My ’86 with a 4.9L engine is idling too fast (don’t know the rpm) and diesels when I shut it off. I read and made the adjustments called for on the sticker but actually everything was correctly adjusted. I think my ECT (engine coolant temp) sensor, which you need to disconnect when you make that adjustments, is defective and giving the controller a bad signal…. but what do I know. How would one check that sensor out? Or is there another obvious problem that I haven’t thought of?
Thanking you in advance for what I know will be good advice.
Bob

 

I would check for a vacuum leak first, and if you can't find one, and have done the carb fast idle adjustment properly, with poor results, go ahead and replace the ECT. They're pretty cheap for your truck. That should solve it.

 

Ah! The dreaded vacuum leak. There is plenty of opportunity for that on that engine and would explain the high Idle and also air to keep the engine running after shutoff.
Good reply... thanks.