A good
vacuum gauge can reveal a lot of different
things if you know what to look for, it
was actually an important tool in the old
days for diagnosis and still can be for
finding mechanical problems in
whats basically an air pump. If your
unlucky enough to get a no start because
of a plugged exhaust, and you wonder if
that's the cause do a cranking vacuum
test, if vacuum is non existent, now you
know,
also if you open the throttle all the way
while cranking and it pops very loud
repeatedly from the intake is another
giveaway, that pressure backs up in the
engine and has to go somewhere.
Hold on,
hold on, there's yet another way to verify
a plugged cat by doing a dynamic
compression test, simply remove a spark
plug and from your compression tester,
remove the shrader valve,
you don't want it to hold pressure at the
gauge. Make sure you ground the ignition
wire or coil, also unhook the injector
wire if equipped, first do a static test
and record the reading,
suppose you see a typical 150 psi, this is
your static reading. Now go ahead and
start it up and look at the running
reading, it should be about 50% of the
static reading, lets say 75 psi,
now briefly snap the throttle to wot and
watch the reading, it should rise to about
80% of the static reading, lets say 125
psi, suppose it goes beyond this reading,
lets say maybe 180 or more psi,
that indicates a plugged exhaust, but lets
just do another cylinder just to be sure,
and see if we get the same result, if so
your exhaust is plugged for whatever
reason.