Full upper grill block on 2007 Jeep Grand Cherokee...Fail
 

I thought I'd try to get one more aero mod in before winter... the almost always positive upper grill block. I doubted that there would be any issues with cooling since I now had a 2 inch lower air dam scooping air up into the radiator inlet below the bumper.
I built a nice block out of 1/8 inch Black ABS and attached it with black zip ties behind the grill with a small opening for the engine air intake
No cooling issues, however, I noticed that it was taking noticeable more throttle to maintain highway speeds. I used the onboard mileage readout, and I had lost 2 to 3 MPG over my highs of 25.5 to 26.5 mpg I had gotten in July after the air dam and new O2 sensors were installed. It looks like I need to do some more testing of partial upper blocks.

In my minds eyes I suspect that since there is about 15 inches completely open space between the front of the engine and the radiator and the extra vacuum created behind the now lower air dam mounted directly below the radiator, that the air coming through the open upper grill was filling this vacuum, so drag losses under the hood were probably greatly reduced over most vehicles where the air coming through the radiator immediately impacts on the engine and accessories. With the closed grill, air flow into the grill now has to be pushed over the vehicle and the increased vacuum formed behind the air dam was creating more drag.
I noticed that the vehicle was much more planted on the highway with the grill blocked, probably due the the reduced pressure under the hood and the effect of increased pressure on the top of the bumper, which acts as a front splitter to create downforce.

Back to a cardboard block next spring for testing.

An interesting result, and diagnosis.

It could be confirmed with a 15" partial bellypan behind the air dam.