98 Ford ClubWagon with 5.4 Liter engine. About 120,000 miles.
I've changed the DPMS sensor a couple times, in prior years, for different reasons... typically the engine would ping under the slightest loads. That problem is NOT happening now.

Any tips appreciated. I am receiving no error codes, on my handheld code reader. There is no "Service engine now" light, lit up on the dashboard.

Idle has been too fast for a year or so. It's annoying, and manifests even while driving down the road. Applying brake will slow the car, and slow the engine.

About 60,000 miles and 6 years ago, I changed my IAC valve, because at that time my vehicle was stalling when it should have been idling. That was easy to troubleshoot. This is different.

I recently removed the IAC, and noticed it looked good, except for light soot inside. I cleaned it out with a gasoline spritz. Nothing changed when I re-installed the same device and tested the engine speed. When I then forcibly pinched the rubber hose feeding air into the IAC, the engine RPMs dropped substantially... in keeping with denial of air-flow. When I pulled the electrical connector off the IAC, this immediately diabled the IAC and the engine appropriately stalled. So I think the IAC is fine, and the engine is idling fast "simply" because the computer is telling the IAC to made the engine run fast.

I know I'm tangling with a fancy feedback control system, so the problem could be in lots of places.


I pulled the doghouse off my engine. I hear no vacuum leaks. I have heard of a clever test for vac leaks: insert a stream of un-ignited, flammable gas (e.g., propane) into a suspected vacuum leak, with engine running, and notice the RPMs increase. I suppose a non-flammable gas, like steam, could induce a less dramatic effect, in an opposite manner.

A website listed the following components as suspect. These parts are all expensive, and I don't want to blindly replace them.

Engine Management
Canister Purge Valve
Coolant Temperature Sensor
Cruise Control Module/Amplifier
EGR Valve
EGR Valve Gasket
EGR Valve Pressure Sensor
Engine Control Computer
Idle Air Control Valve $50 to $100
Mass Air Flow Sensor
Oxygen Sensor
Temperature Switch
Throttle Position Sensor
Vehicle Speed Sensor

Of course a TPS (throttle position sensor) malfunction could cause all sorts of troubles. But wouldn't a TPS fault kick out an error code?? Ditto Mass Air Flow Sensor error? Ditto Oxygen Sensor? Or is the oxygen sensor so fundamental, that the engine cannot tell when the OS dies? I would expect an OS fault to cause a smog test failure... but I didn't fail a year ago, at last required testing.

Any suggestions appreciated.

JohnPeter