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Idle air control valve question. video included

blu3fan

NAXJA Forum User
Location
Arizona
So.
This last 6 months the jeep has required me to hold the gas to get it running.
and for the first 2-3mins of running to keep it from stalling.

Today I changed the idle air control valve.
and we get this result.
Its an improvement.
but not sure why she is doing this.
she revs to 1700-1900 then goes to normal idle.

https://youtu.be/ntj_PgCJ5dM

I have changed.
TPS.
CPS.
cleaned throttle body.

Have not changed map sensor.
thoughts.....
 
What kinda vehicle ?

The high revs will clear up when the computer learns and stores the new parameters.

All those symptoms point to low voltage. The OBD engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will relearn the idle settings after a short period of driving and the engine will often start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the PCM, the ground wires at the coil, and the ground wires at the oil dipstick tube. You must remove, wire brush, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance for smooth starts.
 
What kinda vehicle ?

The high revs will clear up when the computer learns and stores the new parameters.

All those symptoms point to low voltage. The OBD engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will relearn the idle settings after a short period of driving and the engine will often start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the PCM, the ground wires at the coil, and the ground wires at the oil dipstick tube. You must remove, wire brush, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance for smooth starts.

92 xj.
4.6 liter stroker.
aw4.

nost components are brand new in the engine bay now aside from map. and pcm and ecu.

alt tested good.
battery tested good.
All grounds have been cleaned.
I have also created 5 grounding straps along the engine.

my question is.
is the IACV faulty.
causing this surge.
Even now the car is idling too high.
drove it to work today. every start she surges to almost 2k. like the video.
 
Last edited:
The IAC is a stepper motor that the computer moves to adjust the idle speed. It does not adjust the speed immediately when idling. It takes an average over time to adjust the set point. The set point is different at startup and later when warm. It takes a second to adjust.

In order to have the IAC set correctly for start up, the computer needs to set it when the engine is shut off. To accomplish this, there is a ASD relay (Auto Shut Down). Its purpose is to keep power on the computer for a few seconds after the engine is shut off so it can preset the IAC for startup conditions. If this relay is not functioning correctly (common), the IAC is not preset to the correct position for startup and the rpms will be high until the computer eventually compensates and drops the rpms. This is exactly what you describe.
 
Sticking open, leaking fuel injector? Or a leaking FPR Fuel Pressure Regulator leaking fuel into the vacuum line. Also a dirty or bad IAT Intake Air Temp sensor will do that, as will a variable intake manifold leak that changes in size as the engine warms up (or other variable vacuum leak, throttle body gasket....).
 
The fact that idle drops like it does, tells me the IAC is working.

If the O2 sensor is working, the higher rpm at first is likely due to a fuel leak while cranking!!! See my earlier post.

Disconnect the IAC and find out how it acts, cold, warm hot etc!! I think its main job is to raise the idle speed when the AC clutch is engaged, or at stops when turning a corner....

The IAC is not designed to compensate at start up for a fuel leak!!! But the O2 sensor and ECU do!!!

92 xj.
4.6 liter stroker.
aw4.

nost components are brand new in the engine bay now aside from map. and pcm and ecu.

alt tested good.
battery tested good.
All grounds have been cleaned.
I have also created 5 grounding straps along the engine.

my question is.
is the IACV faulty.
causing this surge.
Even now the car is idling too high.
drove it to work today. every start she surges to almost 2k. like the video.
 
Last edited:
So.
This last 6 months the jeep has required me to hold the gas to get it running.
and for the first 2-3mins of running to keep it from stalling.

Today I changed the idle air control valve.
and we get this result.
Its an improvement.
but not sure why she is doing this.
she revs to 1700-1900 then goes to normal idle.

https://youtu.be/ntj_PgCJ5dM

I have changed.
TPS.
CPS.
cleaned throttle body.

Have not changed map sensor.
thoughts.....

Looks perfectly normal to me.
 
The IAC is a stepper motor that the computer moves to adjust the idle speed. It does not adjust the speed immediately when idling. It takes an average over time to adjust the set point. The set point is different at startup and later when warm. It takes a second to adjust.

