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I have a 02 expedition with a 5.4 with about 89k on it. It has just recently been having an idle issue. I noticed it the other day that it would idle around 1000 rpm in drive and in park. Then yesterday my wife drove it and said that while driving the rpm would drop all the way to around 200 and almost stall but then jumped back up to 1500. Now i recently pulled a cop off of it to trouble shoot a misfire on my 99 f150 but put it back on. I made sure that it was seated correctly and no loose connections. This was the #1 cop. I made sure i didnt accidently loosen or knocked something loose. I checked the throttle body intake and it was clean. I did disconnect the battery but it may have not been long enough to reset the pcm. I drove it yesterday for a little and it seemed fine at first but then the high idle came back. If i shut it off and start it back up it will be fine for a bit but then go back up. Now i did not notice any low rpms while driving it yesterday. Could this be a IAC valve issue?
The IAC could be sticking. It doesn't hurt to take it off and clean it. Could be gummed up. It could also be going bad. Do you have a CEL on? Make sure you have a gasket before.