PCM relearn - Battery disconnect or fuse pull?

WarGawd

Original poster
Member
Sep 2, 2012
468
Bought my Envoy July 7th, as of tomorrow I will have put 7000km on it.

I've been lurking the threads and articles on the OS and here for about 8 weeks now. I've learned a lot, but honestly sometimes I can't really be certain that what I think I've learned is applicable to my model year/engine/options etc etc.

I am gearing up to do a bunch of pre-winter maintenance and repairs, and I've amassed a pretty good backlog of questions that I want direct answers for so I can prioritize and budget. Although many are somewhat interrelated I'm gonna try to keep things focused on 1 question per thread (you'll see many in the coming days)

First: One question that's been bugging me but hasn't been answered anywhere AFAIK, I know that the PCM has to do a relearn after a TB cleaning. MOSTLY what I see advocated is disconnecting the battery, but a MAY03LT youtube video showed an alternative that I used when I cleaned mine: pull the PCM fuses.

Given there are apparently some risks (HVAC door mode actuator motor) and mild inconveniences (clock reset) associated with the battery disconnect method, and pullng the fuses is easier and faster, why does the consensus seem to remain battery disconnection? Am I missing something? Why have I never seen anyone in any TB cleaning thread suggest this alternative?

The reason I ask is that post cleaning (removed and meticulously & thoroughly cleaned to spotlessness!), I had a few issues of laggy throttle response, erratic low speed driving throttle response {IE experiencing a brief throttle surge and vehicle forward lurch upon letting OFF the throttle}, one or two instances where vehicle didn't start the way it normally does, and even one time a stall. These have all but cleared up, suggesting the PCM has gone through the relearn and made it's adjustments, but then have to ask myself what adjustments? I was of the understanding that resetting the PCM caused it to go back to factory like conditions using the assumption that the TB is new/spotless, which is the case if I did as good a cleaning job as I think I did.

SO, does fuse removal initiate exactly the same PCM relearn as battery disconnect? Can anyone speculate as to possible causes for the symptoms I saw?

Thx
WarGawd
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
WarGawd said:
SO, does fuse removal initiate exactly the same PCM relearn as battery disconnect?
Yes, and it takes some driving around to allow the PCM to learn the new behavior of the engine it's controlling. The factory algorithms aren't a one-size fits all set of numbers - many of the tables need to be filled in by the PCM while driving in a varied way. That means, you won't get good city performance until you drive around in city conditions. A highway drive will only get good table data for highway conditions.

We normally recommend disconnecting the battery for noobs because there are plenty of other places to put a tool on an energized vehicle that will cause a spark and damage. If an HVAC actuator dies, it was a weak one and deserved to die and it's better to damage it at a time and place of your choosing rather than when you change batteries in the heat of summer or the dead of winter, after dark, in a bad neighborhood with pit bulls eyeing your ass for an after-dinner snack. :eek: :frown:
 

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