I've had a couple of thermostats lately that passed the hot water test but failed to work properly in the car. Both times I had to scratch my head for a while to figure it out. My Duster ran cool but took a long time to heat up. The thermostat worked properly when dunked in hot water but when I ran the engine with the radiator cap off I could see water flowing even when the engine was dead cold. I had kind of the opposite issue on another car where it was hot but thermostat checked out. Evidently in practice it just wasn't opening nearly enough for some reason. One thing I discovered is that there are a lot of super cheap Chinese garbage thermostats on the market these days. Evidently the Chinese can't read blueprints and don't understand QC. My advice is buy high quality and then double check operation in the car.