Well,I was curious enough about the difference in the
two readings to go ahead and swap out my MAP
sensor,but it doesn't seem to be fixed. I did notice a
sound coming from the exhaust at WOT a while ago that
sounded like a clicking lifter,which I haven't heard
before,although I have noticed prior that the exhaust
sounds like crap. I mean,I've had a 3.9 truck of my
own ('99) before my boss bought this 2003 work
truck,and I don't remember it having such a
hollow,metallic,sputtery sound. I'll wait till it
cools down and climb under it,and bang on the cat with
the back off my hand. Hard to see a cat problem
causing power to come and go at random,unless the
brick's bouncing around. Problem does seem
vibration-sensitive though.
Thanks!
Steve P.
--- jon@dakota-truck.net wrote:
>
> 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?
>
>
> Its been a while since I've used a scanner on a
> vehicle so I don't
> recall all of the readings, and I've never used the
> DRB series of
> tools, so I'm wondering - is the scanner actually
> giving you a vacuum
> reading? Bernd mentions "MAP vacuum" (which seems
> like an oxymoron to
> me) as distinct from "MAP pressure", but its
> possible... Is the
> scanner actually giving you a vacuum reading or just
> manifold
> pressure?
>
> The reason I ask is because the computer doesn't
> really know what
> the vacuum is to be able to give such a reading.
> The MAP sensor
> (Manifold Absolute Pressure) is simply a barometer,
> which measures
> pressure, not vacuum, and there is no vacuum sensor.
> It is possible
> for the computer to give you a vacuum reading of
> sorts because when
> the key goes to the on position just before (and/or
> while) the engine
> is cranked, the computer can use the MAP sensor to
> determine the
> current ambient air pressure (barometric pressure),
> and then of course
> once the engine is running, it knows the manifold
> pressure, so to
> determine vacuum, it can just subtract the manifold
> pressure from the
> barometric pressure. (Of course, if the baseline
> barometric pressure
> reading is being taken during cranking, there is
> going to be some
> vacuum in the manifold because the pistons are
> moving, and the
> baseline reading will be at a lower pressure than
> actual barometric
> pressure, which will cause any subsequent vacuum
> calculation to be
> lower than actual.) Another problem with such a
> vacuum reading is that
> it is accurate only immediately after startup,
> because there is no
> ambient barometric pressure sensor, the computer has
> to assume that the
> barometric pressure is still the same as it was just
> before the engine
> is started. If you drive through a weather system
> or up and over the
> Rockies, or through Death Valley, any guess by the
> computer at the
> current manifold vacuum is going to be wildly
> inaccurate.
> (Fortunately this isn't an engine management problem
> because the
> computer doesn't need to know the vacuum to run the
> engine, it only
> cares about the air density, which is adequately
> conveyed by the MAP
> sensor.)
>
> --
> -Jon-
>
> .- Jon Steiger -- jon@dakota-truck.net or
> jon@jonsteiger.com -.
> | '96 Kolb Firefly, '96 Suzuki Intruder,
> Miscellaneous Mopars |
> `--------------------------------
> http://www.jonsteiger.com --'
>
____________________________________________________________________________________
Need a vacation? Get great deals
to amazing places on Yahoo! Travel.
http://travel.yahoo.com/
This archive was generated by hypermail 2b29 : Tue Oct 02 2007 - 15:23:18 EDT