Could it be that the lower temperature effectively lowers the vapor pressure of the gasoline in the tank? Therefore there is a better ratio of liquid to vapor during those cool times?
My car tank is 45 litres however when I full up from empty, the most I can put is 33 litres. I apparently have 12 litres in reserve and at 12km/l fuel economy, I can drive significant distance in my island home using that reserve volume. I want to be able to register more volume on my fuel gauge without having to modify the fuel sending unit.
Also, the vapor vent line has been clogged for the past 10 years. I have tried to fix it but it is permanently clogged due to a defective roll over valve in the fuel tank.
I want to get more fuel consumed between fill ups. Here is my experiment to lower the vapor pressure in the fuel tank without introducing liquid or vapor leaks.
Step 1: Finding the Return Line.
Step 2: Deciding on the Means to Chill the Return Fuel.
Step 3: Getting a Heat Exchanger.
Step 4: Mounting the Heat Exchanger.
Step 5: Testing.
Using my seek thermal infrared camera, I started the engine and measured the return line temperature after 10 minutes. It read approximately 33 Celsius.
Putting the air conditioning on, I tried measuring the same line after 10 minutes. Due to thermal bloom from a nearby power steering engine hose, it was difficult to get a good reading. The tubing however got cold to touch but since it is shiny, my thermal camera is useless to measure it. The air conditioning return line got predictably cold.
Step 6: Adding Insulation and Closing Up.
I installed some thermal insulation to help keep the chilled return line cold. I used plumber's mate and duct insulation to help reduce engine heat leeching into the mini heat exchanger.
I will update this instructables in the coming week to post my real world results.
Step 7: Test Results.
The first pic is after my second fill up after implementation of fuel chilling. I am pleasantly surprised by what my range is at slightly below half tank. I have not gotten such figures before in the 12 years I have owned the car.
Update 26 Nov 2015:
For my third fill up after implementing fuel chilling I got 530km on a tank! On fill up it took 32 litres. My fuel economy jumped to 16.21 km/l. I'm quite happy. I'm hoping this new trend becomes the standard for my car.