The Ford Taurus is a fairly steadfast runabout, but does have a few inherent problems that manifest with age. High or rough idling is often a precursor to more severe failures.Basic DiagnosisAn engine needs a precise mix of air and fuel to run. One way to tell the difference between air starvation and fuel starvation is by seeing if the problem gets better or worse. Fuel problems will usually get worse with RPM, while air problems get better.Vacuum Leaks1990’s Taurus engines are notorious for developing leaks in the rubber vacuum lines that connect to the manifold. If you suspect a vacuum leak, spray the hose fitting with brake cleaner and listen for a rise in RPM, which indicates a leak.Mass Air SensorTauruses use a thin, heated Mass Air Sensor line (located in the intake duct, just upstream of the throttle body) to calculate incoming air. These sensor lines can get coated with dust and varnish, which will cause a rough idle.More Serious ProblemsBlown head gaskets, leaking intake gask