Mounce
Member
Got any local junk yards? It'll be cheaper from a pull-a-part yard than eBay. Try to find one the same year model as yours.
Totally stumped on stumble - any gm master techs out there?
- Thread starter jbwi112
- Start date
Ebay pcm is $85 shipped and programmed with latest firmware and my vin seems pretty cheap to me, also lifetime warranty
I assumed a yard would charge $75+ for one no?
I assumed a yard would charge $75+ for one no?
Mounce
Member
Idk, you'll probably be better off getting the eBay one since it'll have all of the programming done.
I seem to remember people scoring expensive modules cheap from junkers but even if you got a pcm for $20 you wouldn't come out under $85 after programming like the eBay one will have. I hadn't thought about it that way.
I seem to remember people scoring expensive modules cheap from junkers but even if you got a pcm for $20 you wouldn't come out under $85 after programming like the eBay one will have. I hadn't thought about it that way.
Mooseman
Moderator
Don't need eBay for that. Just go to a pick a part yard and get a used one from a donor. All you need is a security relearn. If it works out, you will get a p1345 for a CASE relearn that must be done by a dealer or shop with a Tech 2 or other high end scanner. First check that the replacement PCM works before doing it.
Mounce
Member
But won't the charge for CASE relearn be $50+? That's where getting the one from eBay would be better.
I thought that until I typed this and realized that even the eBay pcm will need a CASE relearn. So, we're back to my initial thoughts of the junk yard being a better deal.
I thought that until I typed this and realized that even the eBay pcm will need a CASE relearn. So, we're back to my initial thoughts of the junk yard being a better deal.
Mooseman
Moderator
Exactly. CASE relearn is always needed after replacement of the PCM. That's why a lot of people like to have their original PCM tuned.
You can run it indefinitely without the CASE relearn. You'll just get the P1345 and CEL. It's just to tell you if there is a variation between the crank and cam angles. After confirming that the new PCM clears up your issues, then spend the money for it.
You can run it indefinitely without the CASE relearn. You'll just get the P1345 and CEL. It's just to tell you if there is a variation between the crank and cam angles. After confirming that the new PCM clears up your issues, then spend the money for it.
Replaced pcm with one out of a 2004, performed relearn. Started truck, stumble remains 
Might as well just deal with it, drive it like i stole until in dies then part it out
I think there is a problem somewhere in the 10 miles of wire harness wire, I should know better then to buy the first year of any product
Might as well just deal with it, drive it like i stole until in dies then part it out
I think there is a problem somewhere in the 10 miles of wire harness wire, I should know better then to buy the first year of any product
gmcman
Member
- Dec 12, 2011
- 4,656
Looking back, you stated the stumble almost went away with the coolant sensor unplugged. Thinking about this it's probably because you were at a higher idle and the stumble wasn't apparent since the engine was already sped up.
I tried to replicate your stumble by unplugging my cam sensor and cpas harness and it ran the same for the most part.
Are you 100% sure, the entire edge of the throttle butterfly is perfectly clean?
What does your air filter look like, is the intake tube free of a rodent nest? Try a quick test with no air filter and see if it stumbles.
This is a vid I just took of my vacuum reading using the throttle body port. The needle is steady except for a slight movement when the TB controls idle. Other than the very, very slight increase and decrease in vacuum for idle control, it's basically rock steady. Have you checked your vacuum and is the needle steady?
https://www.youtube.com/watch?v=FqPjTkJw4OE
Look down at your harmonic balancer and see if the center bolt looks like it has been removed in the past, scratches on the bolt or uneven patterns in the dirt. There was a revision to the very early models in balancer tightening.
Just throwing more ideas hoping to not have to verify timing. I doubt it's timing related if it idles perfectly smooth.
Are you able to upload a vid of the stumble, maybe we can see how severe it is.
I tried to replicate your stumble by unplugging my cam sensor and cpas harness and it ran the same for the most part.
Are you 100% sure, the entire edge of the throttle butterfly is perfectly clean?
What does your air filter look like, is the intake tube free of a rodent nest? Try a quick test with no air filter and see if it stumbles.
