I bought a 2008 a few months back and when off the throttle with clutch in or in neutral clutch in or out and rolling it idles down to 500 rpm which at night has the headlights dimmed down and sometimes idles rough but the odd thing is as soon as the truck comes to a complete stop the idle will ramp up to 750 rpm and maintain that along with the headlights brighter and idling smoother. It seems to be programmed to do this but I don't understand a legitimate purpose.


I'd just like to hear from others with the 2.3L Duratec and manual tranny if your truck does this same thing or if someone knows something about this lower idle during coasting scenarios.

 

Come on, no one has ever observed the tachometer of their 2.3L Duratec with the clutch in and coasting to a stop?


OK, most probably wouldn't pay attention to this unless they have experienced their headlights dimming at night under this condition then brightening back up once the truck completely stops without ever touching the accelerator.


It's a strange occurrence but it's consistent as if the computer is programmed for it to do this, just still baffled as to why it would be.


Still seeking feedback on this.

 

Don't have a 2.3l Duratec

But............emissions dictates RPMs should stay high until speedometer is under 5MPH
"High" is relative but I would say above 1,800rpm

If RPMs are dropping low then there is an issue with IAC Valve or computer

And dimming head lights at RPMs under 1,000 means 1 of the 3 Fields in the alternator has failed, won't hurt anything for now but when another one goes you will be running engine mostly on Battery power, so it just won't start one morning.

No vehicle maker specs in an alternator that can't provide 13.5volts at 600RPM with all vehicle electrics ON, i.e. head lights on, heater fan on high, doors open
So dimming headlights is a sign of an electric issue, not normal, lol

 

My first though when first noticing this occurrence was either alternator or battery issue but I'm no longer suspecting either of those. Parked and idling at 750rpm I can turn all lights, radio and electronics and hit the brake pedal and do not notice any dimming in the lights and I've checked voltage at idle which was around 14volts. This dimming effect only occurs and with constant repeatability when the truck is rolling and the clutch is depressed or the tranny is in neutral yet as soon as the truck comes to a stop or maybe a 1-2mph roll the rpm will increase from around 500rpm to 750rpm which at night is a noticeable difference in the headlight intensity.


Someone could have had some tuning done on this truck before I bought it but I just have no idea why it'd be desirable to have a vehicle idle lower during a coasting scenario vs it's normal idle while sitting still at a traffic light or what ever.

 

At 500rpm lights would dim a bit, spec is 600 for alternators, but your 4cyl should be closer to 750 at warm idle, alternators can't make max amps until engine RPMs are above 1,500

14volt is a bit high unless engine was just started, after running for 10mintes voltage at battery should be below 14volts, 13.5v is spec, it is actually suppose to be 1volt above "at rest" battery voltage, so new battery could see 13.8v or so.

Something is wrong if RPMs are that low when you are above 5MPH
Should be above 1,200 at least, 1,800 maybe

One of the benefits of fuel injection over a carb is that the injectors can be shut off completely when coasting
With a carb when you coasted down hill you would still be sucking gasoline from the jets and even more gasoline if using the engine as a brake, as vacuum in the intake climbs up above 30"
So coasting in neutral with a carb engine was better for MPG, although harder on the brakes, lol.

Fuel injection uses a different strategy, when engine RPMs are above about 1,300 and foot is off the gas pedal, the computer shuts off fuel injectors, so 0 fuel use when coasting as long as you leave it in gear, use engine as a brake.
So if you are "coasting" down hill at 60mph in gear with foot off the gas pedal you are using 0 fuel, injectors are off
As RPMs get close to 1,300 computer will restart injectors at idle levels.

Now maybe something is wrong in your 2.3l Ranger that is causing computer to misread the situation.
My first stop would be cleaning the IAC(idle air control) Valve, this is what the computer uses to set idle RPMs, and for some reason computer isn't or can't set it at 750 when it needs to be.

I assume when engine is cold started the RPMs go high, 1,000+
And then as engine warms up idle goes down and settles in at 750-800 or so
All 4cyl engines have to have a high idle, or they vibrate because of the 180deg firing

If IAC Valve seems to be behaving in Cold Start then could be computer itself has a problem.
You can check with a Ford Dealer, have VIN handy, there may be a software update for your computer, so this is a known issue