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Several possiblities on the idle:
1) The throttle valve isn't closing fully - possibly sticking slightly open. Unlike a carb throttle theidle is not controlled by cracking the throttle valve open slightly. It is supposed to be fully closed. If the fast idle goes away when you wiggle and jostle the throttle under the hood it's something to look at.
2) The idle speed is actually controlled by allowing air around the throttle in two ways. One is the adjuster screw on the bottom side of the throttle. Not much of a problem area there. The other is the cold idle air valve. It is bolted toe the side of the head just in front of the manifold, and connected to the manifold by two fairly large hoses. One hose attaches to the manifold in front of the throttle, so it pulls in filtered air. The other attaches to the manifold behind the throttle, so it wants to pull air in at idle. The valve senses the coolant temp and opens and closes to allow more or less air in past the throttle to raise the idle speed when the engine is cold and cranky. The are rather finicky and your typical 30 year old one is stuck somehow, either open, shut, or somewhere in between. You might check it with the engien cold then hot to see if it is operational.
3) Vaccum leaks - I't doesn't matter how air gets into the manifold. If it's in there the D-jet brain will note the resulting manifold air pressure and inject the proper amount of fuel. So leaks anywhere past the throttle will raise the idle. They can be hard to track down. The intake manifold gasket, each individual injector, the brake booster, etc.
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