to AC. On/off, makes no difference.

[rottenegg]

MitchT said:

rottenegg said:

Does the hissing noise stop when pressing the brake pedal?

No. It s there from engine on to engine off.

You said it "fades away" after engine off, so it's not quite binary is it. I've seen leaking brake servos do exactly that.

[E-bmw]

As it is a diseasel, it could be servo/vac pump or pipework in between, that would be my best guess anyway.

Can you hear it in the engine bay?

[MitchT]

rottenegg said:

You said it "fades away" after engine off, so it's not quite binary is it. I've seen leaking brake servos do exactly that.

Well, no, but it is constant from start to the point of engine off. It doesnt get stronger or weaker or change at all with engine revs, braking or any other factor.

[MitchT]

E-bmw said:

Can you hear it in the engine bay?

I need to check that, possibly after work tomorrow.

It's going to the garage next week anyway as the creaking noise during low speed steering inputs (separate thread) needs investigating and I've tracked that down to a possible top mount bearing... and the glow plugs probably need changing too.

[MitchT]

Just had a nosy under the bonnet. No more noticable, in fact, it's harder to hear because of the engine noise. If anything, it's maybe a little louder in the driver's side footwell.

[E-bmw]

MitchT said:

Just had a nosy under the bonnet. No more noticable, in fact, it's harder to hear because of the engine noise. If anything, it's maybe a little louder in the driver's side footwell.

Likely to be a servo leak then.

[MitchT]

E-bmw said:

Likely to be a servo leak then.

Possibly, though a quick Google says "This leak results in a lack of brake assist, leading to a harder brake pedal and longer stopping distances." and neither of these are factors. The car's braking feel and performance is as normal.

Another thing I've seen mentioned is the interior temperature sensor which, apparantly, is under the dashboard somewhere near the steering column.

[E-bmw]

MitchT said:

E-bmw said:

Likely to be a servo leak then.

Possibly, though a quick Google says "This leak results in a lack of brake assist, leading to a harder brake pedal and longer stopping distances." and neither of these are factors. The car's braking feel and performance is as normal.

Correct, but that is ONLY if the leak is bigger than that required to "assist" when braking is required.

MitchT said:

Another thing I've seen mentioned is the interior temperature sensor which, apparantly, is under the dashboard somewhere near the steering column.

Interior temperature sensor will surely just be an electronic device.

[MitchT]

E-bmw said:

Interior temperature sensor will surely just be an electronic device.

Yes, you'd think so, but Google reveals at least two people saying it has a pipe attached to it and this detaching is what causes the hissing. I'd probably try to investigate but it's going to the garage for a couple of other bits looking at anyway.

[LastPoster]

MitchT said:

E-bmw said:

Interior temperature sensor will surely just be an electronic device.

Yes, you'd think so, but Google reveals at least two people saying it has a pipe attached to it and this detaching is what causes the hissing. I'd probably try to investigate but it's going to the garage for a couple of other bits looking at anyway.

The hose probably goes to the ventilation fan so that the sensor senses the air temp in the car rather than the temperature of the plastic dash that surrounds it.

[E-bmw]

LastPoster said:

MitchT said:

E-bmw said:

Interior temperature sensor will surely just be an electronic device.

Yes, you'd think so, but Google reveals at least two people saying it has a pipe attached to it and this detaching is what causes the hissing. I'd probably try to investigate but it's going to the garage for a couple of other bits looking at anyway.

The hose probably goes to the ventilation fan so that the sensor senses the air temp in the car rather than the temperature of the plastic dash that surrounds it.

Possible, obviously but I don't know if it does or not.

My only thought would be why that would cause a hissing sound?

As you say, investigation required to confirm, good luck.

[MitchT]

I agree with the servo now.

I saw something online that said the brake pedal should be solid with the engine off but go a little soft when you start the engine, if there's enough pressure in the system. It does indeed do this but I noticed that if I press really hard on the brake pedal the hissing goes away. This didn't happen when driving because, during normal driving conditions, I never have to apply a lot pressure to the brake to stop the car, so the hissing just remained constant throughout driving.

So, that's it. Press hard enough on the brake and the hissing stops, so servo it is, I guess!

[MitchT]

Been to the garage. Servo diagnosed. £400 for a new one but he says if the brake feel is normal wait until is starts to feel firm and then do it. I'm looking around for a new car so might not get that far...hopefully. In the meantime, scoping out reconditioned units.