Extremely rich idle condition advice needed

[Neo_11]

Hi all

Some background, car has a stage 2 flash tune and catless dp (2014 MK7 GTI)

Have no engine fault codes but see a long term negative fuel trim of -25% on idle which is extremely high (ie rich condition, ECU cutting back fuel) This value decreases on higher rpms though.

Have no misfires or rough idle to indicate a fuel delivery issue. Actual low and high pressure fuel values meet the requested values by the ECU.

I tested the wideband sensor and it is within OEM resistance values however the post cat O2 sensor's resistance value is almost triple the allowed OEM range. This should be replaced as per the OEM manual.

This is where my dilemma begins. I see there is not much info on what tuners exactly do to the post cat o2 sensor but it seems that they do not eliminate its functionality.

My question is, can the post cat o2 sensor actually override the wideband lambda sensor 1 in our vehicles? Lets assume its stuck in a lean position, could this sensor still request the ECU to add more fuel even though i am flash tuned for a catless dp? I could buy another sensor and it might not solve my problem.

The next step is to start stripping the intake manifold and looking at the HP injectors (could have a leaking injector) but still cant understand why i have no misfires considering im seeing a very high over fueling situation.

Im pretty much stumped with this and it makes its extremely difficult when there are no engine trouble codes, my car still pulls like a train on WOT, with no issues except poor fuel consumption.

I however fear for fuel-oil dilution and piston wash. Will appreciate any advice.Thanks.

 

[Hoon]

Both sensors are used in the fueling calculation. The second sensor is not simply an emissions monitor and anyone who tells you it is doesn't know what they're talking about.

You can simply unplug the secondary on most stage 2 tunes and the car will run fine. Some tuners recommend unplugging anyway, because the lack of cat (or even a high flow cat) changes the fueling calculation and can cause the car to run rich.

 

[Neo_11]

Both sensors are used in the fueling calculation. The second sensor is not simply an emissions monitor and anyone who tells you it is doesn't know what they're talking about.

You can simply unplug the secondary on most stage 2 tunes and the car will run fine. Some tuners recommend unplugging anyway, because the lack of cat (or even a high flow cat) changes the fueling calculation and can cause the car to run rich.

Thanks Hoon, ive read a response which was posted a while back from Chris(Revo) stating that the function of the O2 sensor is not removed when flashed. So technically it should be still used in fuel calculations even though im tuned without a cat?

 

[Diggs24]

Detune it and see if the issue persists.

 

[Hoon]

Thanks Hoon, ive read a response which was posted a while back from Chris(Revo) stating that the function of the O2 sensor is not removed when flashed. So technically it should be still used in fuel calculations even though im tuned without a cat?

Correct, the light is disabled but the functionality is left.

This is for legal reasons.

 

[aaronc7]

In my little bit of testing.... if you have P0420 (cat efficiency) code set, the ecu will add some extra fuel. I drove the car a few 100 miles like this, it was always about 0.3 AFR points rich. Nothing crazy and didn't mess with fuel trims and whatnot. And really not that rich, but definitely a measurable difference. This is what an aftermarket high flow cat... maybe it would add more on a catless setup...no idea, I haven't seen any data on it.

I would pull the rear O2 sensor first and see what happens....reset codes/trims. It will set P0036, but from what I can tell, has no negative impacts other than you'd never pass an emissions test.

If the issue still persists then you know it's something other than that sensor and you can continue to troubleshoot.

 

[Diggs24]

In my little bit of testing.... if you have P0420 (cat efficiency) code set, the ecu will add some extra fuel. I drove the car a few 100 miles like this, it was always about 0.3 AFR points rich. Nothing crazy and didn't mess with fuel trims and whatnot. And really not that rich, but definitely a measurable difference. This is what an aftermarket high flow cat... maybe it would add more on a catless setup...no idea, I haven't seen any data on it.

I would pull the rear O2 sensor first and see what happens....reset codes/trims. It will set P0036, but from what I can tell, has no negative impacts other than you'd never pass an emissions test.

If the issue still persists then you know it's something other than that sensor and you can continue to troubleshoot.

I wonder if having Eurodyne disable the CEL has it ignore the sensor? I have no such lambda deviations in normal highway cruising.

https://datazap.me/u/diggs24/log-1588461881?log=0&data=1-11-12

or are you saying it moves the setpoint?

 

[aaronc7]

I wonder if having Eurodyne disable the CEL has it ignore the sensor? I have no such lambda deviations in normal highway cruising.

https://datazap.me/u/diggs24/log-1588461881?log=0&data=1-11-12

or are you saying it moves the setpoint?

I think Eurodyne actually disables the light AND the trouble code. Cobb relatively recently changed their software to not alter any trouble codes....Light ONLY. I suppose to keep the EPA happy-- in effect they are not altering or tampering with any emissions monitors this way. All tuning suites I've used had 2 different tables: one for DTCs and one for the actual warning light (MIL).

