I was thinking if you really wanted to get technical (and you had the crap lying around like I do) then a simple 555 timer circuit actuating a small servo from a model remote-control plane/car would do the job.
If you made a cam to fit on the output drive of the servo and had it in its normally unactivated position squash a piece of silicone fuel hose closed, and have the timer circuit drive it open for a short time every 1.5 minutes, then problem solved. When the ignition is switched off, the servo would return to its unactivated postion and the hose would be crimped shut. You could fit it all in a small black Jiffy box, even thread the fuel hose through.
Hobby Shops are shops that sell plastic models, kites, model trains, and radio-controlled planes/car/boats and helicopters. They also sell plastic and metal tubing, model paint, balsa wood and all sorts of expensive crap that appeals to the inner boy in most blokes

.
There's three in the city that I know of:
Hobby Place
280- 282 Lonsdale St Melbourne VIC 3000
ph: (03) 9663 4407
Hearns Hobbies
295 Flinders St Melbourne VIC 3000
ph: (03) 9629 1425
Victorian Hobby Centre
1st Floor 21a Swanston St Melbourne VIC 3000
ph: (03) 9650 4817
...but there quite a few scattered across the suburbs as well, just go to
http://www.yellowpages.com.au and put in 'Hobby Shop' and your postcode.
Mick