[Image of a diagram showing the firing order for a 1989 Nissan truck D21]
The firing order for a 1989 Nissan truck D21 is 1-3-4-2. This means that the spark plugs fire in the following order:
1. Spark plug 1 fires
2. Spark plug 3 fires
3. Spark plug 4 fires
4. Spark plug 2 fires
This firing order is important for the proper operation of the engine. If the firing order is incorrect, the engine will not run properly and may even be damaged.