Mysterious vacuum leak & hunting...

[larby]

Hi everyone, I'm new on here having just bought my first merc... a mint black C220 elegance.
I've already sorted a little issue i had with my wiper thanks to the great 'how to' bit on pinning that gear, now i've got a it of an idling issue i'm hoping someone may be able to help with.

I can hear a vacuum leak through the right hand air vent next to my instrument cluster. Not only is it annoying because the hissing comes and goes as i use the throttle, but the car is starting to hunt quite badly at idle.
I've had a brief look in the engine bay and can't see an obvious vacuum hose going into that area, would anyone know what it could be?
It's definately eminating from the vent ecause if i put my hand over it, the noise stops, I'm assuming it's a vacuum leak due to it only making a noise at 0% or very light throttle, and i'm assuming that's the cause for the hunting!

Cheers is advance,

James

 

[vincenz]

You sure it isn't the central locking vacuum?

My W202 C200 central locking used to make a prolonged hiss after it had opened and even upon til the point where you started the engine on the drivers door.

 

[grober]

Its probably coming from the headlight height adjustment switch ( just under the right vent)which is vacuum operated on the older models. Its the knurled wheel beside the main light switch. A vacuum line has probably come off ---replumb it and you should be OK.

 

[miro]

There are several problems associated with this event.

a) your vacuum pump may or has burned out because it never reaches cut-off.

b) The hose itself may be damaged (quite common on 2000-2002 models in 2008)

c) The previous owner may have attempted a headlight change (halogen to xenon) or the hose itself may just have ruptured.

Try turning the adjustment wheel to "3" and see if it stops then.

I would say that your pump has bitten the dust also. Hunting is an idle issue and it is actually a combination of tolerance settings, engine controller firmware and vacuum pressure.

As a basic thing, try changing the air element on the engine intake (right up front along the radiator ). This may relieve some of the air-intake pressure.

Hunting was usually a problem with firmware and it is caused when the self-adjustment in the engine reaches its limit and the software goes "WTF!" and tries to recalibrate. It is just an indicator of it being an early model.

 

[larby]

Vincenz - It's definately not that, i drove er for 7 hours down a motorway and it did it the whole way!

Grober - I rekon you've hit the nail on the head there, seeing as the headlight adjustment doesn't work! I'd just assumed it would've been electric! I'll pull that out this morning and see if the hose is off

Miro - Cheers for the detail here, I probably should've mentioned it's a '96 model. A few questions...

a) Why would it have a vacuum pump being a petrol? Surely it just gets vacuum from the inlet manifold at idle? If it does have a pump then i can only assume it's not burnt out as the hissing is still there and my brakes are fine...

b) I'm assuming this is the problem!

c) Definately don't think this is a worry going by the service history, i don't even think they cleaned it themselves!!

I'll try changing the element and see if that helps... from my experience with my last toy (a civic on throttle bodies) I've seen even the slightest vacuum leak causing havoc with some engines almost always resulting in hunting. Hopefully a filter change and putting this hose back on may be enough to solve the problem

Thanks so much for all the replys everyone, I've never been on such a helpful forum. I must say the car is fantastic too, I'm not sure i ever want to drive a different make again!

Cheers, James

 

[grober]

On these models the vacuum is provided by the engine by a take off on the engine inlet manifold ( or a separate mechanical pump on diesels I think?). Take a look for a small 3? way manifold of yellow one way valves beside the brake servo unit under the scuttle. This vacuum is used to power various devices like the headlamp height adjustment, some emission valves but also to switch various flaps on the heater/ air conditioning units. The central locking vacuum ( including the boot and petrol cap) is a completely independent system powered by an electrical pump located under the rear seat.

 

[larby]

Grand, as suggested i turned the headlight adjustment to 3 and the problem went away! I'll pull the unit out this evening and check where the leak is.

 

[vincenz]

It was just a thought,

Best of luck with the fix!

 

[miro]

Grand, as suggested i turned the headlight adjustment to 3 and the problem went away! I'll pull the unit out this evening and check where the leak is.

It is in the wiring well on the kick panel near your right foot. That is where the hose is hissing.

The fact that it stopped when the wheel was on 3 means that the headlights are disconnected from the adjustment system. Probably Xenons.

you need a red plastic plug that looks like a golf tee (but isnt) and that plugs into a documented and designated position in that tubing network. You can find it by following the hissing and seeing which one is the master line. Split the lines and put the bunt into the main line.

Otherwise leave it on 3.

 

[LoTu5]

Its probably coming from the headlight height adjustment switch ( just under the right vent)which is vacuum operated on the older models. Its the knurled wheel beside the main light switch. A vacuum line has probably come off ---replumb it and you should be OK.

When I picked my car up last week I had the same problem, the leak was making a noise similar to a kids horn every time I started the car. I think you advised me on it then, grober. I've not looked yet but since I've been giving the car regular use some little gremlins have sorted themselves out, including the annoying vacuum noise.

 

[poisas]

how you would describe the vacuum leaking noice ? its like PSSSS or its like GHRrrr ?

funny ya ?

 

[larby]

Seeing as it's raining i haven't had a chance to pull it apart yet... I'll have a look soon as possible and let everyone know what it was. It's a very very quiet hiss noise by the way, you have to put your ear to the vent to hear it, i'm just really fussy!

 

[miro]

Seeing as it's raining i haven't had a chance to pull it apart yet... I'll have a look soon as possible and let everyone know what it was. It's a very very quiet hiss noise by the way, you have to put your ear to the vent to hear it, i'm just really fussy!

Start by looking at the headlights .... if they are xenon then you can plug the violet leads in the engine bay. In that case you will need 2 plugs.

 

[larby]

nope, definately not xenon... unfortunately!