Re: excessively high idle?
There is a vacuum line behind the drivers side head that has a tendancy to rub either the firewall or the trans bellhousing. this is prevailiant in the 87 to 91 trucks because of a harness clip that has a tendancy to break.
Check it out (have had to fix a lot of trucks for this exact problem) and look at all the vacuum lines and also all of the PCV valves. If it still does it, have the Power brake booster checked (could be a torn diaphram.