Engine Rev Up at Stop ! Strange? - Ranger-Forums - The Ultimate Ford Ranger Resource

4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 03-30-2016
Thread Starter
Join Date: Mar 2016
Location: Naples
Posts: 2
Engine Rev Up at Stop ! Strange?

I'm new to the forum, and new to Rangers as well so would appreciate any guidance. Great truck bought through a friend but little history from owner other than 2003 Edge version, 4.0 L and only 37K miles ! So truck like new....but occasionally while slowing down to a stoplight I get an instant engine rev.

To be exact, normally what I would expect is what I get which is take foot off accel. and idle drops slowly down to about 1500, then 1200 and as I come to a stop just under 1000 all running smoothly. Would have thought maybe with the OD tranny it would drop faster but that's ok.

Sometimes I take foot off accel. rpm drops to 1500, then 1200 at around maybe 5 mph, then just before I come to a stop revs jump to 2000 or 2200 for about 2-3 secs. Gives you a startle as it's so noticeable you need to press on brake a bit harder to keep from moving forward.

Very strange, I've worked on cars, trucks and all for years but not sure where to look. Idle positioner sticking maybe? but it goes back up in rpms so doesn't make sense either.

any history of this problem or technical thoughts would be greatly appreciated.

thanks all
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Old 03-30-2016
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Join Date: Jan 2014
Location: Vancouver, BC
Posts: 7,615
Welcome to the site.

Couple of things to check.
Throttle position sensor(TPS), this is attached to the throttle plate rod opposite throttle cable linkage.
It has 3 wires
Top wire is 5volts from computer power supply, used by many sensors
Center wire is the "return voltage", tells computer where drivers foot is on gas pedal
Lower wire is a Ground

I use a sewing needle to pierce a wire to check its voltage while it is connected.

When key is on, engine off and throttle is closed, the center wire will show under 1 volt, .69v-.99v is spec.
As you manually open the throttle this voltage should increase steadily, no jumping around, when you get to WOT(wide open throttle) voltage should be above 4.5v, 4.55v-4.99v is spec
If you detect any jumping around, especially near closed throttle in your case, then TPS will need to be replaced.
If voltage jumps above 1volt then computer thinks YOU tapped the gas pedal, which can increase RPM, but................

Computer can only control Fuel flow with TPS data not air flow, and RPMs are generally controlled by air flow not fuel flow.
An engine is HELD at lower RPMs by choking off its air flow, the throttle plate.

The IAC(idle air control) valve is used to adjust the air flowing into the engine by-passing the throttle plate.
Computer controls the IAC Valve, this is needed on fuel injected engines because an idle screw wouldn't work.
IAC Valve is operated by an electric motor that turns to move a plunger type valve in and out to allow more or less air to by-pass throttle plate.
Computer uses IAC Valve to increase idle when engine is cold then lower idle as engine warms up and to increase idle a bit when AC is on.

If the plunger was sticking, and as RPM were getting lower the vacuum in the intake increases, this pressure could unstick the plunger causing it to open more, RPMs jump up, then computer closes plunger to set correct RPM, which would take a second or 2.

These are fairly easy to clean, 2 bolts to remove, and check that valve/plunger is not loose.
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Old 03-30-2016
Thread Starter
Join Date: Mar 2016
Location: Naples
Posts: 2
Thanks to Ron !

Not sure how to Thank RonD for his response as new to the site so a big Thank you here for the kind of information I was looking for and fully understand where he is coming from. I'll check out the TPS and IAC operation and now understand how they could be casuing this problem. Will let others know how this works out.

Thanks to RonD and certainly the Forum as well.
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