Here's a good example on how a plugged cat and rising vacuum readings work:
Grab your favorite 16oz water bottle. Poke a small hole on the base and
then start sucking the air out of it. You can create a vacuum in it but the
hole in the base will keep it at a certain level. (The base is basically
working like your exhaust). Now, plug the hole and continue sucking the air
out of the bottle - you'll continue to raise the vacuum level inside the
bottle...eventually collapsing the bottle. Same thing in an engine with a
plugged cat (not that extreme though).
Vacuum in an engine (at idle) will be around 16-18". Under light throttle,
it will dip down (numerically) on initial throttle angle increase but will
then go up to about 20" at cruising. Under WOT, vacuum will rise to
atmospheric ("0" for ease of understanding) and will drop slightly after the
load decreases.
Load and throttle angle change engine vacuum readings to the point where
they are all over the place in daily driving conditions. Also....note:
Atmospheric pressure is "0" on a MAP sensor (normally WOT), "high vacuum" is
20"+ and normal vacuum is 16-18" - we're not talking about 2 or 3-BAR maps
here since Dodge engines don't use those anyway.
Scan tools will read manifold vacuum the same way - there just isn't any
other way to read it. What I was looking at was the expected MAP vs.
Load/Throttle Angle...that's was wasn't making any sense.
- Bernd
-----Original Message-----
Ok - I'm going to get a mechanical vacuum gauge and plug it into the
Ram's engine - and see exactly whats happening. Bernd - back when you
said you noticed the MAP readings on the RAM were going the opposite way
of what you expected? I hooked the scan tool up to my neon and noticed
that the numbers behaved in the same way. I'm not sure what that means.
Might mean that the scan tool operates differently? The Neon is running
just fine.
This archive was generated by hypermail 2b29 : Tue Oct 02 2007 - 15:23:18 EDT