Odd findings.
 

Tried to block the lower opening on the sides to prevent some excess flow into the engine bay and the pieces I was using was just enough to block the excess air, but not enough to reduce intake into the radiator.

The oil temp comes up faster and runs 10f hotter, the nose of the car feels VERY nervous and anyone passing me on the hwy (which is everyone) calls for a counter effort to stay straight.

Now, in 2 A/B tests so far, with them adds about 4% better mpg than without, but I took them off and looked at where the air goes and it provides air for ventilation slots behind the tire spats.

It's got me to thinking that some of the 'wheel well air curtains' that some of the german cars have might be something interesting to try.

Odd
 

The data and the experience is 'saying' something.These details are so small though,that a wind tunnel which can provide drag,plus pressure,pitch,roll,and yaw forces would be handy in sorting it all out.
I don't like the sound of the 'stability' issue.I've never encountered anything like it.If you have to do steering corrections for that small of a modification,I'd back off and restore the thing to stock.Safety should dominate the project.

What vehicle? Which opening? Ventilation slots?

Over how many miles was the 4% improvement? It could just be that the engine is warming up faster. In any case, safety first.

I thought most of the mpg improvement was due to the oil warming up quicker. It's on the only car in my garage. It is now without them due to the stability issues and this will be the second 'b' test. Test is over the course of 200 miles.

Is this it?
http://www.blogcdn.com/www.autoblog....review2011.jpg

You blocked off the holes where the fog lights go on some models?

http://cdnedge.vinsolutions.com/deal...dai-Sonata.jpg

Negative, each side about 3" of the lower intake.

Try blocking part of the upper grill instead.

Upper grill is fully blocked, has been for over 3 years.