[brucepick]

Could be that there's massive cooling air going in through the lower "grill". Take a look at the positioning of radiator and a/c condenser also.

My car has the radiator on one side and condenser on the other, with the cat converter behind the condenser (cat is built into the exhaust manifold). I only block the radiator in winter but keep the other side blocked year 'round. ymmv, of course.

A ScanGauge will show you digital engine temp. That beats listening for the radiator fan to tell you that you have too much grill block.