Look here: http://autorepair.about.com/library/firing_orders/bl-fo-9636.htm
I believe it's 1-6-5-4-3-2
For a 1990 Buick Century V6 3.3L engine, the manufacturer's recommended spark plug gap is .060" (1.52mm)
1-6-5-4-3-2
firing order is1432
the firing order is 1342
1,3,2,4 that is the firing order.
diagram of firing order on a 1990 Oldsmobile cutless 3.3 engine
Go to Google.com and type in the words "1990 Geo Tracker firing order" and it will get you to a site that will tell you.
1,3,4,2
Pontiac.on.nimp.org That's should help you with the firing order. KOCKNMOUTH
The correct firing order is 13726548.
1-6-5-4-3-2