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, 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.