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Replacing your IAC? Read this.

Glacier991

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Something I ran across in a FORD service manual. Interesting, and something I did not know.


Idle Air Trim

Idle Air Trim is designed to adjust the Idle Air Control (IAC) calibration to correct for wear and aging of components. When engine conditions meet the learning requirement, the strategy monitors the engine and determines the values required for ideal idle calibration. The Idle Air Trim values are stored in a table for reference. This table is used by the PCM as a correction factor when controlling idle speed. The table is stored in Keep Alive Random Access Memory (RAM) and retains the learned values even after the engine is shut off. A Diagnostic Trouble Code (DTC) is output to indicate that the Idle Air Trim has reached its learning limits.

Whenever an IAC component is replaced or cleaned or a service affecting idle is performed, it is recommended that Keep Alive RAM be cleared. This is necessary so the idle strategy does not use the previously learned Idle Air Trim values.

To clear Keep Alive RAM, refer to «PCM Reset» in Section 2A. It is important to note that erasing DTCs with a scan tool does not reset the Idle Air Trim table.
 



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Brain

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How do you reset the PCM? I did some stuff to the '91 and cleared the codes, but haven't reset the PCM module as far as I'm aware.
 






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I believe that is one of the reasons for disconnecting the negative battery cable. After a short time with no power, the pcm will reset. This is why the manual says it may drive differently for the first 10mi or so after a dead battery. It has to re-learn all the corrections it needs to make for fuel trim, idle, etc..
 






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Erratic Idle

I put an new IAC in my X and then disconnected the battery terminal for 1 hour and I am still having the spuddering and erratic idle. But it only happen afer the truck has been driven all day. The X runs great when cold and has no problem with a 30 minute drive to work and back. After I let it sit for 30 mins and go back out to start it. That is when it happens. It has stalled on me before the engine idles below 400 and sometimes will die. But if I hit the accelorator a few times it tends to get better. Then when I drive it it all goes away. But if I leave it sit say I go into the store for 15 mins and come back out and run it. It does the same crap. I have replaced the IAC, plugs, wires, rotor, cap. took carb cleaner to the intake, even use fuel additives, all coolant hoses. I am out of ideas and do not know what else to replace. I am at my wits end, because I am wasting money and everytime I take it to my mechanic the thing never acts up for him. I do not feel like replacing anything else until I know what is wrong with it. Any ideas would help. Thanks.
 






lwn

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Pull the vacuum line off the pressure regulator, check for fuel, sounds like it may be bad.
 






Glacier991

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Here's the section 2A referenced

"Description

All OBDII scan tools support the powertrain control module (PCM) reset.

The PCM Reset allows the scan tool to command the PCM to clear all emission-related diagnostic information. When resetting the PCM, a DTC P1000 will be stored in the PCM until all the OBD II system monitors or components have been tested to satisfy a drive cycle, without any other faults occurring. For more information about a drive cycle, refer to «Drive Cycles».

The following events occur when a PCM reset is performed:

l Clears the number of Diagnostic Trouble Codes (DTCs).

l Clears the DTCs.

l Clears the freeze frame data.

l Clears oxygen sensor test data.

l Resets status of the OBD II system monitors.

l Sets DTC P1000.

NOTE:
When using the New Generation Star (NGS) Scan Tool to perform a PCM reset, press the CLEAR button. This function is performed only after retrieval of continuous DTC's.


New Generation Star and Generic Scan Tool

Refer to the manufacturer's manual for specific instructions on how to perform PCM reset.


Resetting Keep Alive Random Access Memory (RAM)

Disconnect the battery ground cable for a minimum of 5 minutes. Resetting Keep Alive RAM will clear learned values the PCM has stored for adaptive systems such as idle and fuel trim.

After Keep Alive RAM has been reset, the vehicle may exhibit certain driveability concerns. It will be necessary to drive the vehicle to allow the PCM to relearn values for optimum driveability and performance."
 






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Thanks Iwn I will try that. Are you talking about the fuel pressure regulator or... Sorry newbie here. Trying to learn more about cars.
 






lwn

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Ya, pull the line off after the vehicle has been driven for a bit, see if fuel comes out or is present. the regulator is up on the fuel rail that feeds the injectors.
 






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I understand could it be clogged or just worn out. I know with a 160,000 miles that things need to be replaced but I have replaced so much stuff. Un real. Thanks.
 






cfkane

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Fuel volatility increases as the temperature drops in order to aid starting of a cold engine. This coupled with the use of ethanol can aggravate a warm start condition by inducing vaporization in the fuel rail. Fundamentally anything that causes fuel to be delivered at anything under the required pressure and or in a vapor state can cause this problem. Take a look at the fuel you are using to see if it contains ethanol and if it does try a tank of not ethanol blended fuel.

In addition to the items mentioned in the thread you may want to check the vapor return solenoid. If it is not functioning correctly (common in higher mileage applications) it may not allow air to enter the fuel tank resulting in a vacuum condition making it that much more difficult for the fuel system to overcome any hot start problems. A quick check is to vent the gas cap before starting. If you hear air rushing into the tank and in some cases a popping as the tank expands odds are the valve is bad.

Another simple item is to note if the problem occurs with a full tank or an empty tank. Empty tank occurrences tent to point to the fuel volatility issue as there is more room for vaporization within the tank.
 






mikeh

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Pull the MAF sensor and clean it.

Let me know if I can help. I'm in St. Louis too.

Mikeh
 






Rhett

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With 160,000 mi you will have stuff that needs replacing. I'm not too far from St. Louis either! Lots of MO people in this thread!
 






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Hey MikeH that would great. I am out in West St. Louis County. Where are you at? Would be great if someone could help me fix this problem. This truck just has 160,000 miles on it. I will private message you my number. Thanks.
 






Oz Actionjaxxon

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hey guys,
whats the best way and what do you use to clean the MAF sensor??
 












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