[brucepick]

Do you have a ScanGauge or other that shows the coolant temp? IMHO, analysis only by whether the fan comes on - or by watching a gauge needle - isn't very definitive.

You actually ran with full grill block last summer, driving the same type of roads and speeds and trip length that you are now? And rad fan didn't come on much last summer?

My car seems to need the grill blocking removed when the weather warms up, or the coolant temp will go up and the rad fan does come on. Are there actually drivers driving long runs or highway trips with full grill block in summer?

As I see it, with a full grill block, the only way to get an appreciable quantity of air through the radiator is to run the fan - unless you're running a/c with the temp slider somewhere in the middle, which will cause air to run through the heat exchanger - essentially the same as running it through the radiator.