Terrano II Ralenti Problem

I have a Nissan Terrano II of the 98 engine TD27 Ti in idle the rpm vary from 400 to 800 without touching the accelerator because I could happen this, if someone can help me, thank you very much

Look, first you have to check the air and fuel filters, after discarding them, Czech that you do not have false air inputs, that is, hoses in poor condition, if none of this is the Czech problem the IAC valve, if it is equipped in your vehicle, you have this first and let us know if the problem was solved, if we do not give you more data.