The results you got are typical for a car and shows why grill blocks are so effective. The cooling system airflow on most cars is poorly designed, featuring an oversized, fixed size radiator opening with the various emblems and other visual doodads plastered on the grill, followed by the fan blades and the non-aerodynamic bulk of the fan motor creating a turbulent airflow into the radiator (this flow should be as laminar as possible). Then once the hot air exits the radiator, it has to negotiate its way through a cluttered engine compartment and out of the bottom of the engine bay creating lots of turbulent air under the car in the process.
The following link is about designing aircraft radiators, shows how it should be done. But it would take a bit of work to retrofit this into the average car using a pop-up cooling fan that would be retracted out of the airflow when not in use. When you compare the aircraft version to what you find when you look in the engine bay of the typical automobile, the lack of engineering finesse found in the car is staggering.
Contrails ! Radiator aerodynamics