Help me understand fuel trims.

[Sequoiasoon]

This is in my 2017 Accord, 2.4L Earth Dreams GDI, 6MT. I had a P0420 CEL Catalyst system Efficiency Below Threshold Bank 1. I cleared it, it came back in under 200 miles, cleared and came back again in about 125 miles IIRC. I did the Cataclean and got the cat nice and hot by running at about 4k for 20 minutes on ride home.

My downstream O2 was usually about .7-.8V on highway cruise (cruise on) when I started looking but would drop to .2-.3V randomly. The wideband O2 I don't recall moving from 2.2V when I looked. CEL has stayed off for the last 1000 miles. Backing into driveway I heard a slight exhaust leak so went to shop and it needed a new flex pipe. Today I decided to clean the MAF and TB as it hadn't been done since I had it almost 5 years and 73k.

This is what I'm showing now at cruise (cruise on) and idle at stop light. What to the fuel trims and A/F ratio mean overall? Anything else I should be doing? As long as CEL stays off I guess all is well. It gets 35+ mpg on highway cruise, over 550 miles per tank. Valvoline Restore and Protect running now to try and reduce some oil consumption (1.5-2qts per 5k) which I know is no good for the cat.

 

[ka9mnx]

Was the flex pipe before or after the cats?

 

[Marcozi]

"Fuel trims" are the corrections made by the engine computer in its effort to keep the air/fuel ration at the optimum level, and to compensate for vacuum leaks. There are 2 fuel trims (FT): Long term fuel trims (LTFT) and short term fuel trims (STFTs). LTFTs are the baseline FTs. That is, they are calculated over a period of time/miles to compensate for a permanent "issue" with the engine. STFTs are instantaneous compensations for short term and/or intermittent issues. The A/F ratio is just that: the ratio of the mass of the air and the mass of the fuel. Your readouts shows that the app is using the grams of air per second.

 

[SC Maintenance]

Your long term fuel trims look fine -2.3% at both load and idle.

+/- 5% is pretty much perfect.

Fuel trims are simply adding or taking away fuel from the ECU baseline mixture based on whatever set of conditions. You won't get a lean / rich code until 25%.

 

[Devo242]

Nothing to add on fuel trims, others explained it well. Just wanted to add your O2 sensor voltage wide range is 2.2V in both examples, the sensor itself or its wiring could be an issue. Had similar issue on my Accord years ago, changed with a new genuine Honda sensor and no further issues. Not to overlook that oil consumption issue, that is cause for concern for cat longevity.

 

[Sequoiasoon]

Was the flex pipe before or after the cats?

Flex pipe is after the only cat that is right off motor but before the downstream O2 sensor

View drivers side toward front of vehicle.

 

[Sequoiasoon]

Watching downstream is interesting, 6MT so every shift it goes to 0.00v since off the gas. Does the same in gear, engine braking to stoplights etc.

 

[Trav]

Any exhaust leaking now, the flange on the cat looks a bit sketchy. A leak in that area can cause rear o2 issues.

 

[iCurious]

If it really did had an exhaust leak, your B1S2 voltage would be low not high.

Your fuel trims are fine.

The real problem is oil consumption. It is what kills the catalytic converter. The PCM has been programmed when the catalytic converter reaches to a certain threshold, it triggers the check engine light.

 

[ka9mnx]

Flex pipe is after the only cat that is right off motor but before the downstream O2 sensor
View attachment 293825

View drivers side toward front of vehicle.
View attachment 293826

OK. Before you repaired the flex exhaust pipe, the leaking pipe would skew the downstream O2 sensor readings and function.

 

[cci]

"Fuel trims" are the corrections made by the engine computer in its effort to keep the air/fuel ration at the optimum level, and to compensate for vacuum leaks. There are 2 fuel trims (FT): Long term fuel trims (LTFT) and short term fuel trims (STFTs). LTFTs are the baseline FTs. That is, they are calculated over a period of time/miles to compensate for a permanent "issue" with the engine. STFTs are instantaneous compensations for short term and/or intermittent issues. The A/F ratio is just that: the ratio of the mass of the air and the mass of the fuel. Your readouts shows that the app is using the grams of air per second.

Thank you for this explanation. This is all entirely new to me, and very interesting.

1996 Dodge B3500 5.9 gas, I just this morning plugged in my new Motopower MP69033 to clear the P0441 code, and saw that the STFT is between -1.6 and -15.5 at idle, LTFT is 17.2 and O2B1S1 is .82 and MAP (kpa) is 9. Recently (less than 50 miles ago) changed the air filter which was very dirty.

My question (for now) is when we say "long-term" for the LTFT value, how long is that?

And is the combined STFT and LTFT value more instructive than either value taken individually?

 

[Marcozi]

Thank you for this explanation. This is all entirely new to me, and very interesting.

1996 Dodge B3500 5.9 gas, I just this morning plugged in my new Motopower MP69033 to clear the P0441 code, and saw that the STFT is between -1.6 and -15.5 at idle, LTFT is 17.2 and O2B1S1 is .82 and MAP (kpa) is 9. Recently (less than 50 miles ago) changed the air filter which was very dirty.

My question (for now) is when we say "long-term" for the LTFT value, how long is that?

And is the combined STFT and LTFT value more instructive than either value taken individually?

LTFTs are constantly being evaluated by the engine computer to ensure they reflect the engine's condition. As an example, the engine develops a sudden vacuum leak. The engine computer will at some point (depends on the software) consider that leak as impacting the LTFT, and will incorporate the leak into the LTFT.

 

[Trav]

No feedback from the OP as to if this is fixed or not.

 

[Arc]

Unhook the O2 sensors and the engine will run in the default mode like it does when warming up. Of course you will have to look at the check engine light.

 

[Sequoiasoon]

Nothing to add on fuel trims, others explained it well. Just wanted to add your O2 sensor voltage wide range is 2.2V in both examples, the sensor itself or its wiring could be an issue. Had similar issue on my Accord years ago, changed with a new genuine Honda sensor and no further issues. Not to overlook that oil consumption issue, that is cause for concern for cat longevity.

Any exhaust leaking now, the flange on the cat looks a bit sketchy. A leak in that area can cause rear o2 issues.

No feedback from the OP as to if this is fixed or not.

@Trav Sorry, somehow I missed the reply notifications. No exhaust leaks that I know of now. Car is quiet, P0420 has stayed off since the exhaust repair.

Last week I got a P1157 for pre-cat wideband sensor. I could reset it and start car and was fine for the drive. Next start it came on again. I did that a couple times. I ordered a Denso WideBand from Rock Auto, it came Saturday before the snow. Not a priority with other storm prep. I changed that today, 5 minutes, done. I inherited an O2 sensor socket from my dad and I actually found it quickly .

I tried just starting car but light stayed on. I cleared it, drove a bit. Shut down, started drove 3 more times and light stayed off. Does this coating look like normal for a 2017 Accord with 129k? It had the cataclean when I had original post, it still burns oil, currently burning Valvoline Restore and Protect round 2. It's also seen some Gumout Multisystem, BG44k, over the 5 years and 79k I have had it.

@Devo242 - the new wide band jumps around a lot more now as I step on gas and rev it as well as shift (6MT).

Should I pickup a new downstream to have them both new not one new and one dirty possibly sluggish?

 

[repairman54]

After cat sensor only senses if cat is working properly and nothing else. I'd leave it alone myself as if sensor is weak and cat is weak an it's still not setting a code I'd let that sleeping dog lie.