The coolant temp sensor only really has the ability to make the engine run rich, it really can't do anything other than that. Before replacing it (again, apparently), just check it with a multimeter withe the engine cold and hot. Google up the correct resistance readings it sohuld generate. I doubt this is the problem. It intake air temp sensor is like the coolant sensor in that all it does is richen the mixture when the air is very cold, but its effect is much less than the coolant temp sensor's. Very exceedingly minor tweak to the mixture.

The aux air valve's only job is to open up when the engine is cold and raise the idel speed. It should be fully closed when warm. They tend to get stuck in their old age - in some odd position. I'll bet if you looked under the hood of a bunch of casually maintained D-jet cars you'd see half of them with bypassed and plugged up air bypass hoses.

The throttle valve on a D-Jet should close 100%, not allowing any air by. All the idle air (other than what passes through the aux air valve) passes through a little bypass underneath the throttle - with a tapered needle adjuster facing forward for fine tuning.

Does the engine just stumble and die? Does the idle speed drop until it dies?
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