The Matrix is the wife's work car for the most part and also our trip car. She drives it most of the time and it therefore sits right around the EPA rating. I'm still working on teaching her to drive differently, and its working slowly but surely. Anyway, she makes a lot of trips from company to company throughout the day, many of them are short trips in the city. So, anything I can do to reduce warm-up time should be of benefit not only to fuel economy and emissions, but also to her comfort (yes, bonus points for the husband.) So, I very recently made two grill blocks for it. They've been on the car for almost two weeks now. I haven't pulled her milage book out of the car to see if its helped at all yet. Something to do this weekend.
Before
After. Upper grill block.
After. Upper and lower grill block. I ran out of fasteners so I used the upper ones on the bottom until I ran to get more.
Updates:
The block did work quite well to warm up the engine faster. I'd say it warmed up 20-30% faster than normal. It also increased intake air temp a few degrees (3-5°F) since the intake is behind the driver side headlight. The mileage gain is pretty impossible to tell. I have not done any A-B-A testing with it, and in normal driving it is not possible to notice. The material (foam board) did hold up, but not that well. It used to be black and is now whiteish-brown. I'll be looking to remake new blocks out of coroplast or something similar in the future. However, the foam board is still hanging in there. Its just not pretty.
I have removed the lower block now that its getting warm out because I noticed the radiator fan kicking in quite often when doing city driving.