Thought I had a hole in the exhaust, I was wrong

[jazzdude]

On a cold startup, I was hearing what sounded like a hole in a joint in the exhaust or a gap in the manifold and it would go away after about a minute or so which I put down to the the joint heating up, and closing.

I took it to an exhaust centre and when they examined the car, they said the rear box was corroded, possibly blocked forcing air out of a joint further up. In all honesty, very little seemed be coming out when the engine was revved up and once the new box was on, it seemed to 'breathe'more easily and the car is certainly quieter and more responsive.

But later on when I started it again, the same noise was coming from under the hood, coming from around the exhaust manifold area. Thing is, 20 seconds or so later, the noise just stopped, like it switched off.

I know there is an emissions air pump that comes on when the car starts. Can this make a sound like a blowing exhaust? If so where is the sound coming from and how do I fix it?

 

[WG M-B]

It sounds very much like a pipe from the air pump.

 

[jazzdude]

Thanks, but is it possible to give me a little more information as to precisely where to look?

You see, when the pump cuts out, there is no way to tell where the noise is coming from.

 

[hotrodder]

The air pump is driven by the serpantine belt, it's above the alternator and the symptoms you describe sound normal to me... the pump is fairly noisy on mine too (although i wouldn't describe it sounding like a blowing exhaust), only runs for a min or so on a cold engine to help get the cat upto temp

 

[jazzdude]

It is not the same wirring sound that the pump makes. This was like there was a leak in the manifold and it just stopped instantly as if a switch was turned off.

Does this pump have anything connecting it to the exhaust and can this spring a leak?

 

[jazzdude]

Had a look when I started the car this morning. And the noise is coming from a round hole in the pump that faces Stauffer up. You can put your finger in it and it changes the pitch of the sound.

Should there have been something on this hole that has come off?

 

[hotrodder]

Probably, sounds like you were blocking either it's inlet or it's outlet (which obviously should be plumbed into the exhaut/cyl head by the exhaust ports). I've never needed to pay much attention to it so best i can do is... 14 Intake Manifold, Exhaust Manifold, Emissions Control System - M104 the first two pdfs under air pump might help

 

[jazzdude]

Seems strange that the hole is just open like that. It looks like it is the inlet and looking at it I am surprised there is nothing there but a large hole. Should there be a hose there or a cover?

 

[hotrodder]

I know it's not the easiest thing to photograph but if you don't have a friendly indy/someone that knows w124s well nearby can you post of pic (add an arrow/circle to the offending area as needed)? Going by the drawings in the link my '93 has the 2nd version i.e. i don't remember there being an air intake hose running down behind the alternator and from the quick look i've just had (too damn cold to have a proper nose around) mine has this...

which looks like it could be a baffle/cover over the intake?

 

[StevenN]

On a cold startup, I was hearing what sounded like a hole in a joint in the exhaust or a gap in the manifold and it would go away after about a minute or so which I put down to the the joint heating up, and closing.

I took it to an exhaust centre and when they examined the car, they said the rear box was corroded, possibly blocked forcing air out of a joint further up. In all honesty, very little seemed be coming out when the engine was revved up and once the new box was on, it seemed to 'breathe'more easily and the car is certainly quieter and more responsive.

But later on when I started it again, the same noise was coming from under the hood, coming from around the exhaust manifold area. Thing is, 20 seconds or so later, the noise just stopped, like it switched off.

I know there is an emissions air pump that comes on when the car starts. Can this make a sound like a blowing exhaust? If so where is the sound coming from and how do I fix it?

Hi Jazzdude
Did you solve the problem?
I have a similar noise on my W210

 

[Ade Vic]

Its the preheat system for the cats to do with emissions theres an air pump etc switches off within 120 sec of start up

Ade