How is oil consumption on the engine? If it's got really worn rings, excessive blowby will shoot past the rings, causing an excessive volume of blowby (carrying with it, an excessive volume of oil droplets from the crankcase) to recirculate into the air intake. The oil actually ignites and runs the motor, potentially leading to a destructive "runaway" condition! To test if this might be the problem, try removing your blowby recirculation hose. Rig it up so it blows into a catch-can of some sort, and plug the barb on the intake manifold. Try running the motor with it like that... if the engine speed now returns to idle no problem, you have found your culprit.
If that's your problem, unfortuntely I don't know of an easy and effective system that completely removes the oil droplets from the blowby vapor. I have tried a lot of things that don't work completely - they help but not completely effective. A sure-fire way to fix the problem would be to rebuild the motor to reduce leakage past the cylinder rings. DieselsRCool up here once had excessive oil in the blowby but his compression numbers were all in order. He just replaced the oil scraper rings, leaving the other rings in tact, and reported that it reduced the blowby or oil quantity in the blowby. His motor was I think freshly rebuilt, IE: I wouldn't count on that working unless all your compression numbers test to be good. But that is another thing you could try that is cheaper than a full-engine rebuild.
There is another potential cause of this problem that I am aware of, but only if you have tweaked your fueling level on your injection pump to the max. This I think would be really hard to do on a 1.6lNA.
Hope this helps... good luck