I had the same problem with my sedan kamm experiment:
http://ecomodder.com/forum/showthrea...dan-14330.html
I think what is happening is that now your car is poking a larger hole (wake) behind it! The sedan makes us think that the wake is as big as the rear profile of the car. (rear roofline,c-pillars and all the way down perimeter)
However that must not be the case even on my boxy 94 protege!
If the spoiler/kamm thing would extend futher back to the point where the cutoff point will provide a smaller cross section than the original cross section of the trunk/tail area,AND it did that while flow is still attached,then it would help reducing drag but now I'm talking about a boattail.
I think the sides and top of the kamm does not let air to spill in behind the c-pillars/rear widshield and it does not continue long enough to get the small tail cross section to occur like a partial boattail would, therefore increasing the size of the wake.
All I'm saying is that the aerodynamics on sedans (especially your car) is probably a lot better than we think.
A Kamm works on cars with the station wagon style chopped off rear but apparently not on sedans. Sorry man,I kinda went through the same thing.