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D -> P rpm drop


e21pilot

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Lately, I've noticed when shifting from drive to park (or drive to neutral) in my 92 4.0 Ranger, the rpm drops by 50-100 rpm from the usual 650 when in drive.

I imagine rpm would go up slightly or perhaps stay the same when shifting from drive to park since there is no drive line drag on the engine in park however the opposite happens.

I wonder if this is typical and what component makes the rpm adjustment under the hood? perhaps I need to replace this component.
 


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RonD

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On fuel injected engines, all of them not just Fords, an IAC(idle air control) valve is used to set idle RPMs.

Without jets(carburetor) there is no way to use an idle screw and keep air/fuel mix balanced, with a carb when you opened or closed the throttle plate with a screw adjustment, more or less fuel would be sucked from the jets because of the air flow thru the carb.

With fuel injection computer running the injectors needs to set idle so it can balance fuel mix.

IAC valve is basically a Controlled Vacuum leak.
It sits at the throttle plate on a passage that runs from one side of the throttle plate to the other.
IAC valve controls the air going in and out of that passage.
IAC uses an electric Step Motor(or stepper motor), this is a motor that can be turned XXX number of degrees by one command(one step), IAC motors usually have 100+ Steps that can be called on.
The amount/degrees the motor turns opens and closes the valve.
Computer "learns" what Steps to use on the engine.
On my '94 it might use step 15 to set my RPM at 625
On someone else's it might be step 20 for 625
And 10 years ago it might have been step 20 on mine but with older vacuum lines it may need to close valve more, lol.
So it is an ongoing Learning process for the computer, not static this is it and never changes.

Computer has Target RPMs programmed in to memory at the factory.
My manual trans has warm engine idle of 625.
Your automatic should be closer to 700 in park.
And when you shift to Drive the computer is notified via the NSS(neutral safety switch) and will increase RPM(open IAC valve a bit) to compensate for the drop as transmission engages, when you shift from Drive to Park same thing should happen, idle should go up then drop down to 700 again, as computer closes IAC valve a bit.

Same if you have AC, computer will see AC compressor being activated and increase idle a bit to compensate.

IAC valve can be cleaned and not a hard thing to do at all.
Give computer a few drive cycles to relearn "steps" if they have changed from the cleaning.

IAC Valve is also used when engine is cold, RPM is temp related, usual cold idle is 1,000-1,100, then that slowly drops as coolant warms up to target warm idle, coolant above 190degF.
Computer has it's own coolant temp sensor for this, a two wire sensor near the ONE wire temp Sender used by the dash board temp gauge.
 
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e21pilot

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Thanks for this details reply. I wonder if I could disconnect IAC valve and then set the throttle plate stopper screw. I changed the throttle body out and never adjusted the screw.
If I did so, what rpm should I adjust the screw to assuming the engine is warm.
 

RonD

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That is an anti-diesel screw, and can be used for slight adjustment of TPS(throttle position sensor) closed throttle voltage, 0.6v to 0.9volts.

With engine fully warmed up unplug IAC valve, it will close, RPMs should drop to 500, or engine may even stall, either is normal.
It also means no vacuum leaks.

You can adjust it open a bit if engine stalls, so it will idle at 500 with IAC Valve unplugged, but no higher than 500 or IAC Valve can do it's job.
But if you do also check center wire on TPS, with key on and throttle closed, the center wire should have under 1volt, .6-.9v is spec
 

enjr44

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I am going to chime in here because of what happened to my 02, 4.0. I don't have any idea if it applies to a 92 4.0 (different systems and different engines).

This happened out of the blue. Shift from drive or reverse into park or neutral it would drop from 750 rpm to around 600. Sometimes it would recover to 750, usually not. Occasionally, it would stall. I replaced the IAC, reset the computer and no change.

After a few months of this, when cold on the first start of the day it would not go to the normal 1500 rpm (once in a while, maybe). It would go about 11/1200. The rest of the starts for the day were completely normal. And it ran normally except for the dropping rpm going to P or N. It was driving me nuts!!! Wife said either take it in or quit bitching.

I couldn't figure it out and neither could a very good local shop. Everything tested good and they cleaned all the usual stuff (MAF, TB, Injectors, etc.) These guys are good and honest to a fault (since they didn't fix it, they didn't charge for any of their labor).

Anyway, after another month or so, I broke down and bought a fuel pressure tester, don't know why, just something to do.

Since you depressurize the system when you connect the gauge, it of course read zero. Turned key to run, went to 10 psi, cycled key again and it went to 20, again 30, and again got 45 and that was it, no higher (it think it should have been 60/65). Started the truck and it stumbled and fumbled and then stalled. I did not know at the time that the computer turns off the fuel pump if rpm drops below 400. Anyway, cycling the key would not pressurize the fuel system again.

That was the last time it started until the fuel pump assembly was replaced. It gave up the ghost right there in front of me.
 

e21pilot

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Thanks for the voltage readings to look for. I will give it a try this weekend.

I should also mentioned that I replace the fuel pump with a factory Ford one earlier this year so I'm hoping fuel pressure is not the issue.
 


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