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We are having two occasional problems, not at the same moment. But I thought I would mention both, just in case they are inter-related.

We have a 2003 Windstar with 3.8L, automatic transmission, approx. 120k miles.

With the engine at normal operating temperature and on basically flat terrain, we will have the speed control set in the area of 60 to 70 mph. The RPM when normal will be around 2000 to 2200. Occasionally, we will hear small surges in the engine and see the tach jump about 200 to 400 RPM, then backs off with in a few seconds. The transmission never shifts in or out of overdrive when this happens.

The other problem, which I'm not sure if it is connected or not. Occasionally the engine will idle at a real high (1200 to 2400) RPM. Twice when this was happening, I also noticed that when climbing a moderate hill the transmission downshifted out of over-drive and would not shift back into overdrive when we got over the top of the hill. I figured the shifting problem might be caused by the high engine speed, but I thought I would mention it in case it might light a light bulb. When we have the high engine idle, if you shut the engine off and restart it right away the engine idles at the correct (800 RPM) speed.

The engine idle problem started last fall when the daily high temperatures were in the upper 40's to low 60's. We replaced the Throttle Position Sensor. Then the temperatures turned colder and we didn't have the problem again until this spring when the temperatures turned warmer again. Which might be or might not be coincidental. But, I thought it might light up another light bulb.


I don't have any idea if the problems are related or not. But, I thought it worth mentioning as one post.
 

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I would try cleaning the IAC valve for the high idle.They require cleaning about every 30K due to blowby gases from the pcv system venting into the air intake hose.As far as the surge of 200-400 rpm,The cruise is trying to maintain set speed.a slight incline or a good headwind may be your cause.200rpm isn't alot.
 

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Curt,
My first thought just reading your title, was
vacuum leak.
How-To: Locate a Vacuum Leak - Ford Forums Online - Ford Discussion & Enthusiast Forum
But then reading further, my next thought was TPS throttle position sensor.
But reading further, you had already replaced the TPS.
So I guess I am back to vacuum leak.
You can backprobe the TPS and it should move smoothely from 0.5 - 1.0 volt TO just less than 5.0 volts as you go from idle to wide open throttle. The transition must be so smoothe that you might miss it with a digital voltmeter and the only way to really check it is with an osicilloscope.
Some after market products are not as good as Ford original equipment.
 

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