Been searching and reading, But there has to be a way to beat the high reving at start up.. Turn the key, Start, Varooom.. 1800-2000 rpm for a second or two. Then down to normal. Feel guilty when it is really cold out. There has to be a mod or somthing we can figure out. I have a new AIC. And truck runs perfect after that crazy start up. Any Idea's? Folks say it is normal, But i don't like it.

Thanks

2002 4.6l

 

Like it or not this is the way it works.
The colder it gets the higher the idle and the longer it takes to come down.
If it did not work this way you would be complaining of stalling and having to wait to drive off cold.
.
You need to be sure there is not a fault because 2000 is a bit high.
Try another IAC
.
Here is how cold start works.
Key to 'run' computer looks at the cylinder head temperature sensor and the intake air temperature sensor resistance values as the only way the computer knows it's to be a cold start.
This sets open the IAC, sets amount of fuel and ignition timing therefore the high idle.
Once the OX sensors heat to 600 degrees they take over control of the fuel.
.
You can't alter or mod any of these functions and have proper cold starts because they are all the same for hot restart that would also be affected.
Only difference is the temperature detected on hot startups and Ox sensors come into fuel control much faster.
.
You could check the CHT and IAT for resistance values within spec tolerence limits that might tend to stack up on the cold side and cause the computer to set idle higher. A rare occurrence.
As a note, do not make any adjustment to the base idle setting screw on throttle body or other issues are likely.
If it has been messed with a set up procedure needs to be done.
Good luck.

 

Thank you Sir.. Good info.

Could we fabricate a pot arrangement for the CHT and IAT on a switch to bring the temperature up to say 60. Make it think it is a tad warmer but not to warm it will give cold starting issues? Or do the same with IAC?

No adjustments on the base idle. Never do, i know that is bad news.

I can plug in and display the output of the CHT and IAC.. They both seem to be somewhat accurate.

I have had two IAC's in there. Both reacting the same way.. The cold idle is working great. It will idle around 800/900 when it is cold out.. Say 30 degrees. Which is nice for warm up. But that initial hit from starting is bothering me.

 

If your saying hot idle is 800 to 900 rpm, that is a/the problem.
The program table value is supposed to keep it at 650 +/-50 RPM when fully heated as monitored by the crank sensor.
Have you looked at the throttle linkage and Cruise control cable for some kind of hang up?
Look at the throttle plate for being open to far off it's idle stop.
Good luck.

 

Sorry, I meant high idle around 800/900 when it is cold. It comes down to 650 ish when she warms up.

What i am after is somthing similiar i did to my lawn tractor.. You had to raise the throttle above the rabbit to activate the choke.. Once she fired you back it down but if you were to slow with the lever she would race like a rabbit. That thing sounded terrible starting at such high revs cold. I put a manual pull choke on it.. Now i can start it at a WAY lower rpm. Off idle. Want to do somthing like this to the truck so i don't get a crank,crank,crank- VARROOOOOOMM 1800/2000 RPM

 

Last reply!.
You have a normal operating system that 'has' to operate the way I described it to you.
I cannot for the life of me understand what is so objectionable about 900 rpm cold starts then dropping to the normal 650.
It has to be you.
The old carbed cars with electric chokes, step idle cams and dash pot worked the same way to accomplish the same end result.
Maybe I should not tell you but your transmission won't even go in overdrive until the transmission fluid warms up. It's again designed to work that way for a reason.
All other makes work the same general cold start routine so you would not be happy with any of them..
Good luck.

 

I understand the high rpm on cold start and the length of high rpm idle being longer the colder the temp, but mine revs. At temps of about 40-45 degrees and below, I start the engine and the rpm's immediately shoot to about 1800 hold for 5 sec or so then drop to around 700. This repeats itself till the O2 sensors take over and the rpm's smooth out.

This normal or malfunctioning IAC. I have cleaned it twice but no real change in the performance at the stated temps.

 

The only way any gas engine changes RPM is with a change of air flow.
This is called a variable efficiency engine.
This is why all have a foot operated throttle to change the airflow rate in some physical manner for the engine to make power on demand..
The IAC either has to change opening area for some reason or there is a gross uncontrolled leak of air that changes with engine vacuum pulling it open then closing up as engine RPM changes the intake vacuum level.
As the intake air increases the manifold vacuum goes down.
This is usually a soft rubber part that has split or cracked open.
This kind of fault could result in a cycling action if the fault is just the right physical manifestation.
Good luck.

 

wonder if the MAF sensor is a bit dirty??. 1800RPM does seem a bit high on initial start up. Its like 1deg this morning...doubt that my truck will hit 1800 RPM after lighting up, 1200-1500 maybe.(I'll be more attentive this morning & double check) and it sits in a unheated garage so not quite out in the cold .

but yes it normal to spike RPMs after initial start up...Bluegrass7 is pretty spot on about these things and it could very well could be a slight vacuum leak or a lazy IAC...or slightly out of wack resistance values from the CHT.

Presume that it runs fine other than this?

 

It has to be the iac. I've cleaned every piece of emissions system I can get my hands on down to the manifold. Including boring out the egr port in throttle body elbow, cleaned every hose and connector inside & out replacing any that appeared damaged. Replaced o2 sensors cleaned maf twice. Truck runns pretty good.

It started the morning just fine & have to be 40 degrees or below. The issue seems to directly correlate with true freezing temps. Bluegrass7, I wonder if the rubber grommet in the iac could be somewhat rotten in places holding moisture & freezing or at least not expanding & contracting evenly. Could that small a leak be the source? I've had it off twice, there isn't a gross difference I can see.

You guys know any way to test the iac function if its working correctly electrically?

? Definitely have plenty of cold weather on the way to assist in troubleshooting.

Thanks guys.

From phone key pad.

 

Crank Crank.. Varoom.. 1800 to 2000 rpm every time i start, A cold or warm engine. Been paying attention to it. Been 15 to 40 degrees around here lately.

After a few seconds she comes down to either 650 (warm motor) or hovers around 800-100 cold and when she warms up it will idle at 650 after a cold start.

The truck runs perfect other than that.

Can i build a timer box to close the IAC on initial start up and return to normal operation after 3 seconds. Would that mod do it?

 

Not sure who I am responding to?? daskool or tckboater ?? seems like one in the same??

Well I checked my truck....it starts , dead cold, hits 1500-1600 RPM for maybe 2 seconds at the most, then drops to 1100-1200...and then does the slower decline to 650-750rpm (normal Idle) within 10-20 seconds depending on how chilly it is.

Oh BTW, it is 7 degrees out...well below your 15-40.

 

Its your IAC. If you didn't buy Motorcraft ,the others are different.mine does the same thing with a cheaper brand IAC so now i'm buying a motorcraft one to fix the problem.