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Q: idle surging on 1998 Ford Expedition

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My 1998 Ford expedition has a surging idle when topped or in park. It will fluctuate between 400 - 1500 rpm. I have replaced the mass air flow sensor and the vacuum line coming from the PCV valve to the rear of the air intake manifold. There is no "check engine" light on so there is no codes to look at. Does anyone have any other ideas?
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I would be looking at the idle air control valve, or a vacuum leak at the intake manifold, or something similar to that.
Could also be a Throttle positioning sensor or TPS going bad too I think my firebird needs either one of these checked on it too.
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