[DMCForum] Re: [DML] My first tune up
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[DMCForum] Re: [DML] My first tune up


Joe Kuchan and I have been exchanging messages on this subject off
list. He sent me some valuable insight and we both thought it would be
good to copy the list for anyone else who might be having this


---------- Forwarded message ----------
From: Joseph Kuchan <josephkuchan@xxxxxxxxxxx>
Date: Feb 7, 2006 12:32 PM
Subject: Re: [DML] My first tune up
To: ryanpwright@xxxxxxxxx

Well there is always the remote possibility that you have another issue like
a cross-fire or misfire, but based on your description, especially since you
say that it happens on deceleration, I am 99.999% sure of what it is and
yes, it is normal. It might improve a bit if your air fuel misture happens
to be way off and you are able to get that dialed in right.

This even happens on cars with quiet exhausts, by the way, although it is
less audible due partly to the quieter exhaust. It is caused by a sudden
extreme vacuum condition in the intake that happens when the throttle shuts
when you decelerate the fast runing engine. It results in a mixture that
can't burn completely in the motor. Fuel that did not burn in the motor
accumulates in the exhaust and continues to burn there especially when
ignited by puffs from cylinders on their exhaust strokes.

By the way, that is one reason why there are little "backfire prevention"
valves in the throttle plate. When the throttle slams shut there is a
collision between the moving air and the barrier of the closed throttle. The
backfire prevention valves allow a bit of air to pass through, reducing the
extreme vacuum condition in the intake and also reducing an extreme rich
condition downstream from the throttle.

When you decelerate and the throttle snaps shut downstream from the throttle
it is actually getting extremely rich. That's because there is still a bunch
of fuel flowing through the injectors because the motor was running at high
speed. Now suddenly the throttle snaps shut and except for the air passing
through the backfire preventers, there's not enough air for it to burn
completely. The unburned fuel ends up in the exhaust and burns there. That's
what you are hearing. I have even seen flames shoot out of exhaust pipes
from this in extreme cases.

Not sure if this is clear, but rest assured all is "normal". A quieter and
more restrictive exhaust would probably get rid of this noise. From
listening to your sound file I would say you have a mild to moderate case of
the gurgles. If you are unhappy with it, the cure is to make sure your CO
adjustment is correct and then get rid of the exhaust you have and get a
quieter one.

By the way, one thing that can really aggravate and even cause this is an
air leak somewhere in your exhaust. Even a small leak can let in enough air
for the combustion process to really take off inside your exhaust system.
Make sure the system is tight. If it isn't, simply fixing any leaks in the
exhaust may stop much of the racket.

BTW, this answer may help others who are experiencing the same issues, so if
you want to post it to the list feel free to do so.

-Joe Kuchan

>From: Ryan Wright <ryanpwright@xxxxxxxxx>
>To: Joseph Kuchan <josephkuchan@xxxxxxxxxxx>
>Subject: Re: [DML] My first tune up
>Date: Tue, 7 Feb 2006 11:30:24 -0800
>Really?!? That's normal? I don't like it. If I switched to a factory
>exhaust, do you think it would go away?
>I mean, people ask me what's wrong with my car. Passengers grimace and
>say, "Wow, she really needs a tune up." Friends say, "When are you
>going to get the engine fixed?"
>On 2/7/06, Joseph Kuchan <josephkuchan@xxxxxxxxxxx> wrote:
> > The sound you are hearing is absolutely normal for the exhaust system you
> > are describing. It is definitely a low back-pressure, free flowing exhaust.
> > The sound you hear is actually unburned fuel that is entering and
> > accumulating in the exhaust and then burning there. I am short on time now,
> > but I will follow up with a more detailed explanation of what you are
> > hearing and why you are hearing it.
> >
> > Personally, I would describe it as a "gurgle" or "burble" or "popping" all
> > of which are pretty common ways to describe that sound. A lot of people
> > (including me) like it when it isn't overly loud. Its sort of a
> > "performance" sound that is very common to low restriction exhausts.
> >
> > I'll send you more info later today or perhaps tomorrow.
> >
> > In the meantime, don't worry. It's all normal.  :)
> >

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