This is a vid I just took of my vacuum reading using the throttle body port. The needle is steady except for a slight movement when the TB controls idle. Other than the very, very slight increase and decrease in vacuum for idle control, it's basically rock steady. Have you checked your vacuum and is the needle steady?
https://www.youtube.com/watch?v=FqPjTkJw4OE
Look down at your harmonic balancer and see if the center bolt looks like it has been removed in the past, scratches on the bolt or uneven patterns in the dirt. There was a revision to the very early models in balancer tightening.
Just throwing more ideas hoping to not have to verify timing. I doubt it's timing related if it idles perfectly smooth.
Are you able to upload a vid of the stumble, maybe we can see how severe it is.
Thanks for the vacuum video upload, I actually hooked up my gauge a few days ago and tested, its reading normal and reacts as expected to rpm changes. ALso checked intake manifold bolts, all tight.
I decided to learn more about fuel trims, o2 sensor and map sensor data so I could make use of the information coming from my scanner. 3 hrs and 10 youtube videos later I knew exactly what fuel trims are, expected map voltage and 02 voltage at different rpm's, and how these are important to figure out what is going on here.
My fuel trim is -17% at idle and get better as rpm increases. The upstream 02 is reading a low amount of oxygen at idle and is "trimming or lowering" the fuel -17%.
I am almost positive its a leak at the exhaust manifold before the first 02 sensor. At idle the 02 reads lean, as engine rpm's increase, the leak becomes less of a issue due to more exhaust flow and the fuel trims get back to 0
I will check the exhaust manifold bolts, I think they may be loose....
I decided to learn more about fuel trims, o2 sensor and map sensor data so I could make use of the information coming from my scanner. 3 hrs and 10 youtube videos later I knew exactly what fuel trims are, expected map voltage and 02 voltage at different rpm's, and how these are important to figure out what is going on here.
My fuel trim is -17% at idle and get better as rpm increases. The upstream 02 is reading a low amount of oxygen at idle and is "trimming or lowering" the fuel -17%.
I am almost positive its a leak at the exhaust manifold before the first 02 sensor. At idle the 02 reads lean, as engine rpm's increase, the leak becomes less of a issue due to more exhaust flow and the fuel trims get back to 0
I will check the exhaust manifold bolts, I think they may be loose....
gmcman
Member
- Dec 12, 2011
- 4,656
-17 is a lot. 
Generally you try to keep a value of 10 at the highest but 7 is ideal with the short term making up the difference of the long term. If you have loose exhaust bolts then yes the exhaust will suck in air resulting in a rich mixture. However if you don't hear a pronounced tick, tick, tick from the manifold then you may not have this issue.
What fuel trim is -17? STFT, LTFT?
Edited to add, sounds like you have a lean miss. With -17, the PCM is pulling out A LOT of fuel and when you hit the gas it needs to transition back to a steady fuel flow. This may explain the stumble.
I'm hoping it's leaking/dirty injectors and they are dribbling a little instead of a fine mist at idle.
What is your fuel trim with a STEADY foot at highway cruise, level ground?
Generally you try to keep a value of 10 at the highest but 7 is ideal with the short term making up the difference of the long term. If you have loose exhaust bolts then yes the exhaust will suck in air resulting in a rich mixture. However if you don't hear a pronounced tick, tick, tick from the manifold then you may not have this issue.What fuel trim is -17? STFT, LTFT?
Edited to add, sounds like you have a lean miss. With -17, the PCM is pulling out A LOT of fuel and when you hit the gas it needs to transition back to a steady fuel flow. This may explain the stumble.
I'm hoping it's leaking/dirty injectors and they are dribbling a little instead of a fine mist at idle.
What is your fuel trim with a STEADY foot at highway cruise, level ground?
gmcman
Member
- Dec 12, 2011
- 4,656
Have you tried an Italian tune up yet? If you have ran some of the 44K through, I would take it out on a highway, put in 2nd gear and from 3 or 4K, run some WOT pulls to about 5500to 6K and let it slow down in 2nd to about 3 -4 K again...then hit it again. This will give the injectors 100% duty cycle and if you have any carbon will work some of it out. Watch the rear when you hit the gas and see if any puffs come out, may only happen after the first decel.
