Re: [DML] Cold Idle Problem

["Joseph Kuchan" <josephkuchan@xxxxxxxxxxx>]

For the benefit of the list, I respectfully disagree.

The vacuum that an engine produces is a
result of  how much air it pumps (referred to in some contexts as its  
"volumetric efficiency") AND the restriction to that air flow posed by 
anything upstream
from the intake, most notably the throttle plates at or near idle. That's
why vacuum decreases as the throttles open. Look at a vacuum/boost gauge at
or near WOT and you'll see this. Actually, as soon as the throttles open the
vacuum drops almost instantaneously. Thus, vacuum at idle depends a great
deal on having those throttle plates closed.

Another example: A jet engine flows LOTS of air. Is their a huge vacuum in
the engine? I'm not a jet engine expert, but unless there is some
significant restriction to the flow of that air, then the answer would be
no. Now, slap a piece of plywood over that engine's intake. What will
happen? The vacuum will instantly rise to the point where the plywood and/or
the engine will implode.

A final example: Draw air in through a straw. Draw a LOT of air. Is there
much vacuum in that straw or in your mouth? Probably not. Now, while drawing 
in the same
amount of air (you're now the "engine" with a fixed volumetric efficiency) 
place
your finger over the end of the straw.
Now that you've closed the "throttle" - just like magic - you have a lot of 
vacuum  in your mouth and the straw!

It takes air flow AND restriction to create that vacuum. It isn't "magic" at 
all. It's a phenomenon
known as Bernoulli's Principle. It's what makes venturis in carburetors 
work.

Diesel's don't have those upstream throttles, and guess what? They don't 
make vacuum. That's why they have vacuum pumps to run accessories. Unlike 
gasoline engines with their closed throttles diesels will run away from you 
on a downgrade if you let off the accelerator. Why? Becuase with no throttle 
plates to close there is no limit on the amount of air that gets sucked 
through the motor. In a gas engine the closed throttle restriction (which is 
VERY significant) creates vacuum (drag) that slows everything down.

-Joe Kuchan

>The engine produces the same amount of vacuum whether it's idling off
>cracked throttle plates, opened manifold screws, or CIS. All three
>accomplish the same thing.
>
>One thing cracked throttle plates will do that neither CIS nor the
>manifold screws will is open ported vacuum. On a stock DeLo setup that
>only affects charcoal canister purge.
>
>Bill Robertson
>#5939
>
> >--- In dmcnews@xxxxxxxxxxxxxxx, "Joseph Kuchan" <josephkuchan@xxxx>
>wrote:
> >
> > You should start by readjusting the throttle to the proper position.
>You
> > will never fix it if you make "masking" adjustments, although I know
>you did
> > that just to get by with the intention of fixing it properly at this
>time.
> > By turning the throttle adjusting screw you almost certainly reduced
>the
> > amount of vacuum the engine produces. That can affect your braking,
>among
> > other things, so start by getting that set properly.
> >
> > Next have a look at the idle speed motor and its controls.







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