Symptoms - At just about 72,000 miles my end-of-year Mountaineer suddenly got an extremely rough idle and low RPM stumbling after starting cold or hot. It felt like it was about to stall when stopped at a light. Occasionally it would take a lot more cranking than usual (3-4 secs instead of 1) to start. There seemed to be an occasional miss, and sometimes the exhaust smelled rich. The engine 'seemed' to smooth out over 2,000 RPM. There was no CEL (check engine light) nor any OBD-II codes using an ODB-II reader.

I read through the forums searching for similar issues and found many. I noted the recommended steps then searched for any related "how-to" threads and printed them down.

Steps Taken -
1) Tried Dry Gas and Fuel Injector cleaner with no change.
2) Performed "routine maintenance". Replaced Air Filter, PCV Valve (OEM different from store listings... straight thru, not elbow), Oil and Filter (latter two only because they were due).
3) Tried to change Fuel Filter but this too was "non-standard". It was held in place by c-shaped spring clamp (no cover to un-bolt... it's fully exposed) and the connectors did not use the hairpin method described in Chiltons (photos). Need to get "removal rings".
4) Checked the ignition wires physically end-to-end then visually looked for blue flashes in the dark. Normally I would have changed the wires but had done so less than nine months ago.
5) Pulled each plug (also had been changed nine months ago) to check the condition and looking for signs of fouling (photos). Verified gaps were correct, added dielectric grease to boots when reconnecting the wires.
6) Looked and listened for any disconnected or cracked vacuum hoses.
7) Cleaned MAF sensor (photos and method) leaving battery disconnected for the duration of the procedure (~ 1 hour).
8) Cleaned IAC (with silver pressed-in cap over filter and crimped-on solenoid) and Throttle Body (photos and method) leaving battery disconnected for the duration of the procedure (~ 2 hours).

Problem Solved! Running smooth as silk!

Thanks to all forum members who shared their problems with others in need. Without reading all of your suggestions and methods I never would have thought of nor tried many of these procedures... including the ones that solved my problem!

Now to move on to the blank radio display and the wiper that cleans 1/4 of the rear window...

 

Do you know which of the suggestions fixed your rough idle or was all of them needed?

Mason

 

I tested between each step and it wasn't until I did the IAC and Throttle body that the problem disappeared. It's impossible to definitively say which of these two was the cure but I would lean towards the IAC because the throttle body just didn't look that dirty.

 

What is the IAC and where can I find information on how to work on it? I am having similar difficulties.