Not too late to join in though,Phil. Especially this
thread!
My readings were taken at idle. I thought the MAP was
bad after getting these two "opposing" readings. Found
out that a 3.9L Dak uses the same MAP sensor as an
4.0L Jeep G.C.,and so I thought I found an easy way to
finally get the truck figured out without wasting
$90.00. What happened was I found yet another part
that isn't causing a problem,which is just tremendous.
Thanks!
Steve P.
--- Phil Jenkins <bugnik@yahoo.com> wrote:
>
> Sorry if I'm late to this.
> Were your readings taken at idle? If so they are
> inline with mine.
> With key on, engine off, MAP reading is 29.
> At idle MAP reading is 10. Vacuum gauge is 19.
> The MAP sensor and vacuum gauge should not have the
> same reading. HTH
>
> Phil
>
>
> --- Steve Preston <steveophonic@yahoo.com> wrote:
>
> >
> > This helps me a little bit. But it creates a new
> > question in my mind: should a person have the same
> > reading from their MAP sensor via scanner that
> they
> > would have from a vacuum gauge? Because I,in
> > fact,did
> > not get the same readings. The scanner said 10 in
> > HG,and my gauge said about 19. But your
> subtraction
> > that you came up with is making me wonder whether
> > there should be a disparity?
> >
> > Thanks!
> >
> > Steve P.
> > --- Phil Jenkins <bugnik@yahoo.com> wrote:
> >
> > >
> > > Ok, I hook up my scanner with engine off. I have
> > 29"
> > > of barometric pressure from MAP sensor. Start
> > truck,
> > > at idle I have about 11" showing on the scanner.
> > > 29-11=18" of vacuum. I plug the exhaust and have
> > 20"
> > > on the scanner. 29-20=9" of vacuum. Less vacuum
> > with
> > > exhaust plugged, higher reading on scanner. Does
> > > that
> > > sound right? (Pouring some gasoline on the
> fire.)
> > >
> > > Phil
> > >
> > >
> > > --- "Bernd D. Ratsch" <bernd@dodgetrucks.org>
> > wrote:
> > >
> > > >
> > > > Proper wording may not be quite right and
> we're
> > > all
> > > > over thinking it now.
> > > > To make it easy - grab a scanner, plug your
> > > exhaust,
> > > > and put a load on the
> > > > engine under light throttle - watch the MAP
> > > reading.
> > > > Does the same thing
> > > > every time.
> > > >
> > > > - Bernd
> > > >
> > > > -----Original Message-----
> > > > From: jon@dakota-truck.net
> > > > [mailto:jon@dakota-truck.net]
> > > > Sent: Sunday, September 23, 2007 3:15 AM
> > > > To:
> dakota-truck-moderator@bent.twistedbits.net
> > > > Subject: Re: DML: RE: Re: Vacuum 101
> > > >
> > > >
> > > > "Bernd D. Ratsch" <bernd@dodgetrucks.org>
> wrote:
> > > >
> > > > > Again, refer to the plastic bottle post.
> > There
> > > > are things such as
> > > > positive
> > > > > and negative pressure.
> > > >
> > > >
> > > > I don't know that the bottle analogy holds
> in
> > > > this case though
> > > > since it concentrates only on the vacuum side
> of
> > > the
> > > > engine and
> > > > ignores the pressure events. That is, we're
> not
> > > > trying to suck
> > > > through the exhaust, we're trying to blow
> > through
> > > > it. (Read on below
> > > > for my more excruciatingly detailed thoughts
> on
> > > > that...) ;-)
> > > >
> > > >
> > > >
> > > >
> > > > > The pistons pulling the air into the
> cylinders
> > > > (via the intake) does keep
> > > > > vacuum in the intake stream.
> > > >
> > > >
> > > > Yep - that's the *only* reason there is any
> > > > vacuum in the engine; the
> > > > pistons are pulling against the back of the
> > closed
> > > > butterflies at idle
> > > > with only a tiny IAC passage through which air
> > can
> > > > pass through; its
> > > > basically the same as trying to suck air
> through
> > a
> > > > straw - there is a
> > > > vacuum created in your mouth. As the throttle
> > > > blades open, the straw
> > > > gets bigger, but there is still some vacuum in
> > > your
> > > > mouth until the
> > > > straw gets so big that the restriction becomes
> > > less
> > > > than the power of
> > > > your lungs.
> > > >
> > > >
> > > >
> > > >
> > > > > The opening of the exhaust valves releases
> > > > > that though the exhaust piping. Plug the
> pipe
> > > and
> > > > you can't release the
> > > > > vacuum.
> > > >
> > > >
> > > > I'm not sure that I buy that; or, more
> > > correctly,
> > > > I guess my
> > > > problem with it is your considering vacuum to
> be
> > > > something that can be
> > > > "released". I'm more comfortable with the
> > notion
> > > > that a vacuum is a
> > > > *lack* of that something; its a void. You can
> > > > release pressure, but
> > > > vacuum is something that you relase *into*,
> not
> > > > something that is
> > > > released itself. (Well, I guess except for
> > > exposing
> > > > a weak vacuum to
> > > > a strong vacuum, you might think of the strong
> > > > vacuum being "released"
> > > > into the weak vacuum, but I'd prefer to just
> > think
> > > > about it as a
> > > > pressure equalization. Whenever I try to
> think
> > > > about vacuum as some
> > > > entity moving from place to place, my brain
> > flips
> > > > out on me.) :-)
> > > >
> > > >
> > > > Anyway, back to your point. Regardless of
> > how
> > > > one thinks about
> > > > vacuum or pressure, I still don't think your
> > > > description is correct
> > > > because of the combustion event between the
> > > opening
> > > > of the intake and
> > > > exhaust valves. Let me try to explain what my
> > > > thinking is by an
> > > > example. Lets consider an engine which has a
> > > > certain vacuum level in
> > > > the intake manifold. The intake valve opens
> and
> > > the
> > > > piston drops,
> > > > which creates a vacuum in the cylinder which
> is
> > > > greater than the vacuum
> > > > in the intake manifold. This allows air (and
> > > fuel)
> > > > inside the intake
> > > > manifold to be sucked into the cylinder (and
> > > > likewise, air is being
> > > > sucked through the throttle body into the
> intake
>
=== message truncated ===
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