Could be:
1. One or more cylinders are not firing.
2. Broken motor mount.
3. Needs a new air filter.
4. One or more spark plugs have fouled.
5. Bad spark plug wires.
I believe that was an ( A4LD ) automatic transmission with the 4 cylinder engine on a 1989 Ford Mustang
In a 2000 Ford Mustang , 3.8 liter V6 , automatic transmission : The automatic transmission fluid level dipstick is by the firewall , in the area near where the transmission bolts to the rear of the engine , on the PASSENGER side of the engine in the engine compartment
On a 1996 Ford Mustang : The automatic transmission fluid level dipstick is located in the engine compartment , near the firewall , on the passenger side of the engine in the engine compartment ( use a funnel )
The Ford Mustang GT has a V8 engine, while the Ford Mustang has a V6 engine.
The 2006 Ford Mustang has a V6 engine.
The 2009 Ford Mustang has a V6 engine.
The 2007 Ford Mustang has a V6 engine.
The 2008 Ford Mustang has a V6 engine.
The 2012 Ford Mustang has a V6 engine.
The 2005 Ford Mustang has a V6 engine.
The 2003 Ford Mustang has a V6 engine.
The 2013 Ford Mustang has a V6 engine.