Good morning. Wondering what more experienced folks than myself think about this.

My car came with exhaust modifications. I don't know brand names for any change. Smaller CATS.

The STFT hover around ZERO +/- (spike sometimes but not much or often) rarely over 5 +/-... At the same time my LTFT rarely go below 8 and hover mostly around POSITIVE 11, 12, 13, 14 - especially at idle. They always match, less and more, bank one to bank two.

I've looked for vac leaks. Found one or two, nothing major, fixed, and that hasn't made much of a difference.

I have no codes and the CATs come back as GREEN for drive cycle complete-ness and ready-ness... I'm open to doing tests (smoke, maybe a spray while idling listening for changes etc) but I'm wondering if folks who have made modifications to converters have experienced this and is higher than normal LTFT are normal with a high flow/mods to CATs and something to just live with?

What now?

 

Check the vacuum line that goes into the right side of the throttle body. There is a o-ring in there. I finally located it by spraying brake cleanser on it.

 

i’m at like 7 or 8 it’s not a big deal.

you need a smoke machine to find this stuff or just throw more parts at the problem. brake booster line, EGR, oil cap and dipstick seal come to mind.

 

I'll look...
And test.

Does it make sense that upper sensors are so in spec and the lowers aren't? Is that a thing?

 

I'm no expert but have tried to get my head around fuel trims in the past. I believe both STFT and LTFT are measured by the upper heated O2 sensors, the lower (after CAT) sensors just test the effectiveness of the CATs. Positive fuel trim indicates that fuel is being added and suggests a vacuum leak. However, you stated that the LTFTs are much the same on both banks so you need to look for an issue common to both banks, perhaps a MAF sensor issue? However, something else that occurs to me is that if your CATs are non-standard it may be that the high LTFTs are simply adjusting for the non-standard CATs. This is perhaps bourne out by the fact that you are getting good (around 0) STFTs.