Water and unburned fuel in exhaust while engine is running

I bought my 1995 4runner over a year ago. I finally had a chance to remove the plenum and replaced the injectors and replaced all the heater hoses on the engine. I finally got a chance to drain my radiator and ran water via water hose into two heater hoses to flush out the rust inside the engine and heater box.
After the flushing, engine ran rough as if water was inside the engine.

As the engine was warming up and idled very poorly , water mixed with gasoline (only gasoline, NO oil) shot out the exhaust muffler onto the floor. Within minutes, more water mixed with gasoline was over the floor.

I turned off the engine, and pulled the spark plugs and discovered that plugs from cylinders 4 and 6 were covered with water mixed with gasoline. All other plugs seemed relatively drier.

Questions:
1) Engine ran fine (not perfect) before the removal of the plenum. Did I screw up with the reassembly of the plenum that allow water got into the cylinder?
2) Where is the damage (head gasket, cracks) that caused the water/gasoline n the exhaust?
3) why there is NO engine oil mixed with water coming out through the exhaust pipe?
4) Did the previous owner add chemicals to the engine cooling system to keep engine running before I flushed the chemical out which caused water in the exhaust?
5) Is there anyone else on the foum has similar problem like mine?

Any comments are greatly appreciated.

Thnk you