I also don't think you understand what contaminated metal is... dirt will not harm your metal, and oils will flare off... contaminated means, you're using metals with alloys you're totally unfamiliar with and are prohibitive of producing a quality of cast you desire using the methods you have at hand. Those wheels are chocked full of magnesium and manganese, both will give you problems if not dealt with under strict controls. Not to mention the cast product you do end up getting not only will have gas bubbles but will also be extremely brittle. Pistons and crank case metals will give you similar issues, as well. But... degassing and lower temps with pistons and crank cases will give you a workable product. Those blue cakes will work for these metals. I've suggested you to try that blue cake and yet you adamantly affirm what you're using will do the job... or should, at least, which implies you're not going to go that route. Learning this trade requires experimentation. What won't work every time for you should give you a clue it just may not be what you ought to be using. Those blue cakes are cheap as candy... it won't hurt you to try it.
Inside that furnace you need your crucible encircled by the flame and your melt not exposed to the combustion of gas and air, this is why I suggest to leave the skin on the melt till you're ready to pour, this helps keep the metal protected from the furnace combustion elements. This is true regardless what metals you're melting down. And as for melting wheels to cast into ingot, expecting them to be "cleaner", does not eliminate the difficult alloys, nor does it purify the aluminum. The ingots you make will have the same properties as the metal you've been casting. Shit in... shit out... in other words. Don't know how to explain it any clearer than that...
Congrats my friend... Now, you're learning. The visual acuities will take some time to develop. Photos don't do justice in showing folks what to look for... ya just about have to be stand'n there watch'n and memorizing as it's occurring to burn it (so to speak) to memory. Dial in all the other issues and all you'll have left is to witness what occurs within the pot, during the pour, etc., and burn every successful event to memory. Once you get far enough down that road of self experimentation you'll appreciate the two decades of results and conditions I've stuffed into my head from thousands of trial and error pours. The more you learn, the less fuel and time you waste. Start first with a full understanding what you have in your sand, too much?, too little?, need an additive??? Do research and be trusting of those with experience that can give you some insight how to improve your processes.