Doug - that don't necessarily mean its bad. Your vacuum gauge is reading direct
pressure/vacuum. With the hose hanging out in free air - the needle is reading
0 regardless of what the outside/ambient pressure is. Map stands for manifold
absolute pressure and your scan tool is probably reading the difference between
the inside/manifold pressure and outside/ambient pressure - hence the name. It
gets a reading of the ambient or outside pressure whenever the key is turned to
the on position (same time it runs the fuel pump for a few secs). It is this
base pressure that is being compared to after cranking and while driving.
Basically the theory is that if you know the temperature and pressure on each
side of an orifice of known size, then you can compute the air/gas flow through
that orifice. This is also part of the reason why honing TB's produce such an
effect on our engines. Changing the orifice size in effect changes one of the
systems base formula's for calculating fuel requirements.
The best way I've found to test the MAP is with it off the truck - While
supplying 12V and GND (I use a pc's power supply) read the MAP's signal output
voltage w/a voltmeter. Using a handheld vacuum pump slowly increase vacuum and
watch the voltage output. It should be very linear and read the full scale (1
Bar). This pdf outlines the procedure:
http://www.championauto.com/championauto/pdf/49.pdf
Latr,
Shane
Doug Myers wrote:
> Hi Bob,
>
> So that was your scan tool reading that. When I hook my scanner up, the
> MAP is only sensing 12" of vacuum. I got the 15" reading directly off the
> intake manifold. Could I have a bad MAP?
>
> --
> Doug Myers
> -------------
> '99 Dakota RC/SB 5.2L/5-speed/3.92SG
> K&N Gen II, B&F Billet TB, MP cam, MP headers, MP catback,
> MP plug wires, Accel Supercoil, Custom Computer by Ron Zimmer,
> 180 T-stat, Centerforce Dual Friction Clutch
>
> "Bob Tom" <tigers@bserv.com> wrote in message
> news:5.0.2.1.0.20010521111858.009ed160@bserv.com...
> > The previous figures came from a freeze frame from the scanner while I was
> > waiting for the okay to go around the water pit on one of my runs.
> >
> > With an idle of 640 - 672 rpm (goes back and forth), gear in neutral,
> > baro. reading of 29.4 iHg, coolant temp. at 172F, the vacuum read
> > is between 15.9 to 17.6, with variance about 17.0 and then about
> > 16.0 and then back again about 17.0 etc. with the occasional read
> > of 15.9 and 17.6.
> >
> > Bob
> >
-- '96 IndyRam-HisIndy-MPI/TB/Pulleys/AccelCoil/MPComp/HookerSuperComps/CompTAs '96 IndyRam-HerIndy-numbered(#142)"Track Truck" '74 Triple-Black Dodge Challenger Rallye 360 home-brew EFI R&D vehicle '68 Black Corvette Convertible 427 (For Sale)
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