Vauxhall Corsa C - Hissing sound under the bonnet

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Mon 12 Dec 2016 15:50
Vauxhall Corsa C - Hissing sound under the bonnet - Josh Brice
Hi all,

After picking up a 2004 Vauxhall Corsa yesterday, I've discovered a hissing noise coming from the top corner where the brake fluid reservoir is. It sounds like air escaping from somewhere however I can't seem to find where it's coming from.

After doing a bit of research I can't find much on the issue. The air conditioning system works fine along with the heater etc. It begins as the engine starts up and stops as soon as the engine is switched off. As I'm driving along, if I accelerate a bit harder then it will become a lot less noticeable and as soon as I ease off again it comes back. The same thing happens when I brake and as soon as I ease off the brakes, the sound comes back. The brake fluid, coolant and oil levels are all fine so I'm suspecting it may be a burst hose? The engine check light has come on as of today and luckily enough the garage that I got it from is willing to sort it out for me. Is it something I could fix myself though? The car drives fine.

Many thanks!
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Mon 12 Dec 2016 15:55
Vauxhall Corsa C - Hissing sound under the bonnet - Railroad.

You have a vacuum leak on the pipe from the inlet manifold to brake servo or on the servo itself. Spray Carb cleaner around the area the noise is coming from to locate the leak precisely. When the engine note changes you know you've found it.

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Mon 12 Dec 2016 16:07
Vauxhall Corsa C - Hissing sound under the bonnet - Josh Brice
Ok thanks. Do I need to get under the car to do this? Is the inlet manifold in this area?
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Mon 12 Dec 2016 21:20
Vauxhall Corsa C - Hissing sound under the bonnet - FP

Careful. The advice from RR implied (but did not say) that you do the spraying while the engine is running and listen for a change in note.

The inlet manifold is NOT accessed from under the car, and you would be taking a huge risk getting under a car while the engine is running - DON'T.

The inlet manifold is where the fuel and air mixture gets into the car (air, basically, with modern injection systems). So identify the air cleaner/filter and work out what this is attached to. Youshould be able to access this easily from the top of the engine with the bonnet up. You still need to exercise a lot of caution in case the car suddenly moves - do not stand in front, only at the side.

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Mon 12 Dec 2016 21:27
Vauxhall Corsa C - Hissing sound under the bonnet - elekie&a/c doctor

The inlet manifold,vac pipe and servo are all accesible from on top of engine bay .No need to get under the car.The plastic cover over the air intake plenum,by the wipers, is the only part that needs to be removed.

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Tue 13 Dec 2016 09:55
Vauxhall Corsa C - Hissing sound under the bonnet - Railroad.

Careful. The advice from RR implied (but did not say) that you do the spraying while the engine is running and listen for a change in note.

Well there wouldn't be much change in the engine note if it wasn't running would there? And air only passes though the inlet manifold. Fuel is injected just prior to entering the cylinder head. Vacuum is present when the engine is running with the throttle closed, hence why the noise is prominent then and disappears when the throttle is opened. This problem cannot be difficult to find. It's either the servo pipe or the servo itself. Both have happened before.

Pull the pipe out of the servo and put your thumb over the end with the engine idling. If the noise is gone then the problem is the servo. If it's still there the problem is the pipe which will be punctured or split.

Edited by Railroad. on 13/12/2016 at 10:00

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Sat 17 Dec 2016 02:45
Vauxhall Corsa C - Hissing sound under the bonnet - edlithgow

I'd think on a modern car spraying carb cleaner around might damage plastics.

An alternative is to use a gas blowtorch, if you have one. Unlit, naturally.

Both are obvious fire hazards (the gas perhaps slightly less) so it would be a good idea to have some extinguisher capability (purpose made or improvised) handy.

The theory is that the engine sucks the combustible gas or vapour in via the leak and then burns it, causing the revs to increase slightly.

You could also try smoke, running a joss-stick along your pipework and seeing if the smoke gets drawn in to any gaps

A more efficient but harder to improvise method involves blowing smoke under pressure into your plugged air intake, engine off.

If the smoke leaks out, air will leak in at the same place when the engine is running.

If you can hear it, you've got a big leak so these methods might not be necessary. Try a piece of plastic or rubber tube as a stethescope to help locate the sound.

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Sat 17 Dec 2016 07:55
Vauxhall Corsa C - Hissing sound under the bonnet - hardway

Just look for the brake fluid res'

usually a bright yellow cap,

The res' is bolted to the brake servo and there will be a plastic pipe going into the servo,

kind of like an upturned cooking pot.

it's a fair size pipe around 20 mm dia or so,

check where it fits into a rubber grommet into the servo and the pipe where it runs down into the inlet manifold.

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Sun 18 Dec 2016 10:56
Vauxhall Corsa C - Hissing sound under the bonnet - Big John

Er - spraing flamable liquids / gas towards a running engine is potentially VERY dangerous indeed

Very good suggestion to remove the vacuum pipe from the brake servo and block the end to see if the noise goes away - which would show it''s a brake servo issue. As the noise comes from around the brake master cylinder something like this likely to be the issue. Check the condition of the vacuum pipe as well.

Edited by Big John on 18/12/2016 at 10:57

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