my van is a 1989,is idle adjustment on throttle? with a turn of a screw.or is it

Pretty sure your year has an Idle Air Control valve screwed into the side of the TBI. (Do you have a TBI - Throttle Body Injector?) Anyway, if you do have this "IAC" then that controls idle automatically via the computer. However, I believe there is also an idle screw that CAN be adjusted, but which the manual says DO NOT ADJUST. ... If you were thinking about adjusting the idle because you think your idle is too high, consider the possibility that you have a vacuum leak. Friend of mine had a damaged gasket where the TBI sits on top of the intake manifold, and air was escaping through that area, causing pretty high idle. Simply replacing that gasket (not a difficult job) fixed the problem all by itself. Without screwing unnecessarily with the factory-set idle settings.