Hi,
In the idle relearn procedure, it got "Start the engine and allow the coolant temp to reach a minimum of 176 degrees F".
Do we have to start the cold engine or can we drive it a bit to easily get to 176 degrees and start the "idle relearn"?
Thanks,
-Mike

 

It's best to do from cold start if the battery has been disconnected. There are values retained in PCM memory below 176* - these are lost with disconnect.

 

I have always done it from a cold start.. doesn't take long...

 

If you just forget the idle relearn and drive the car you will be fine.

Bill

 

Thanks yall!!!

I may not think so because I went on vacation last year and took out the battery and when I got back, the idle got messed up

 

So?? Get in the car and drive it for a few miles. When you get back it will have learned all of the engines parameters it needs to know. By the way it can't learn them all if it is sitting and idling. All you are doing is allowing the computer to re-learn the individual operating characteristics of the engine it is controlling. Most of those have nothing to do with idling. I just loaned my battery to a friend so he could drive his C5 back from an event while I trailered my car. He returned the battery so I could get my car off the trailer and I never bothered with a relearn. Did the same thing when I installed my Blackwing.

Bill

 

My experience has been that the more mods a car has the longer it takes to re-learn the idle--So it would benifit to do the re-learn procedure--on a basically stock engine with just a few mods like CAI and catback-- the idle will relearn on it's own pretty quickly--so the procedure would not be needed- A car with several mods like hds/cam etc. WOULD benefit from starting all over as the LTFT's are way off---The only exception being a modded car that has been carefully retuned with the mods and has the LTFT's retuned close to "0" will learn pretty quickly on it's own as well just as a stock type

 

I just let mine run in neutral (parking brake on) for around 5 minutes. The idle was relearned and smooth.
I did the idle relearn after I replaced my starter. I had disconnected the battery to replace my starter which caused my idle to drop when I was stopping, then shoot back up once I stopped.

 

 

It has been my experience that LTFT's play a very small part in idle learning. If they are at the extremes it will certainly affect idle quality but they aren't what the idle learn procedure is about. As Bink touched on above the idle learn procedure is designed to reset the Short Term Idle Trims and the Long Term Idle Trims. They are airflow values that are added or subtracted to the idle airflow final values. These values are kept in non-volatile RAM in the PCM (static ram) and are constantly being updated when the engine is in idle mode (cell 19 0r 18). These values are used in conjunction with intake air temp, engine coolant temp, idle airflow parked, and running airflow, in addition to several other modifiers. The end result is an airflow offset value that is converted to a throttle blade offset that is applied to the throttle blade opening at idle. The reason for this is charge density will vary from day to day, due to air temp and humidity. Since idle airflow rates are small these factors will influence engine characteristics quite a bit. Since these are values that are being updated constantly when the engine is in cell 19 or 18 the car will learn it's idle even if driven normally. The reason the idle learn is performed is if the battery has been disconnected, the LTIT (long term idle trim) values are lost from static memory and are zeroed out.The reason for letting the car reach 176 degrees and doing it from 'cold' is there is a cell in the PCM that enables idle learning only after a certain engine coolant temperature has been reached. I think it's 60 degrees C in a stock PCM. There is also a 'cold engine timer' value that is only applied if the engine coolant temp is below a certain value. So doing it from a 'cold' start and running the idle learn will give you a new set of LTIT's before you drive off and the car should transition to and from idle nicely when you start to drive it.
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I think the idle learn procedure was designed for the tech that works on your car and has to disconnect the battery. If it isn't done, the car may idle transition poorly at first when the customer comes to pick the car up and make it appear the car is running worse than when delivered for service.