NOTE: this exploit no longer works with the latest version 1.0.7 (updated 17 July 2012)
The step will only work when you're upgrading your equipment (any parts). At certain level, you can chose to buy the upgrade using cash or coins. If paying using cash, it'll then ask if you want to send the parts immediately by paying a small amount of gold coins (sometimes 1 or 2 gold coins). This is where the bug located.
Try to follow these steps:
STEP 1 http://i1151.photobucket.com/albums/o622/funnytone/aed86264.jpg example over here. I wish to upgrade my Intake valve from level 4 to level 5. USE CASH.
STEP 2 http://i1151.photobucket.com/albums/o622/funnytone/076c6307.jpg the game will then ask for confirmation. SELECT BUY.
STEP 3 http://i1151.photobucket.com/albums/o622/funnytone/a968df35.jpg It will then ask that you pay 2 gold coins For immediate use unless you're willing to wait 6 minutes. DO NOT PRESS ANYTHING.
STEP 4 http://i1151.photobucket.com/albums/o622/funnytone/d4992ec1.jpg Go to your Date & Time setting And adjust forward one month. In my example, I adjusted the month to August. RETURN BACK TO THE GAME.
STEP 5 http://i1151.photobucket.com/albums/o622/funnytone/a968df35.jpg SELECT DELIVER NOW
STEP 6 http://i1151.photobucket.com/albums/o622/funnytone/9c8126ff.jpg If you do it correctly, you'll see a handsome reward and set yourself for life.
Glad you guys could still use it. Wanted to keep shush about it to avoid the developer making a fix, but I guess having tons of gold coins would probably raise some notice. But since the cat is out of the bag, might as well share it with the rest.