Fuel Trim wonky... where to start

[project_x]

I recently did a ton of maintenance on my Suburban.
I installed a new transmission, transcooler, transcooler lines, and rad.
I cleaned my K&N Air filter.
I also installed dual CATs (from a 6.0 Yukon Denali)... instead of the y-pipe that was on.
I don't currently have a muffler(s) on, so haven't driven very much.
My short term fuel trim is 26% both sides, and long term is 25%...

Where should I start? I don't believe I had any issues prior to making the above changes....

 

[justirv]

I recently did a ton of maintenance on my Suburban.
I installed a new transmission, transcooler, transcooler lines, and rad.
I cleaned my K&N Air filter.
I also installed dual CATs (from a 6.0 Yukon Denali)... instead of the y-pipe that was on.
I don't currently have a muffler(s) on, so haven't driven very much.
My short term fuel trim is 26% both sides, and long term is 25%...

Where should I start? I don't believe I had any issues prior to making the above changes....

Since you're charting trims one can assume you don't have any codes. My first thought is your K&N (freshly) oiled, and is there any chance of MAF contamination? If thats good, your ECU is compensating for a bit of extra air. Any chance of vacuum leak(s)?

 

[Fless]

Ditto on the MAF potentially needing cleaning due to K&N air filter.

Are these trims reading this way at idle, or at higher rpms? What do they look like at 1800 or 2000 rpm?

 

[project_x]

I will clean the MAF.
They look about the same... And I've seen as much as 30%

 

[project_x]

MAF is cleaned no change.
My upstream 02 Sensors are mostly reading 0... but I can't believe both are bad... I have a spare that I will replace and see if it makes a difference...

As far as a vacuum leak, where should I start looking based on the work I did?

 

[rockola1971]

MAF is cleaned no change.
My upstream 02 Sensors are mostly reading 0... but I can't believe both are bad... I have a spare that I will replace and see if it makes a difference...

As far as a vacuum leak, where should I start looking based on the work I did?

O is a no go. They should be switching a mv range continuously. Have you verified that your vehicle even goes into closed loop once warmed up. I bet it doesnt.

 

[project_x]

My scanner says it switches to closed loop.

 

[project_x]

I also see voltage on one of them (bank 1) sometimes... Bank 2 seems to be 0 or 4mV

 

[project_x]

I replaced Bank 2 Sensor 1 and it looks like that corrected the problem.... now I need to get Bank 1 Sensor 1 loosened... it is stuuuck

 

[rockola1971]

I replaced Bank 2 Sensor 1 and it looks like that corrected the problem.... now I need to get Bank 1 Sensor 1 loosened... it is stuuuck

May have to heat the bung with a torch.
FYI Your upstream sensors are what the PCM uses for rich/lean condition to adjust your fuel injector pulse width. Your downstream sensors only job in life is to tell you when your cats are bad.