1985 318i Will Not Idle Properly (HELP!)

I recently aquired a 1985 BMW 318i. It will not idle properly. When the engine is not in gear and in motion it will jump constantly between 900 and 1100 RPMs. Everything else seems to be running properly. I've
been told by some that the idle control unit needs to be replaced. Is this likely? I have heard also that there is an idle control valve; this is not the problem? I cannot locate the idle control unit. Someone told me that it is between the glove box and dash, but I can't find it. Will someone provide me with detailed instructions or a diagram? I cannot afford a mechanic and am not savvy with cars. I cannot afford a manual, but wonder whether or not this information is provided in the Bentley manual.
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