Ya try cap and rotor wouldn't hurt.The 91 have a problem with the fast idle thermo valve coming loose(its one the intake it has 2 coolant hoses to it no electrical plug in).You can take the 2 bolts loose of the top and tighten the wax element down with a flat head screw driver.Be gentle.Dis connect you battery for a while re connect it and let the car idle for 30 min to re learn the idle.Also the iac(idle air control)can leak coolant internally causing you engine to suck coolant.Take it off(two coolant hoses and one electrical plug in on the intake)look for coolant in the air passages.A compression test might tell you a lot and some times injectors leak causing a rich condition.To test for leaking injectors hook up a fuel psi gauge run it and shut it off you should not see a fuel psi drop for an hour or 2.If it drops right off take apart you intake and look for a wet port that port would contain your problem injector.