Within this post I will included ways t make gold with engineering, and trust me there are countless ways to make gold with this wonderful profession, hang in there, you're in for a long read!
Selling pets is one of the best ways to make gold with engineering. As of recently blizzard has added two more engineering pets, once for each specialization, they are the De-Weaponized Mechanical Companion mechanical companion and the Personal World Destroyer. I have made a killing selling the Personal World Destroyer, I craft them at ~5-600g and they can sell upwards to 2-3k each.
If you can manage to snag yourself a lifelike mechanical toad schematic for cheap ( 1-2k or less ) learn it! I can sell these toads for 150g each, and they sell like hot cakes. I sell at least four or five pets a day, by simply selling this amount selling the pets has paid for the schematic and then some.
The Mechanical Yeti is also an excellent way to make gold with engineering, especially since the schematic is a little harder to obtain now, you need a reputation of revered ( correct me if I'm wrong ) to even start the quest, allowing you to control the market and set your own price for this pet.
Selling any of the other pets ( Pet Bombling, Lil' Smoky ) also make you some excellent gold, the materials are cheap and the pets can selling anywhere from 50-150g each, they do sell a little slower, but profit is profit!
Selling the New "Fish Feast"
Engineers now can make the new "fish feast" of cataclysm, it is called the Goblin Barbecue. Selling these at 75-100g per is great profit, especially if you're on a realm that raids constantly, they'll want the best buffs that they can acquire, and this is by far one of the easiest to get.
The Goblin Barbecue requires 2 Elementium Bars, 2 Toughened Flesh, and 2 Lavascale Fillets to craft. You can farm most of these mats to help reduce the costs and help increase the final ROI ( I recommend you farm the fish for the fillets )
Kaye Toogie - The Wormhole Ordeal
If you don't know about this now, go read up on it over at The Gnomish Coin. The way it works is, everytime you use your wormhole you have a chance to get the option "Underground", which takes you into a sewer where there is a vendor that sells rare engineering schematics. There are three total schematics, but one of them is what makes it all worth it, it is Schematic: Arcanite Dragonling. As of the shattering, this schematic is NO longer dropped by mobs and this is the only place to obtain it, I personally have sold two of them, each at 4,500g each. I use my wormhole whenever it's up, and I've gotten underground now three times, it's an easy way to make 3-5k within seconds.
Farming Primal Air in Outlands
I've found that farming Primal Air in Nagrand by using the Zapthrottle Mote Extractor is an excellent way to make gold! The place to farm them is in Nagrand, there are roughly 10-12 "clouds" that can be farmed.
What is this Zapthrottle Mote Extractor? Where do I get it?
The Mote extractor is a engineering device that "extracts" motes from Mini clouds that are all over Outlands and Northrend. To obtain the schematic, you have to go pick up a quest in Zangarmarsh. The quest can be started at Mack Diver, for the horde, and K. Lee Smallfry. The quest is fairly easy to do, it can be done in under 5 or even 10 minutes.
I have the schematic where do I go??
You are going to be flying around Nagrand, There are 10-12 clouds around that can be farmed continuously, especially since not many people know about the mote extractor. Click this map to view where the motes "spawn"
So why does primal air STILL sell? This stuff is from BC!
The main reason this stuff still sells is because people are doing the transmutation master quest, to either keep it and then drop it and changing it to elixir or potion spec, but either way, they need 5 of almost every primal that is available in Burning Crusade. Another great reason this stuff still sells is because of the twink enchant, 15 agility to gloves. I must admit, not many people have this enchant but the few that do have it, are most definitely buying primal airs, rather than going out and farming them.
Selling Fused Wiring
Check out this awesome post by yours truly over at The Gnomish Coin. Selling fused wiring is an awesome way to add a little extra to your daily income. Fused Wiring only costs silver to make, but sells for 1-5 per, and you get 5 of them per craft, if you haven't been selling them, I suggest you go do so.
Selling Engineering Mounts
This is another way to gold about making gold with engineering, it does require that you do have a more gold, but the return on all of the mounts is rather nice especially the choppers.
I'm not simply talking about the chopper, but crafting and selling the two engineering flying mounts is another excellent way to make gold, I bought my 280% chopper for 2.5k just because I didn't want to get all of the materials, time is money after all.
One last tip, selling the engineering bots is another excellent way to make gold.