I have a 5.0 GL 2000 model. Left valve is getting very hot and smoking. Right side seems fine.
new water circulator, thermostat, water pump.
I just cleaned and reinstalled manifolds and risers.
New plugs also.
Check the water flow to your left (port) side exhaust riser/elbow, if that check valve is getting that hot the riser may not be getting enough water to stay cool. Make sure the water outlet in the thermostat housing that feeds that side is not clogged.