Lean or Rich? Lambda reading .8 but LTFT is +28%???

[PierceS4]

I have a 2013 Audi S4.

My Bank 2 LTFT is reading +28% but the lambda reading is .8 and the post cat sensor is showing around .2v... What could be the problem here? All 4 O2 sensors are brand new although I'm not ruling out the possibility of a faulty sensor. I don't have tons of experience but i can't see why the lambda value would be .8 which is rich and still be telling the car to add more LTFT?? COuld this be an ECU problem? or an MAP sensor problem? All 3 MAPs are new though too

Please take a look at the following logs to see if they can help diagnose the problem.

Sorry they're just screenshots since I can't load actual logs...

[irish07]

Units of lambda and O2 sensor voltage entirely different. An O2 sensor will not exceed 0.9v and never go below 0.1v. The sensor will always read in between these values, outside this boarder will indicate a circuit error and flag either the sensor or entire circuit as faulty.

Units of lambda is what I see monitored, that will just give you the AFR ratio if used 14.7 as stoichiometric.

To know if your running lean or rich by fuel trims, you take LT + ST. So if you have a LTFT +28% and a STFT if -23%, you would have a +5% value, meaning adding fuel. I doubt you would see a -23% in real life in STFT with +28% LTFT but I’m using those values as an example only.

Regardless, anything over +/-25% will trigger the MIL light, so in theory your light should be on flagging an issue. On average you shouldn’t see the LTFT more than %5 and generally anything near zero is preferred. As the engine ages I’ve seen values near 10% but usually start flagging issues with stuck rich/lean on upstream sensors.

So by looking at your LTFT and if you monitored the O2 voltage, you would see the car running lean, meaning the sensor voltage will average below 0.45v. It will most likely reach 0.19v and not breach past 0.69v. You can’t view this by looking at numbers, best is by using a scope or viewing live data as a graph when logging pids. Just don’t monitor stuff like rpm which uses a lot of data and solo out the O2 sensors.

To diagnose, shoot for things that bring in air or stuff that monitor air. The car has a MAF sensor so might be wise to start there first, especially if you have an oiled filter. If nothing is found, start looking at fuel pressures . Your getting to much unmetered air(hole in intake tube after MAF) OR not enough fuel(low fuel pressure) and limits are being exceeded to try and meet a mixture of 14.7:1


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[MSq5]

Units of lambda and O2 sensor voltage entirely different. An O2 sensor will not exceed 0.9v and never go below 0.1v. The sensor will always read in between these values, outside this boarder will indicate a circuit error and flag either the sensor or entire circuit as faulty.

Units of lambda is what I see monitored, that will just give you the AFR ratio if used 14.7 as stoichiometric.

To know if your running lean or rich by fuel trims, you take LT + ST. So if you have a LTFT +28% and a STFT if -23%, you would have a +5% value, meaning adding fuel. I doubt you would see a -23% in real life in STFT with +28% LTFT but I’m using those values as an example only.

Regardless, anything over +/-25% will trigger the MIL light, so in theory your light should be on flagging an issue. On average you shouldn’t see the LTFT more than %5 and generally anything near zero is preferred. As the engine ages I’ve seen values near 10% but usually start flagging issues with stuck rich/lean on upstream sensors.

So by looking at your LTFT and if you monitored the O2 voltage, you would see the car running lean, meaning the sensor voltage will average below 0.45v. It will most likely reach 0.19v and not breach past 0.69v. You can’t view this by looking at numbers, best is by using a scope or viewing live data as a graph when logging pids. Just don’t monitor stuff like rpm which uses a lot of data and solo out the O2 sensors.

To diagnose, shoot for things that bring in air or stuff that monitor air. The car has a MAF sensor so might be wise to start there first, especially if you have an oiled filter. If nothing is found, start looking at fuel pressures . Your getting to much unmetered air(hole in intake tube after MAF) OR not enough fuel(low fuel pressure) and limits are being exceeded to try and meet a mixture of 14.7:1


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Does the 3.0 TFSI engine have a MAF sensor? I thought it did not and that MAF air mass was only a calculated value.

[bhvrdr]

Interesting. I didnt catch any of your trims in the logs you posted but to try and give you info...

Your lambda that ranges from .9 to .8 is normal. The car will enrich under WOT and run up to around .80 by redline under full load. Thats completely normal for a FI engine.

If your fuel trims are showing +28% that means the car is trying to add that percentage in order to meet the requested air/ fuel ratios. So your car is in danger of running lean. Keep in mind that doesnt mean its running lean. your lambda is monitoring the actual AFR.


Can you let us know what your LT and ST fuel trims look like for both banks? Is this only happening on one bank?

Thanks
mike

[vpls4]

Units of lambda...Audizine

Thanks so much for sharing...I've been wanting to learn more about fuel logging.

?? Does anyone know on avg what these values should be w/engine off ? Car won't start so logged these but don't know what to compare them to.

20 kpa rel = Fuel Pressure
477 hpa = Actual Fuel Rail Pressure
977 hpa = Fuel Pump: Actual Fuel Pressure

[irish07]

Does the 3.0 TFSI engine have a MAF sensor? I thought it did not and that MAF air mass was only a calculated value.

