It grenaded. There wasn't a good way to stop them. I forget what it was but I think it was a fuel shutoff was how you normally stopped them. But if the engine ran away the fuel shutoff didn't kill it.
Everyone got a safe distance away and we just watched it blow a hole in the side of the block.
I've heard of some mechanism on some engines that floats the valves open as an emergency shutdown, but otherwise if its scavenging fuel from the sump, all bets are off!
What's a "safe distance"?