Hole by Engine Coolant Cap/Best Cylinder to purchase?

[06YarisRS]

I am going to speculate that you have a blown head gasket. I say that with reservation because the frothy appearing substance on your radiator is nearly pure white. White smoke coming from a tailpipe often means that engine is ingesting coolant. You suspect that you've overheated a couple/few times. I still can't explain the hole in your radiator overflow outlet/inlet. Can you pull your oil dipstick and take a picture of it? If it's at all milky, that would suggest coolant in your oil. Maybe also removed your radiator cap and take a pic of that as well as down inside the radiator filler neck. 'Frothiness' there would suggest oil in your coolant. Given all of your symptoms, it doesn't sound good. That said, the 1.5L engine is very tough. Again, I would not drive the car anymore. It needs to go to a mechanic. It may be salvageable, but more driving is likely to be a death sentence. It's possible that a good cleaning, new fluids (in their proper locations) might fix things. Is there a possibility that whomever did your service put the wrong fluids in the wrong places? That's definitely happened before.

I think I ran Denso Iridium 3324 spark plugs in both my 2006 and 2008 Yari.

Edit, I keep looking back at the picture of the white stuff on your radiator tubing. It almost looks crusty, like salt. Is it crusty and dried? I know it sounds ridiculous, but could someone have put salt water in the cooling system?