Help with resetting long term fuel trim

Ross Reynolds · January 15, 2020

So I have been chasing down a lean issue and have effectively brought my short term fuel trim down to bouncing between -3 and +3, but long term is stuck maxed out at +29 on both banks. There has to be a way to reset my long term fuel trim without a tech2. Can anyone please help?

Ross Reynolds · January 15, 2020

I know there was a way with the ECM fuse to reset long term memory on earlier models, but not sure the steps or if that will work with the newer models. I have 173,XXX on my truck so after I found the fuel pump and three or four sensors that were barely working but not throwing codes, the pump was not delivering enough pressure, test voltage and ohm readings on the sensors were low, so I decided to change them all. Truck runs better than I have ever had it and like I said earlier I got my STFT down to where I’m very happy but after driving for awhile I’m still throwing lean codes on both banks and I think it is the LTFT retripping the codes.

Ianf · January 15, 2020

I reset mine using a VXDIAG for GM and my PC. If you have a flex fuel vehicle the ethanol % may be way off as well. It can also be reset. I found the test port of the manifold cover was removed which was giving me high long term fuel trims. What codes are you getting?

CamGTP · January 15, 2020

Unhook the battery, both cables.

Let it sit for a bit, then take the two cables and touch the ends together. That should help reset the trims. Or just use a scan tool.

riverbanks · January 15, 2020

Still a problem, check freeze frame,what rpm is it at when lean code sets,that's the rpm you need to look at,lt trims should be dropping if problem gone

swathdiver · January 15, 2020

17 hours ago, Ross Reynolds said:

So I have been chasing down a lean issue and have effectively brought my short term fuel trim down to bouncing between -3 and +3, but long term is stuck maxed out at +29 on both banks. There has to be a way to reset my long term fuel trim without a tech2. Can anyone please help?

You should also reset the fuel composition sensor with trims that out of whack too. Make sure the fuel is E10. But there is no way to do this that I know of without a bi-directional scan tool.

I know guys that use little Dell tablets with the VXDIAG Nano and GDS software for cheap or with a $350 Tech-2.

riverbanks · January 15, 2020

Cam,you still touch cables together?on the newer ones,I'm hesitant,I'll leave the key in run position,door open, headlights on,I feel safer that way

Ross Reynolds · January 16, 2020

Thanks for the info everyone, the more I learn the better. After more research last night, instead of looking for short term to bounce around 0+/- it should be reading rich while driving the long term down. Well, last piece of the puzzle will be injectors. It’s the only part of fuel and evap systems that aren’t brand new.

Gotta say though that I want to change them all as part of the refresh project but 6-800 bucks is really hard to come off of right now so I’m weighing getting the rock auto house brand for a quarter the price. They claim to test them just like the bigger names. I try to only put Delco or Denso parts on the truck but that price tag is killin me...

Any input about injector brands?

Ross Reynolds · January 16, 2020

I also meant to say that I do have a good reader but it’s not bi directional. I will be looking into the vxdag. Also my truck is not hybrid so I don’t think I have to worry about the alcohol content , is that correct?

riverbanks · January 16, 2020

Flexfuel is the one,I would be looking at vacuum leak first, remember 02's determine trim,if lean code shows up, vacuum problem

Ross Reynolds · January 16, 2020

That was my first thought as well, but I had it smoked and nothing was found. Changed out the intake gasket anyway with one of the thick Delco Pro gaskets.

All my hoses, connections look good.

sefiroxx · January 16, 2020

Cam,you still touch cables together?on the newer ones,I'm hesitant,I'll leave the key in run position,door open, headlights on,I feel safer that way

Yes, touchIng cakes together is safe.

Memory are 3 types.
- Short term is maintained by battery with key on
- long term is maintained by capacitors and battery. Touching cakes together for 5-10 seconds will complete the circuit and drain down the capacitors quickly.
- permanent memory is not battery dependent and will be retained on power loss. This includes the basic programming of the car, but not anything tied to stored sensor readings.

swathdiver · January 16, 2020

16 hours ago, Ross Reynolds said:

I also meant to say that I do have a good reader but it’s not bi directional. I will be looking into the vxdag. Also my truck is not hybrid so I don’t think I have to worry about the alcohol content , is that correct?

Is this a GMT800 or GMT900? What engine do you have? L33, L59, LM7, LMG, LC9, LY5?

Ross Reynolds · January 17, 2020

First off thank you for the battery/capacitor break down, that makes perfect sense.
Secondly to James, it is a GMT900, but I do not know which engine I have. How embarrassing. It is a 5.3, gas not hybrid and Canadian, vin code J. Would engine type be stamped on the block?

swathdiver · January 17, 2020

2 hours ago, Ross Reynolds said:

First off thank you for the battery/capacitor break down, that makes perfect sense.
Secondly to James, it is a GMT900, but I do not know which engine I have. How embarrassing. It is a 5.3, gas not hybrid and Canadian, vin code J. Would engine type be stamped on the block?

J is for the LY5 which is not a FlexFuel engine. I do not know if it would calculate for the alcohol content in fuel, I'm only familiar with the FlexFuel Engines. Yours is near identical to mine save for the larger injectors on mine, yellow gas cap and programming. Fix your signature Ross, you are driving an NNBS (GMT-900).