[mwebb]

on the vacuum gauge , wander is ok , normal even
if it had flutter the needle would be a blur of motion .
now
measure engine vacuum at 2k rpm hot
should be 2" above idle value
then WOT snap decel
vacuum should be around 24" as the engine slows down from peak RPM of the WOT snap
WOT = Wide Open Throttle
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Long Term Fuel Trim is one of the PIDs, Parameter ID s that is present in
OBD2 Generic Data

i have used scan gauge 2 only 1x and there is a way to scroll thru various PIDs

somewhere in there
is LTFT , Long Term fuel Trim .
a rule of thumb is up to +10% is ignition misfire
above that is fuel related
so 11% and up is fuel related ....

at 2500 rpm hot or better still at cruise at 40 to 50 mph
and
at hot idle
i would like LTFT , 2 values , which may be the same
if the value is zero , the system is really pissed off and is in open loop
in closed loop LTFT will never be zero . may have a + 0r a - value

these are not carved in stone values and are useless with multiple cylinders misfiring ....
but
i am trying to use what you have to solve the problem ....
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if LTFT is adding high percentage at idle
but close to zero at 2k rpm or cruise , you have a vacuum leak after the MAF sensor .
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we can also use calculated load to see if your cat is clogged
but , one thing at a time