Issues with IAC valves

[spag1]

I went on a road call to a client's location the other day because his car wouldn't stay running. He had replaced the battery because it was dead, but couldn't get his car to stay running. I reset the PCM and started it up without any issues. My client was frazzled and frustrated and asked me what my magical trick was. I informed him that sometimes when a battery dies it will fool the IAC and PCM into hot mode rather than cold mode. So for future reference keep this in mind. Hope this helps, because not everyone knows about that. I worked as an auto mechanic years ago but now I work as a diesel mechanic.

 

[mattd]

what actually happens is the PCM learns the idle with a carboned up throttle body. Once he replaces the battery, the PCM is reset and thinks the T/body is clean. Since it is not, the throttle blade is not opened enough to keep the engine running. Cleaning the throttle body will solve this problem.

By any chance, is this vehicle a toyota?

 

[LeakySeals]

 

[spag1]

Originally Posted By: mattd
what actually happens is the PCM learns the idle with a carboned up throttle body. Once he replaces the battery, the PCM is reset and thinks the T/body is clean. Since it is not, the throttle blade is not opened enough to keep the engine running. Cleaning the throttle body will solve this problem.

By any chance, is this vehicle a toyota?

You raise a valid point, but the throttle body was cleaned about a month ago and it was still clean upon opening the butterfly valve, but I have seen that happen in that scenario. Nope it wasn't a Toyota, it was on a 2000 VW jetta with a 1.8 gas pod.

 

[Vikas]

Can you tell us why the PCM did not reset automatically when the battery was replaced but you had to do it explicitly?

I am presuming you really meant to say "relearning idle" after the battery replacement and not really meant the PCM reset as you said in your post.

 

[spag1]

Sometimes pulling terminals off of the battery will not necessarily re set the PCM. Sometimes it needs to be flashed with my laptop.

 

[Vikas]

Then may be we are using different terminology. PCM gets reset when the power is removed to it. Using computer, you can instruct the PCM to reset itself but that is NOT called "flashing" it.

It is certainly possible that the guy who installed the battery, "glitched" the connections so that PCM went in to weird state but that would be low probability event.

Since you posted this almost as public service announcement, it is only fair that you elaborate on what you are trying to convey.

 

[SteveSRT8]

Originally Posted By: spag1
Sometimes pulling terminals off of the battery will not necessarily re set the PCM. Sometimes it needs to be flashed with my laptop.


Exactly. The VW software is known for being a bit "cranky" (pun intended).

These days yanking the battery can cause issues with many cars. Everyone is different, so many will never experience this problem at all...

 

[meep]

yanking a battery on a later model BMW can result in absolutely ridiculous behavior.

test drove one that had been sitting, had to charge it to start it. It ran on maybe 4 of 6 cylinders, air blew cold but zero control over vents, dash was useless. Had to keep foot in gas to prevent stalling.

Got back. shut it down, restarted 5 minutes later, ran like a dream.

 

[SwampSurvivor]

Originally Posted By: SteveSRT8
Originally Posted By: spag1
Sometimes pulling terminals off of the battery will not necessarily re set the PCM. Sometimes it needs to be flashed with my laptop.


Exactly. The VW software is known for being a bit "cranky" (pun intended).

These days yanking the battery can cause issues with many cars. Everyone is different, so many will never experience this problem at all...



I replaced a headlight on a friend's honda accord. The battery had to come out.

Removing the battery deactivated the radio / clock / HVAC system.

What an asinine idea.

Good thing the previous owner had the radio code noted in the owners manual. I'm assuming the previous owner went through the same thing when they installed the new battery.

 

[threeputtpar]

Originally Posted By: Vikas
Can you tell us why the PCM did not reset automatically when the battery was replaced but you had to do it explicitly?

I am presuming you really meant to say "relearning idle" after the battery replacement and not really meant the PCM reset as you said in your post.


VW and Audi have a bit of power reserve for when a battery is changed so you won't lose the radio or the presets or the time on the clock. Having power disconnected for a little bit won't reset the ECU.

They also have a procedure called throttle body adaptation that may have been done to fix the problem from the original post. I've had to do it a couple of times when I sent out my ECU for reporgramming. Usually the TBA will be done when the ECU is removed and reinstalled, so if one doesn't have the proprietary software for VAG, pulling the ECU and plugging it back in will accomplish the same thing.

 

[Clevy]

Originally Posted By: threeputtpar
Originally Posted By: Vikas
Can you tell us why the PCM did not reset automatically when the battery was replaced but you had to do it explicitly?

I am presuming you really meant to say "relearning idle" after the battery replacement and not really meant the PCM reset as you said in your post.


VW and Audi have a bit of power reserve for when a battery is changed so you won't lose the radio or the presets or the time on the clock. Having power disconnected for a little bit won't reset the ECU.

They also have a procedure called throttle body adaptation that may have been done to fix the problem from the original post. I've had to do it a couple of times when I sent out my ECU for reporgramming. Usually the TBA will be done when the ECU is removed and reinstalled, so if one doesn't have the proprietary software for VAG, pulling the ECU and plugging it back in will accomplish the same thing.


I've found the same thing. I have to purposely discharge the system by disconnecting both battery cables and touching them,or leaving the cables unhooked overnight.
My hemi actually held a charge from evening til the next morning and didn't reset my radio. I was trying to get some zip back in the pedal.
My mustangs are the same way.

 

[mattd]

Originally Posted By: Miller88
Originally Posted By: SteveSRT8
Originally Posted By: spag1
Sometimes pulling terminals off of the battery will not necessarily re set the PCM. Sometimes it needs to be flashed with my laptop.


Exactly. The VW software is known for being a bit "cranky" (pun intended).

These days yanking the battery can cause issues with many cars. Everyone is different, so many will never experience this problem at all...


I replaced a headlight on a friend's honda accord. The battery had to come out.

Removing the battery deactivated the radio / clock / HVAC system.

What an asinine idea.

Good thing the previous owner had the radio code noted in the owners manual. I'm assuming the previous owner went through the same thing when they installed the new battery.


for future reference, access the honda headlights through fender liner, especially driver's side. takes 2 seconds, there may be only 1 or 2 if any clips to remove.

 

[Pop_Rivit]

Our Subaru has a "procedure" to relearn the idle after the battery has been replaced or disconnected for maintenance.

After the battery has been reconnected turn the ignition switch on for 10 seconds before starting the engine. After 10 seconds start the car, and let it idle with no load for 10 minutes or so. It will idle very low, or the idle will hunt until the computer gets the settings adjusted, then it will start idling normally. Once that happens it will drive and idle normally.

If the battery is changed or disconnected without doing the above, it takes a while for the computer to adjust the engine settings, and it stalls every time the clutch is pressed while coming to a stop. It takes an hour or so of real driving before it will relearn the idle if I just take off and go after a battery disconnect.

 

[Merkava_4]

Originally Posted By: spag1
Sometimes it needs to be flashed with my laptop.


Which software are you running on your laptop to do that?