Your right, just ran out and checked mine. It does have a weird plastic adapter into the air box(fast look I thought was a MAF) but it’s just a size adapter to fit the filters larger opening radius to the tube.

But it wouldn’t be a calculated value, just uses a different method to monitor intake temp and manifold pressure. I’ve not looked in depth in Simos but most likely monitors pressure pre-charge and post-charge.. that wouldn’t require a MAF


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[bhvrdr]

Your right, just ran out and checked mine. It does have a weird plastic adapter into the air box(fast look I thought was a MAF) but it’s just a size adapter to fit the filters larger opening radius to the tube.

But it wouldn’t be a calculated value, just uses a different method to monitor intake temp and manifold pressure. I’ve not looked in depth in Simos but most likely monitors pressure pre-charge and post-charge.. that wouldn’t require a MAF


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The simos provides calculated MAF values. It has a MAP sensor.

Mike

[MSq5]

The simos provides calculated MAF values. It has a MAP sensor.

Mike

Indeed, my VCDS has a pid that generates a calculated value for MAF air mass flow. I don't know whether it is accurate in an absolute value sense, but it is useful for comparison to baseline when making changes.

Yes, direct injection forced induction engines run pig rich AFRs when under load at WOT. It is protective. The extra fuel provides cylinder cooling that lets the engine have more timing advance under high boost without encountering knock.

[PierceS4]

Yeah so I've replaced both map sensors and verified that it is running lean because one day my bank 1 Post cat voltage will be .1 and the next bank2 will be .1 and it flip flops back and forth One Bank Reading lean and then the other pretty randomly. The only thing I haven't checked is the fuel pressure with an actual test kit. It also idles rough after a while. Live check for intake leaks and exhaust leaks as well as taking it into a specialist to do so and there are none so at this point it's got to be either the map sensors or fuel pressure right? I can't imagine what else it would be

[PierceS4]

I also get stuck Rich codes for the primary sensors randomly. Thank one will be stuck rich and then I'll clear the codes and a day later bank2 will be stuck rich. Could this be an ECU issue? Or possibly a wiring harness problem. If the primary sensor gets stuck rich it would choose to lean out the system right? And that's why my post cat would be reading lean on that side. All 4 O2 sensors are new so it can't be an actual sensor malfunction unless it's wiring

[irish07]

Thanks so much for sharing...I've been wanting to learn more about fuel logging.

?? Does anyone know on avg what these values should be w/engine off ? Car won't start so logged these but don't know what to compare them to.

20 kpa rel = Fuel Pressure
477 hpa = Actual Fuel Rail Pressure
977 hpa = Fuel Pump: Actual Fuel Pressure

You have literally no fuel pump pressure. Your tank pump should prime with key on and you have only 977hpa which is about 14psi. Not really sure which pid is which sensor, but which ever value you use, 20kpa, 477hpa, 977hpa..all way to low.


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[irish07]

I also get stuck Rich codes for the primary sensors randomly. Thank one will be stuck rich and then I'll clear the codes and a day later bank2 will be stuck rich. Could this be an ECU issue? Or possibly a wiring harness problem. If the primary sensor gets stuck rich it would choose to lean out the system right? And that's why my post cat would be reading lean on that side. All 4 O2 sensors are new so it can't be an actual sensor malfunction unless it's wiring

Your logs at top, show you a value for intake and a value for charger, are those in units of hpa? Cause +/-2210hpa would be around 32psi absolute. Comes out to around 18psi from your charger. I thought these can only do stock around 13.5psi @ sea level.

Do you have any aftermarket parts or tune?


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[PierceS4]

Yeah I'm running EPL stage 2 with a bunch of mods but I dropped it all to the stock tune and the problems still remain. I did notice that if I WOT the long term trims will both sit at like 5% after for a bit before one randomly goes to like 25% or more

[PierceS4]

Soooo.. today we discovered that the guys that installed my headers and exhaust crossed the wire extensions to the post cat sensors and they've been on the wrong banks all along....

[Rodizzle]

Soooo.. today we discovered that the guys that installed my headers and exhaust crossed the wire extensions to the post cat sensors and they've been on the wrong banks all along....

Out of topic but to install your headers ? How many hours ? Did the drop the engine or tranny ? Notice ur in Santa Rosa too


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[bhvrdr]

Soooo.. today we discovered that the guys that installed my headers and exhaust crossed the wire extensions to the post cat sensors and they've been on the wrong banks all along....

Arent the post cat secondary sensors primarily cat monotoring sensors. Theyre not giving you meaningful afr data anyways. Make sure they didnt mess up your primary sensors too and that youre logging the right channels

Mike

[kelseysautobody]

Soooo.. today we discovered that the guys that installed my headers and exhaust crossed the wire extensions to the post cat sensors and they've been on the wrong banks all along....

Including that you had headers installed would have been good info in the original post. What headers went in? Do you have a CEL or any codes? After doing headers I had to replace both secondary O2 sensors, then found one of the new sensors was faulty or failed again. But having the wires mixed up I'm sure is a big part of your issue.