The firing order for a 1990 GMC Safari van depends entirely on the engine it has. GMC Safari vans of that year came with several different engine options (e.g., 4.3L V6, 5.0L V8, 5.7L V8). You
must know the engine size to determine the correct firing order.
Once you know the engine size, you can find the firing order in your owner's manual or online through a repair manual specific to your van's engine.