Over the winter my car has always started on the first try without having to press the throttle. The last few days have been very hot and when I went to start it yesterday it fired up, then died right away. I was able to get it going by keeping the revs up for about 20 seconds, after which it idled perfectly at 1000rpm. If I let off the gas, it would stall. This morning it was cooler, around 40f, and it started up fine. By the afternoon it was around 75f and I had the same problem. Any ideas what I should check first?