Engine Spins...Won't Fire
 

Had to replace the water pipe between water pump and thermostat. This required loosening the thermostat mounting bolts and pulling the thermostat back to make room for the new pipe. To make it easier, I also removed the intake manifold, and while I was there, walnut blasted the intake valves (Gunked up pretty bad after only 1.2k miles since last cleaning - with a catch can!). The problem is: The motor will spin over fine but will not fire. I have, in the past, replaced the thermostat and cleaned the valves, with no starting problems afterward. I've double-checked all the sensor connections, etc. The check-engine light is on but only code shown on ScanGauge is p115c, which was there before i did the work. The only thing I did differently is I used the crank pully to position the valves for cleaning and I MAY have rotated the engine backwards at some point. I read somewhere that doing that is a big No No. Am I screwed? Can I (we) get me out of this? The nearest Mini capable shop is 100 miles away.