What causes the idle speed to run erratically between normal and 2500 RPM all the time?
Look for a vacuum leak. It may be on the throttle body or it may
be associated with the EGR valve assembly. Sometimes you can spray
a small amount of starting fluid at a suspected vacuum leak and if
there is a leak at that point, the engine will suddenly start to
sound different while it burns off the starting fluid. For a
vehicle to have a pulsating idle from approx. 800 rpm to 3000 rpm
approx. the issue is not with vacuum but with the TPS ( throttle