Engine idles rough and even misses. At higher rpm engine smoothes out. At idle with foot on break engine loads up and will bearly run. Hard on fuel. Is this a vacuum leak or what?

my guess, car unseen.
car has an A/T trans, and all A/T jump from park to drive
selection is made on a cold engine, due to the very fast idle speed.
what was the park and cold idle speed. and out door temps.
the lower out door temps, make idle speed , cold engine, even higher
but when the engine reaches 180F or more and holds, the RPM
is about 800 RPM (the FSM tells the exact amount and does vary for the unstated engine option) so at 800 the tranny don;t lurch, did it on a hot engine. so if idle speeds are wrong say so and say it with engine temps. or its all meaningless.
next :
runs rough, the engine (even parked?) is called misfire.
misfire on a good engine (full compression)
is:
1: bad spark (or never had a tune up ever) every 60k miles.
2: bad fueling. too rich, too lean (low idle) bad injectors (stuck or clogged, or wrong fuel pump pressure or bad Fuel pressure regulator)
3: bad fuel.

there is a test for each possible failure. all very simple test.
it takes $80 in gauge tools to find engine problems.

1996 mustang You just go to the link I attached I'm sure you will get the solution of your problem.
Check it out once. it's instantly downloadable

This can be a vacuujm leak or a cylinder missing.

check for a vacumn leak around the cyl head by spraying carb cleaner near head (caution do not spray flamable fluids on hot exhaust) listen for engine speed changes or smooth running/ engine faultering / stall, ....... could be broken vacumn line also......

Possible Idle air control, simple to change on the throttle body

• does the engine miss? bumps is a rythmic pattern?
• an engine miss will appear to diminish at higher rpm due to centrifical force.
• other possible problem broken or flat motor mount.
• is the idle rough in reverse?
• if not
• open hood
• start engine
• Left foot on brake (hard)
• start engine
• watch engine as you select drive
• still holding brake with left foot / throttle gas with right foot
• does engine dramatically move?
• if so replace motor mount on offending side
• if not possible engine miss or transmission problems

With that kind of symptoms you should be displaying a check engine light have the codes extracted and get back to me let me know what codes and I will give you a better idea of what is wrong with your vehicle.

when you turn on the a/c the ecm increases engine fuel to compensate for extra load could possibly be a vacuum leak

run a fuel pressure test, if that checks out fine it may be a faulty coil pack or ignition control module

You have a vacume leak somewhere. You can try to listen for it or you can get a can of brake parts cleaner and spray it around the engine. Anywhere there is hoses or connections of any kind. When you find the leak the engine RPM's will increase. If thats not the problem it could be your EGR valve is clogged or your map sensor could be bad. Those things would usually cause a check engine light. Vacume leaks wont. Good luck