Tempering spring steel has me scratching my head over color changes

I've been working on a project using old leaf springs to make some simple tools. When I temper the steel after quenching, the colors don't show up like they should. I set my oven to 400 degrees Fahrenheit, but the steel turns blue way too fast instead of that straw yellow I'm aiming for. I even bought a new oven thermometer to double check the heat, and it says it's right on. Maybe the steel has different stuff in it from being in a car for years? I tried lowering the temperature a bit, but then the steel stays too soft. Has anyone else had this happen with recycled spring steel? I'm really curious if there's a trick to getting the temper right on this kind of material. Any tips would be awesome.