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I've a 1997 F-250 (old style) and just recently I noticed…

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I've a 1997 F-250...

I've a 1997 F-250 (old style) and just recently I noticed that when I take it out of gear, the engine revs to 2,500 RPM. When I turn the engine off and restart it, it revs at 800. I changed the module in that control the idle air - but that apparently was not the issue.

Mechanic's Assistant: How many miles are on the car? What size is the engine?

98% of the time it idles correctly - and it only seems to do it right after I start up The truck has 185,000 miles and runs like it was brand new ...

Mechanic's Assistant: What is the model of your '97 Ford?

It's an f-250 HD with I thing a 5.8 - the mid size V-8

Mechanic's Assistant: Are you fixing your F250 yourself? What have you tried so far?

Actually I took it to my miccanic and itwas he who changed the the air trottle control valve?

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

No ... that's all

Submitted: 8 months ago.Category: Ford
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Answered in 1 hour by:
7/29/2017
Ford Mechanic: kalamykid, Shop Owner/ Tech replied 8 months ago
kalamykid
kalamykid, Shop Owner/ Tech
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Satisfied Customers: 5,085
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q
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The first thing you need to check will be to see what the coolant temp sensor readings are when the engine is cold. If the ect is reading that the engine is much colder then it really is, then the ecm will richen the fuel mixture up to heat the engine up faster. The ect"coolant temp sensor" is for the computer, not the gauge on the dash. The gauge uses a differnt sensor, so the gauge may be reading correctly, but the ecm thinks the engine is to cold.

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Customer reply replied 8 months ago
Already check that - its reading fine - what next?
Ford Mechanic: kalamykid, Shop Owner/ Tech replied 8 months ago

Did you check it after the truck sat over night and had not been started?

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Customer reply replied 8 months ago
I check it while it was hot.I'm still trying to trend when it does it - it's doesn't do it all the time, and if the engine is 'racing' all I need to do is shut the truck off and restart it and it idles correctly.I drove it for a hour after it did it today and was unable to get the problem to recur
Ford Mechanic: kalamykid, Shop Owner/ Tech replied 8 months ago

Does it only "race" when it is first started?

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Customer reply replied 8 months ago
Good question, no, but it did seen to idle a little funny - but it only races after its been driven at hight speed - over 50 mph
Customer reply replied 8 months ago
I'm going to be driving it again in two hours - is there anything I should be on the look out for
Ford Mechanic: kalamykid, Shop Owner/ Tech replied 8 months ago

Check all of your vacuum lines and see if any of them are craked, or loose, or if any of them have come off.

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Customer reply replied 8 months ago
Did it already - by the way, I'm a nuclear engineer - so I looked for all the obvious items.And if it were a vacuum hose - why would shutting the truck off clear the issue?
Ford Mechanic: kalamykid, Shop Owner/ Tech replied 8 months ago

If it is just a small leak, the iac would over come the leak by closing more to let less air in. When you are driving down the road, the iac will go to full open, then close as you decress your speed.

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Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago
Edward
Edward, ASE Certified Technician
Category: Ford
Satisfied Customers: 3,569
Experience: ASE certified. Auto Transmission rebuilder for Ford Motor Co. from 91 to 2000
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Hello, I see you requested another expert, so I will take a shot at it. One thing I want you to try is this, after the truck is idling at 2500 and it's acting up, pull over somewhere, get out, and by hand take the throttle lever and see if you can manually move it back to where the throttle lever sits on the idle stop screw. I'm thinking possibly the throttle is sticking open at times, or the throttle cable is frayed causing it to hang open, or the throttle body itself may be dirty causing the butterfly to stick open slightly in the throttle body. It's either that or the Idle air control motor is going to full bypass for some reason. It's getting way too much air at times and the only way it could be idling that high at times is it the throttle plate was stuck open mechanically due to dirt or a sticking throttle cable, or the IAC motor is bypassing way too much air. If that doesn't help, try this, with a screwdriver handle and it acting up and idling at 2500 rpms, tap on the IAC motor lightly, smack it with the screwdriver handle and see if the idle comes down any. if it reacts to that impact, it may be a sticking IAC motor. clean the throttle body with an old toothbrush and carb cleaner and wipe it out good if it's dirty, that could free up the throttle plate if that is what is causing it to stick. let me know what you find there.

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Customer reply replied 8 months ago
A couple of things.Testing the throttle cable was the first thing I tried - it's free. Moving it has no effect. I even sprayed WD-40 on it, no effect. My mechanic also checked it and said it was fine.What is the IAC? We replaced a device that he said controlled the idle speed - but that had no effect - also, note that I can come to a stop with the engine racing at 2,500 RPM, turn the truck off, restart it, and the engine starts up and revs to 850 ... so it's an electric signal somewhere that is causing something to cause the engine to rev - The cause of the the RPMs could still be the IAC - but the trouble is not a bad IAC, but something feeding into it.So the question is, what feeds into it?That said, I will still check the Throttle plate - that thought did cross my mind - but why would turning the truck off cause the trouble to go away if the issue was mechanical?
Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

Alright, If the throttle cable is free then that rules any linkage issue out. the IAC is the Idle Air Control motor. and it looks like this.

The PCM controls the idle speed. While it's idling at 2500 rpm's, pull the wiring harness OUT of the motor and see if the idle speed knocks down to 850 then.

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Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

Did your mechanic replace or test anything else? just trying to get as much history as to what has been done. Thank you.

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Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

The PCM is the powertrain control module, and it controls the idle speed via input to the IAC motor. many sensors give input to the PCM on where to put the idle speed such as temperature, whether the A/C is on or not. etc. Did he check for codes stored? if so, were there any stored? Did you ever notice the check engine light coming on after this problem started? thanks.

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Customer reply replied 8 months ago
Good input.I have the old style code system - so there is no check engine light on my version of the truckFord changed styles of the F-250 in 1997 - I have the old style.But again, you have given me good advice. Next time it happens I'll pull the connection to the air control value and see if the issue goes away - that will answer some questions
Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

If you just turn the key ON, and leave the engine off, you should see the check engine light come on. It does have a check engine light in the instrument cluster from the factory. If the idle speed doesn't slow down when you unplug the idle air control valve, then I may suspect a intake gasket leak or possibly evap vent solenoid leak. it's getting too much air somehow if it still does it. Look on page ten in the owners manual for information on the check engine light. thank you.

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Customer reply replied 8 months ago
Again, good advice - but the key thing to remember is all I have to do is turn the truck off and if I immediately restart it - the truck idles correctly.So cutting electricity to the engine cures whatever is causing the issue.
Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

I understand. but your also cutting electricity to the pcm as well when you turn the key off. as well as all the sensors and actuators. Let me know what happens if you can get it to act up and pull the connector at the iac while it's idling at 2500.

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Customer reply replied 8 months ago
I will!That's the best advice I've gotten to date
Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

Absolutely, thank you.

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Ford Mechanic: Edward, ASE Certified Technician replied 8 months ago

Any luck with that test of pulling the harness to the IAC while it's idling high by chance? thanks.

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