Negative LTFT, maybe running rich? (2zz)

[inside]

Here we go again, fiddling with my 2zz swapped mr2.

Recently I've noticed my longterm fuel trims on idle are very negative. Idle LTFT is usually -10 to -20. Since I'm paying attention its one day -10, other day -18, even with same weather and temperatures.

Additionally, ecu has been throwing P0420 (catalyst efficiency below threshold) for few months now. At first I thought its related to high flow cat (its been fine for one year), however now suspect it runs too rich. Also, my exhaust pipe is quite black.

Car pulls strong, idle is solid ~800 when warm, no bouncing, generally smooth and no hesitation. Its using 2003 celica ts ecu. Intake is using oem maf housing for 2zz, and enclosed filter with cai. Exhaust consist of adapted oem 2zz header, then magnaflow high flow cat, then custom backbox, o2 sensor on oem-like locations.

Is it possible that it runs fine with these trims, aka ECU adjusts so the afr is optimal?

What could be the cause, what should I check and how to diagnose this? By all the logic, with these mods it should run leaner (trims should be positive), right?

Thanks in advance!

[inside]

What I've done so far:
- replaced MAF sensor with new denso a month ago
- doublechecked intake in couple of different configurations, even without filter, always with stock maf housing - no change
- tested tps, reads fine, no flat spots, gradually increases/decreases
- checked PCV, if I block the pipe to TB, st fuel trims go positive... I will replace it
- purge valve for CCV, works, blocking it doesn't change anything... will leave it
- cat was checked by mechanic with camera on both sides, says it looks fine, not clogged
- o2 sensors look fine to me on obd2 graphing... primary was replaced two years ago (denso oem)

[Carolyn]

Those PCV valves are just sneaky.

[inside]

I've just been on drive with wideband attached to post cat bung. Confirmed it runs rich in WOT (open loop). Lift and WOT is 11.3-11.5 afr. Closed loop is ecu adapter to stoich... so o2 sensor doing its job.

I've just replaced PCV but haven't tried. However just releasing it to atmosphere didnt change much on the trims.

Fuel system, injectors, maf housing is stock. Just free flowing exhaust and intake. Baffling...

[men3cac]

Holy thread revival Batman

Did you get anywhere with this @inside ?

I have a very similar setup and issue. Long term fuel trims are sat on -12 ish and I'm struggling to figure out why

[inside]

Holy thread revival Batman

Did you get anywhere with this @inside ?

Yes, I got everything sorted but still running rich.. In the end bought adjustable maf housing and used slightly bigger diameter, which effectively balanced out the rich mix. This eliminated some surging I was experiencing lifting-off in highers revs (7k+).

[J88TEO]

In the end bought adjustable maf housing and used slightly bigger diameter

Not heard of this before...mind showing some photos or link?
Cheers.

[Dev]

What kind of intake was on the car before. What was the diameter.

[inside]

What kind of intake was on the car before. What was the diameter.

It's was OEM maf pipe - cutout from 2zz celica airbox. Also tried new maf, didn't change a thing.

[inside]

Not heard of this before...mind showing some photos or link?
Cheers.

It was spectre 9405 kit: https://www.ebay.de/itm/264592743133

[J88TEO]

It was spectre 9405 kit: https://www.ebay.de/itm/264592743133

Thank you!

[Dev]

It's was OEM maf pipe - cutout from 2zz celica airbox. Also tried new maf, didn't change a thing.

 You did it right as can be. Two other things I would check would be the spark plugs and coil packs. If the coils are old and tired it can give you negative fuel trims without noticing anything abnormal.

[Alex Knight]

Here we go again, fiddling with my 2zz swapped mr2.

Recently I've noticed my longterm fuel trims on idle are very negative. Idle LTFT is usually -10 to -20. Since I'm paying attention its one day -10, other day -18, even with same weather and temperatures.

Additionally, ecu has been throwing P0420 (catalyst efficiency below threshold) for few months now.

Just so that I understand correctly, you have an ECU error code (which is known to retard timing for safety), and you are wondering why your car is running rich also?

Did it not occur to you that they may well be related? Prevention being better than cure and all that.