When my 1997 Taurus LX began running poorly, I installed new spark plugs and wires, cleaned the upper intake and the secondary intake butterflies. The IAC passage was almost fully blocked. I also replaced the PCV valve with a new Motorcraft one and replaced the U hose connecting it to the intake. I've checked and no vacuum lines were left disconnected.
What happens is it won't stay idling at a proper RPM. Sometimes it's 900, or 1000, or 1500, or 2000. In town it often wants to run at 1500 so I have to keep hitting the brake to slow down to 25 MPH. If I stop and hold the brake with it in gear it will drop to 900 and stay there, perhaps for a while, perhaps not.
I've installed a new IAC valve (the old one was very dirty) and new TPS. Each one seemed to make it behave, for about a day. Until it started to run rough it had been fine. All I've had to do to it before was replace the transmission fluid and filter and the oil pan gasket that went at 93K miles.
What makes this problem better or worse?
It's random. Engine warm or cold. Check engine light is never on, never any codes.
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