I once put cardboard in front of the radiator on my 77 F150. Then we had a freakishly warm day in Febuary (65 deg.). My truck overheated soo quickly that I did'nt even have a chance to pull over before the pressure blew the top off the radiator (sounded like a shotgun blast). It is cheaper to learn from others mistakes then from your owen. my $0.02

 

The cardboard blocking the radiator obviously won't make the engine warm up faster but it will allow it to hold the heat in the radiator once it is warm by reducing the airflow across the core. Many of the over the road tractors have a louver set up in front of the radiator that does the same thing. The cardboard is the Rube Goldberg version....

 

I WAS talking about warming up the whole engine block faster AND keeping it warmer long in EV mode.

It was 41'F this AM and I had EV mode in 5 blocks, and 3 minutes after an all night cold soak.

You cannot drive in EV mode unless the water is over 140'F.
If you are in EV mode already, the car will stay in EV down to 125'F then force the engine on, even if you have the brakes on at a light, or are in Park.

Blocking the radiator helps A LOT in winter if you do mostly low speed, in town driving, where you want EV mode as much as possible. It's much less help ( but still a little ) if you do mostly high speed highway driving, and EV would not be possible anyway.

With my aftermarket temperature gauge, I can say for sure, that my in town, slow speeds water temp without the carboard hovers between 125'F ( EV mode ) and 160'F ( ICE mode ). With the cardboard block, its about 30 degrees warmer. Never over 190'F around town. ( Over the highway it's 198'F at 70 MPH @ 50'F ambient )

To be honest, above 190'F the electric fans speed up, and draw more electricity, so over 190'F water temp may be counter-productive. This will never happen in my Colorado winter though!

-John