Vacuum Leak?

[kampala]

I think I need a little help.  The car is a later 250sl, manual with power steering and no AC.

After warm-up, I am hearing a hissing sound from the right front area of the engine bay, around the air filter area.  it's not the hissing while cold we hear from the injection pump air filter but sounds just like that but from the air filter area.  Is this normal?

I am asking as the car does not run as it did ... the idle is a little rougher and a little lower at about 600 rpm and from a standstill, the car does not run well until its over 2,000 rpm.  Almost like it missing ... but It seems fine above 2000 rpm.  During low revs, I need to allow the revs to get higher before I can engage the clutch.  In addition, I am noticing more backfiring ...

I have tried to check each hose coming into the air filter area but can't seem to find an obvious leak ...the hissing sound seems louder if I listen from the grill but it's clearly in the right front corner of the engine bay ... if this noise is normal, I will attack elsewhere, but wanted to check this first since I just don't recall hearing this before ... but may have just not noticed. 

Your suggestions would be appreciated,

Thanks

oz

[Benz Dr.]

It's probably the hose going to the idle air screw.  Get that screw adjusted in just the right spot and it will make quite a bit of noise.

[garymand]

I'm thinking a vacuum tubing leak would be too small to change your idle that much or cause a studder up to 2K.  The idle air adjuster comes to mind, it makes a loud hiss if the hose is off but that wouldn't give you the studder sypmtoms, it wouldn't effect the running of the motor at all, it would just make an obvious loud hiss.  You might want to take that hose off the air filter canister and see if that is as loud as the noise you are hearing, just for comparison.

Sounds more like an intake leak maybe a gasket, but a volume of ~1/4 inch total leak.  Is it should be really loud, obvious hole sucking air, (or is it just a tiny soft hiss)?  Either way you need to find it.  Use a few feet of hose, one end to your ear and the other end -hunt around the engine until you locate the source.  Stay away from the fan blade.  Check around the butterfly and around the intake manifold gaskets.  The sound will be really loud when you get the hose near it.

[stickandrudderman]

A smoke test will certainly find any vacuum leak that you might have.

[Benz Dr.]

A smoke test will certainly find any vacuum leak that you might have.

Great machines, aren't they?

[garymand]

I've always wanted to see someone use a smoke machine around a car.  I guess you'd also have to be a bee keeper.  I saw a guy use deoderant once. when the doderant got to the leak the motor reved up.  You can probably more easily find a propane torche in a garage.  open the torche but don't light it !  I think the rpms go up.  I've never found a leak that way.  Just remember, don't light it 

[Benz Dr.]

Smoke machines are the cat's ass when it comes to this job. It fills the intake manifold, or anywhere vacuum would be present, with a chemical smoke. The smoke isn't like regular smoke in that it seems to disappear quickly after it comes out of a leak.

You simply attach the hose from this machine to the brake booster pipe fitting and then flip the switch. Smoke will pour out of every leak, big or small, until you shut it off. Great for locating the almost impossible to find leaks under the dash in later model cars like a 107 or 126. The smoke leaves no smell, no fumes, and no residue. And, there's no chance of blowing you or the car up.

Down side is that it's an exspensive machine and it has to be refilled with a cartridge once in a while. I can't justify buying one so I borrow one once in a while if I need it. Great for finding leaks on V-8 manifolds. I don't think you could do it any other way as effectively.

[kampala]

Appreciate all the advice ... since I don't have a smoke machine in my back pocket,  I resorted to some basics.  I did try using a hose as a stethoscope and was able to zero in on a small leak.  I don't know if this is the only one impacting the poor running as it seems small and I'm not entirely convinced that this is 100% of the hissss I hear as I just didn't have time to pursue it further today ... more checking tomorrow.  Here's what I have found so far.  I am hearing a leak where the flex air pipe connects to the venturi, this is the pipe between the air filter and the venturi.  I hear some hiss on this connection.  Snugging it up does not help as it feels bottomed out.   I sprayed some starting fluid with the engine running on this area and I expected the idle to go up ... it did effect the idle but the idle went down ... I will retest and remove and reseal.  I tried to put a some plastic tape over it just to see if sealing it improves the running.  It seems to help a little.  Will go after this first and see how it reacts.  I checked the idle screw hose and seems okay.  The engine breather hose where it connects to this area is a bit cracked so I will replace that just in case.   Thanks very much for the help ... hopefully I am on the right track.

[stickandrudderman]

Dan, my smoke machine is my very favourite tool!
As you say, great for v8s, but also good for finding wind noise or water ingress problems too.
Yes, a bit on the pricey side, but if you're in the game it's a must-have.
See here:
https://www.youtube.com/watch?v=ACq1DaPtOt0

[garymand]

I was going to mention the air breather hose.  It won't cause the hiss, but it cracks on the side you don't see.  Then incoming dirty air bypasses the filter leading to early piston and ring wear.  It is expensive ~$90, but critical to keeping your engine young. 

Thanks guys for the smoke tutorial.

[garymand]

I found some nice links to how to make your own smoke machines.  I really see how valuable they are for W126.  Take a look at this guy> http://www.bing.com/images/search?q=homemade+evap+smoke+machine&qpvt=homemade+evap+smoke+machine&FORM=IQFRML#view=detail&id=9651B8295A9964D6D68642A00AAAF69200A960D8&selectedIndex=37

I think he's blowing into the hose... :