pretty sure the auto air vent is inside the combustion area on the side of the heat exchanger on a 24i which presents a problem as you should be a RGI to go in there as you need to prove all is well before allowing the boiler to run again.
I think its probably not allowing the air out keeping a nice blanket of air through your exchanger which is why its overheating.
dont think you'll get the old fella running until thats ruled out.
iain
September 2008