In order to have the IAC set correctly for start up, the computer needs to set it when the engine is shut off. To accomplish this, there is a ASD relay (Auto Shut Down). Its purpose is to keep power on the computer for a few seconds after the engine is shut off so it can preset the IAC for startup conditions. If this relay is not functioning correctly (common), the IAC is not preset to the correct position for startup and the rpms will be high until the computer eventually compensates and drops the rpms. This is exactly what you describe.
I will look at this.

I will also clean the throttle body again just to make sure junk isnt stuck.

anyone saying this is normal clearly has only owned an xj with issues...
11 years owning this car. It has never surged on start.
 
The fact that idle drops like it does, tells me the IAC is working.

If the O2 sensor is working, the higher rpm at first is likely due to a fuel leak while cranking!!! See my earlier post.

Disconnect the IAC and find out how it acts, cold, warm hot etc!! I think its main job is to raise the idle speed when the AC clutch is engaged, or at stops when turning a corner....

The IAC is not designed to compensate at start up for a fuel leak!!! But the O2 sensor and ECU do!!!


The fuel leak comment is interesting.
I have an inline fuel pressure gauge.
Ill get a video on here of what she does. but there is fluctuations when you rev the engine quickly
just not a lot.
should pressure be consistent or no ?
 
You need to look for any excess fuel pressure drop with out the engine running, after the FP is primed. Even then a small slow leak may not be detected by the FP Gauge. It may slowly leak fuel into the intake, that shows up in the first few seconds as a rapid idle. You will not see that kind of small leak with the engine running on the FP gauge.

Do not ignore the comments below on the relay that stays powered to reset the IAC when ignition is turned off. Easy test might be to disconnect the IAC while running at idle right after a cold or hot start, then restart right away and see what happens in both cases. Does the fast idle fix itself with out the IAC connected, does it even show up ???? Then prime it, wait 5 minutes, prime and start it again (IAC disconnected, the power connector) to see if the high early idle shows up then!!!!

If it does (fix itself with no IAC connected), it is likely a mini fuel leak FPR into the vacuum line at the nipple (just pull the Vacuum line after priming but not starting, if you see, or feel a wet fuel spot or smell fuel, the FPR Diaphragm is leaking. I am pretty sure your year rig still had one on the front of the fuel rail!!!??? With the Vacuum nipple... 96 and newer did not have the vacuum operated FPR, the FPR was moved to the gas tank to make it easier :D to replace (LMAO)

It is also possible it is a vacuum leak at start up that IAC compensates for after a few seconds.

The fuel leak comment is interesting.
I have an inline fuel pressure gauge.
Ill get a video on here of what she does. but there is fluctuations when you rev the engine quickly
just not a lot.
should pressure be consistent or no ?
 
You need to look for any excess fuel pressure drop with out the engine running, after the FP is primed. Even then a small slow leak may not be detected by the FP Gauge. It may slowly leak fuel into the intake, that shows up in the first few seconds as a rapid idle. You will not see that kind of small leak with the engine running on the FP gauge.

Do not ignore the comments below on the relay that stays powered to reset the IAC when ignition is turned off. Easy test might be to disconnect the IAC while running at idle right after a cold or hot start, then restart right away and see what happens in both cases. Does the fast idle fix itself with out the IAC connected, does it even show up ???? Then prime it, wait 5 minutes, prime and start it again (IAC disconnected, the power connector) to see if the high early idle shows up then!!!!

If it does (fix itself with no IAC connected), it is likely a mini fuel leak FPR into the vacuum line at the nipple (just pull the Vacuum line after priming but not starting, if you see, or feel a wet fuel spot or smell fuel, the FPR Diaphragm is leaking. I am pretty sure your year rig still had one on the front of the fuel rail!!!??? With the Vacuum nipple... 96 and newer did not have the vacuum operated FPR, the FPR was moved to the gas tank to make it easier :D to replace (LMAO)

It is also possible it is a vacuum leak at start up that IAC compensates for after a few seconds.

smart way to do it.


I am pretty sure I know where my leak is. causing the small drop.
I replaced the fuel pressure regulator.
and the O ring it came with was the wrong size.
I replaced it with the ****correct one. but it still wasnt right. even swelled. the oem one was bigger.
I need a thicker O ring for that fpr might be it.
 
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