This means that the spark plugs fire in that order, starting with cylinder #1 and ending with cylinder #2.
To determine the cylinder number sequence:
* Stand in front of the engine.
* The cylinder closest to the driver's side is #1.
* The cylinder closest to the passenger side is #4.
* The remaining two cylinders are #2 and #3, and their positions can be figured out by following the sequence from cylinder #1.
It's important to note that this firing order is specific to the 4-cylinder engine found in the 1990 Ford Mustang. Other years and models may have different firing orders.
