OMG! Exactly my story, even forgetting to put the radiator cap on before I left the house. The check engine light came on, I pulled over and turned it off. I popped the hood and sure enough, I forgot to put the cap on and it was sitting where I left it. There was heavy steam coming out of my radiator so that tells me the radiator had water, (water = steam). IF it was without water, there would be nothing coming out of the radiator. Now the car won’t start back up. It was running perfectly before I turned it off. It cranks but doesn’t fire. I had it towed to the garage and the mechanic tells me I blew the engine. The engine turns over, it just doesn’t start. How can I have blown my Toyota engine that quickly? And there was water and oil in my car at the time I turned it off. Is it totaled as he said? I find that hard to believe. It had been needing coolant more often that it should so there had been a preexiting coolant/water issue before this, but nothing major. It NEVER pegged straight up, the lever only went 3/4 up and not 100%- ever!