Click&Clack , I find it hard to imagine these guys both have degrees from MIT, recently wrote about cardboard in front of the radiator. I agree it does help your vehicle warm up quicker, but it does not make it run hotter. The thermostat controls the temp, it may cause the fluid to remain in the radiator longer as it will take it longer to cool, but it will not increase the temp of your heater output after initial warm-up My suggestion is to place a piece of carpet, not rubber backed between the radiator and the condenser or in front of the radiator if you don’t have air. A sample is just the right size. This will not block the air flow but will restrict it and with today’s tight engine compartments I don’t think the air flow should be blocked totally. The other method if you can’t finds a piece of carpet is to use cardboard and cut a hole in the middle about the size of a cup saucer. Keep an eye on the cardboard as they tend to get wet and make a mess that is difficult to clean out of the radiator and or condenser fins. I know summer is coming and this is out of season but I felt I had to comment.FYI. Before it is AC time check the radiator and condenser for any junk that has built up, also check between the condenser and radiator as this is where a lot of problems originate.