GM LS1 Turning short term fuel trims on causing car to run rough and lean

Why is it when turning short term fuel trims back on after tuning ve table car runs rough and lean in spots ? when short term trims turned off car runs good 5 to 10 % rich how i have tuned the ve table logging with AEM wideband gauge and serial port to laptop.

There's a lot of variables at play. What modifications? If it's something with an agressive cam, you can run into issues with the cam overlap causing havoc with closed loop fueling (and to a lesser degree, long tube headers and sensor location) and they just seem to run nicer being slightly rich around idle and some light load stuff. This is of course assuming the o2 sensors are good and closed loop fueling is setup correctly to begin with (they probably are in this case)

Hi thanks for reply. car modifications are 3 inch exhaust no cats otr intake cam specs Duration @.050 - INT 231 - EXT 243 Lobe separation 113 + AdvValve lift INT 0.617 - EXT 0.623. have AEM 30-0300 have sensor just before the cat which is hollow , 02 sensors are just in front closer to engine. i have just redone exhaust gaskets in case they were leaking. i do have long term fuel trims disabled. was just going to run short term trims . my ve table is 5 to 10 % rich but i will be running car in speed density no maff sensor. going to try re tune Ve table closer to 14.7 AFR this time see if it help

That is a decent size cam. With that combo, I'd probably run it open loop with the AFR's you currently have for the sake of drivability. You might be able to get it better by bumping up the switching voltage a little higher for the closed loop. I've not had much success personally with the LS1 PCM and getting closed loop fueling nice with bigger cams. Not to say it can't be done though.

Thanks for the information. I did have 2 other guys on different forums tell me the same thing it may have to stay in open loop due to the size of the cam. Is that a thing you have to leave them in open loop once the cam size gets to big? All new to tuning i assumed that every time you tune a car you have to re enable the closed loop. If not it is classed as a bad tune and unsafe for a daily driven car that won't have a laptop on the seat all the time.

Running open loop on 'big' cams is definitely a common thing in the industry, especially on gen3 stuff. As for what qualifies as 'big', I can't give an exact answer, I just go by feel. GenIV stuff is better and seems to be able to cope with 'bigger' cams running closed loop still.

I always leave closed loop on for anything that will tolerate it, which is most cars really. I've never found any downside to it other than the occasional drivability issues at which point I'll force open loop (as I would in this situation). As for whether or not that makes a 'bad' tune, it's a personal thing. If you paid someone to tune your car for you, would you rather get it back driving nicely and possibly using slightly more fuel, or get it back driving crap but with the promise that closed loop is active and that's just the way it is with a big cam installed? (both these situations are quite common in the tuning scene).

In my mind, by the time you get to the point of fitting a larger camshaft, it's not a daily driver and you will likely tolerate the extra fuel usage and possible fuel smell from sitting there idling, for the sake of having a car that drives nice.

Thanks for the information that clears a few things up. I'm going to be back tuning hopefully in the next week. Going to retune ve table as close to stoich 14.7 as i can possibly get and see what the car does if i have the same issue, I'm going to leave car in open loop and re tune ve table 5 to 10 % richer and roll with it.