Toyota Corolla Engine Smell

After Running Engine 10-20 Minutes. Smells Like Eggs

My Son's Corolla Reg 53 Developed An Odd Smell From Under The Bonnet, Almost Like Burning Plastic (no Sign Of Smoke). He Checked It Out And...

The Engine Starts But Won't Catch. There Was A Slight Fume Odor But No Leak Found. The Battery Tested Fine