Fuzzo

I can't remember if this was ever mentioned here but i noticed that my car sound very rough when I first start it. It goes away quickly enough but there is a noticable difference in the sound at first startup...anyone else notice this...

mdp c230k

Mine sounds like a diesel truck at startup! It goes away after about 30 seconds. I hope it goes away with time but I doubt it.

vadim

No, it won't. I was told that that's the way the 111 Kompressor engine warms up the cat. converter - by sending more air to it via opening up a gate from the supercharger.

Not sure if the C240's V6 does the same thing - it just might.

One other thing: under load (even mild acceleration) the car souns like there's an exhaust leak (manifold gasket?)... That characteristic sound: ds-ds-ds-ds... Can be heard very well when I'm driving close to a wall or a concrete divider with the windows down or the sunroof open...

Anyone else noticed this?

P.S. I'll try to borrow a battery-powered Marantz and record it some time next week...

BlackC230Coupe

to me when ever i floor it it sounds like shi* i hate the way it sounds it sound like its a desiel. i love the car but i just wish it sounded better. i guess i need to get a new exhaust soon hopfully it will help that problom.

Spyke

My first question to whoever told you that would have been "How do you warm up the converter by supplying it with (relatively) cool air?"

vadim

More air = more oxygen, helps burn unburnt products of in-cylinder combustion. It is no longer needed once the cat. converter has reached its operating temperature. Plus, supercharger-compressed air is warmer than the ambient air (it is drawn before the intercooler).

Fuzzo

personally i dont know if changing the exhaust will change much because what i noticed was that the noise was coming from the engine itself...

it was funny because when i first got the car .. this cutie at work came over one day and was talking to me and then she complimented on how quiet my car is.... now just the other day she mentioned to me how loud it is and if i didnt anything to it... so that why i noticed the crappy noise too....

Spyke

warmer than ambient air, yes, but that's why I added the "relatively". There is no way the "air" going to the cats is anywhere near ambient temp.

I'd have to check with one of the converter geeks at work, but I'm not buying the unburnt products argument. Anyway, for what it's worth every other manufacturer I've worked with that's trying to heat up a converter faster will move it closer the manifold (may even put a converter IN the manifold) and/or lower the mass of the material in the system before the cat so it doesn't take as long to heat up.

I guess if my car ever shows up I could just take a look at the dang thing!

vadim

It's up to you, of course, but that's what the converter does: burns unburnt products, to begin with. And more oxygen means more intensive oxygenation which in turn heats up the converter rather quickly. In my Volvo S70 the same effect is achieved by using an air pump, which can be clearly heard when the engine is cold.

shtatc

I really do not hear anything the first time I start it up in the morning, but I do notice it at start-up after I have been driving for a bit, then stop for a short period of time, then start it up again...seems like the motor is making alot of noise. It does not run rough, but sounds like alot of clattering. But then goes away after a bit.

Rick

It's interesting that all the people posting about this seem to have the C7 package, which includes a lower restriction exhaust system. Personally, my non-C7 car sounds smooth when I start it, unless it's a very cold day. Then I seem to get a more of a whining from the Kompressor until it warms up.

Spyke

I'd have to say this is a very oversimplified description of what a converter does (They need precious metals simply to burn unburned hydrocarbons?). Plus the fact it's called a catalyst would imply it does more, but that's getting further off topic and in fact we've got PhD's here to worry about that stuff so I don't have too.

But... Rick's comments rang a bell. His pointing out that this seems to be occuring with C7 cars reminded me that years ago I added a resonator to a system to address a particular issue; there was this raspy sound on start up and cold drive-aways that went away after the system heated up. I had forgotten all about that, but could very well be what's going on here. It sounded like something was loose and rattling in the exhaust system. Very annoying.

If I had my dang car I could talk about it more intelligently, I suppose I should just shut up until then!?!?:o

vadim

Rick

Not to antagonize you, but please remember that the C7 exhaust replaces the "front resonator" with a straight pipe, decreasing back pressure and increasing the "throatiness" of the exhaust note. I'm no expert at exhaust systems, but this seems to be the root issue. (I ommited that from my original post because I thought it was a moot point)

All the people complaining of this own C7 cars, and perhaps the issue is that the muffling effect of that resonator has been removed.

Spyke

Well, no, the precious metals are not there to aid in the combustion of unburned hydrocarbons. You've missed the point I was trying to make, but thanks for printing the definition of catalyst which addresses my point; which is that being a catalyst, it enables a chemical reaction rather than simply burns things off. Hence the precious metals, which wouldn't need to be there if burning things was the only goal. Unless of course you are refering to the chemical reaction as a form of combustion.

As for asking the Mercedes tech for sources, thanks but no thanks. The guy two offices down from me is currently working on the cats we'll probably see in about 7 years. No disrespect to anyone intended, but there aren't a whole lot of people around that know more about catalytic converters. (But if you've ever talked to a PhD about chemistry you may understand why I'm hesitant to start that conversation! You know, the tendancy to describe how to build a watch when all I asked was what time it is? )

All that being said, I'll stop the speculation and shut up about this until I have my own car and hear/don't hear it for myself...

vadim

Spyke

Yes Vadim, combustion is a chemical reaction. This does NOT mean that all chemical reactions are a form of combustion. So your comment is correct, but I'm struggling to figure out how it's relevant to what I said.

The website you listed does give a very basic overview of what converters do, but no info on HOW they do it. And not to nipick, but ceramic beads haven't been used in cats for about 20 years now. You'll notice the site never once mentions combustion. There is a reason for this. Because combustion is NOT the main reaction cats use to convert the nasty stuff into friendly stuff. (In fact burning raw fuel at the cat is to be avoided at all costs but let's not take this even further down that path)

My last comment on this, a re-cap: You stated "It's up to you, of course, but that's what the converter does: burns unburnt products, to begin with." I said that was an oversimplification. EVERYTHING you've posted since can be used to support my statement, and gives no mention to any form of combustion.

And that's all I'm going to say on the this cuz to be honest this whole subject is getting way off topic and in fact truly boring... so fire away, you get the last word.

vadim

mdp c230k

Not to add another variable to a great theory, but, I don't have C7 and my car makes the noises described. By definition, doesn't a cat cause a reaction to just increase in speed without being consumed itself? All the chemicals needed in the process must be present, right? Just curious.

renncpe

Not to question anybody’s information posted here. The Coupe and the SLK 230k both divert air to the exhaust system during warm up it may be more pronounced on the C-7 cars but it does the same for both. It really sounds like an exhaust leak for the first 60 seconds or more. As soon as the valve closes the sound changes.
Randy

90integraGS

My first car was a 1981 Pontiac Trans Am with the Pontiac 301 V8. It, too had an air pump that injected fresh air directly into the exhaust manifolds. Gotta keep that fuel burning!! It averaged 13 mpg and had pathetic power but the exhaust smelled clean!