: Long-term fuel trims up during deceleration

08-29-08, 10:04 AM
Seeing the long-term fuel trims up during highway deceleration up to +10 to13%. Normally at 70mph I see Bank1 LT at 3% and Bank2 LT at 3.9%, which are fairly close to 0%. Short-terms bounce between +-2% which is fine. I'm monitoring them on the fly with my scanner in effort to diagnose a p0420 code. I thinking either an exhaust or air injection leak, since it happens during a high vaccuum senario. Both O2s on the manifolds switch fine, but the one behind the cat doesn't seem to follow any kind of pattern. After the first 2minutes of starting the rear sensor will switch hi and low, then once on the road it hovers around 600mv, give or take 100mv. If I sit and rev it after running a while, it'll start switching again, but once on the road, it'll just hover around 600mv. Could it have a flakey heater I'm wondering?

08-29-08, 04:07 PM
Its possible the heater might be going out, but I wouldn't worry too much about the rear O2 readings. The computer only uses them to monitor the operation of the cats, not the engine parameters. If one falls out of spec, you'll get a code for "Catalyst inefficency bank #". On my T/A, I had edited them out with a custom tune as I kept throwing codes from the aftermarket cats on my longtube headers.

As for your LTFT's, it's been awhile since I've datalogged that kind of thing, back in the days when I used to use a MAF translator to tune my car. From my recollection, when monitoring LTFT's, you ignore the idle and decelleration fuel trim cells (I want to say like #0 and #20 or #22 - again memory real foggy - maybe #22 was the WOT cell). The important cells are 1-19, they are the various fuel trim cells for cruise speed, and ideally should be as close to zero as possible.

08-29-08, 05:55 PM
I am going to make a STICKY about P0420.
A P0420 is strictly about the ratio and voltage spike of the post O2 sensor and that relates into the efficiency of the converter. :mad2: