Ideling wil cause most engines to raise in temp.
Does the temp just keep rising or does it level off.
What temps are you reaching?
How and where are you measuring the temp?
BTW...
there is more to fans than diameter.
The contributing factors are:
Diameter
Number of blades
Blade pitch
Blade length
Blade shape
Example...You could get more air flow out of a smaller diameter fan that has a steep enough pitch