Check your codes is the best way to tell. If sensor plugged in and no codes...they hard disabled it. Eurodyne and smaller companies can probably fly under the radar, but who knows, they may eventually change their ways too.

When I had Ecutek on the car it totally disabled the code, I didn't have to unplug the rear O2 sensor.

I have some datazap logs but I can't access it right now. I did both idle and constant cruise logs. AFR set point was unchanged, but it was clearly about 0.3 richer all the time in these steady state conditions. I'm thinking maybe the AFR set point param that we are using isn't actually the final commanded AFR.

 

[Diggs24]

I never have any O2 sensor codes when I scan. Had a persistent CEL when on JB4 so I know my setup (bigdaddysgarage minicat in front of sensor) wasn't making ECU happy prior to going Eurodyne.

 

[aaronc7]

yeah, then it's safe to say eurodyne completely disables it and you don't have to worry about unplugging anything. It is really a cobb specific thing...and I guess any other tuners that start doing the same thing. Of course with the sensor unplugged you'll get a different code and never be able to pass emissions.

 

[aaronc7]

I wonder if having Eurodyne disable the CEL has it ignore the sensor? I have no such lambda deviations in normal highway cruising.

https://datazap.me/u/diggs24/log-1588461881?log=0&data=1-11-12

or are you saying it moves the setpoint?

Here's the logs I have uploaded, I think these are all at idle fully warmed up. Did same test at cruise speed, but those must be saved on my PC somewhere. Should be 3 logs total in the drop down.

https://datazap.me/u/aaronc7/second-o2-sensor-test?log=0&data=1-2-14

 

[Diggs24]

Interesting, so it doesn't change setpoint just runs rich. Is 0.3 even noticeable?

 

[PacDawg]

Hi all

Some background, car has a stage 2 flash tune and catless dp (2014 MK7 GTI)

Have no engine fault codes but see a long term negative fuel trim of -25% on idle which is extremely high (ie rich condition, ECU cutting back fuel) This value decreases on higher rpms though.

Have no misfires or rough idle to indicate a fuel delivery issue. Actual low and high pressure fuel values meet the requested values by the ECU.

I tested the wideband sensor and it is within OEM resistance values however the post cat O2 sensor's resistance value is almost triple the allowed OEM range. This should be replaced as per the OEM manual.

This is where my dilemma begins. I see there is not much info on what tuners exactly do to the post cat o2 sensor but it seems that they do not eliminate its functionality.

My question is, can the post cat o2 sensor actually override the wideband lambda sensor 1 in our vehicles? Lets assume its stuck in a lean position, could this sensor still request the ECU to add more fuel even though i am flash tuned for a catless dp? I could buy another sensor and it might not solve my problem.

The next step is to start stripping the intake manifold and looking at the HP injectors (could have a leaking injector) but still cant understand why i have no misfires considering im seeing a very high over fueling situation.

Im pretty much stumped with this and it makes its extremely difficult when there are no engine trouble codes, my car still pulls like a train on WOT, with no issues except poor fuel consumption.

I however fear for fuel-oil dilution and piston wash. Will appreciate any advice.Thanks.

Did you pull and inspect the plugs? I had a leaking injector so car was running super rich at idle. Checked the plugs and new exactly which injector was leaking. I had hard start issues with missfire and rich codes though. Also I'd try unlpugging the rear o2 sensor plug and reflash tune to see what it does if plugs look fine.

 

[aaronc7]

Interesting, so it doesn't change setpoint just runs rich. Is 0.3 even noticeable?

Not really... I mean on my corvette which runs open loop fueling at WOT, I would be pretty happy with average AFR error being 0.3 lol. It's just weird seeing it on these cars where fueling is closed loop all the time and always super accurate.

If you got 25 mpg, running 14.4 AFR would be 24.3 mpg in theory........... not significant.

At the end of the day not a big deal IMO, but its also super easy for me to just pull the rear o2 plug in the engine bay too.

 

[Neo_11]

In my little bit of testing.... if you have P0420 (cat efficiency) code set, the ecu will add some extra fuel. I drove the car a few 100 miles like this, it was always about 0.3 AFR points rich. Nothing crazy and didn't mess with fuel trims and whatnot. And really not that rich, but definitely a measurable difference. This is what an aftermarket high flow cat... maybe it would add more on a catless setup...no idea, I haven't seen any data on it.

I would pull the rear O2 sensor first and see what happens....reset codes/trims. It will set P0036, but from what I can tell, has no negative impacts other than you'd never pass an emissions test.

If the issue still persists then you know it's something other than that sensor and you can continue to troubleshoot.

Thanks Aaron, i like this idea, will definitely pull the plug on the 02. Reset and monitor LTFT. I have a feeling that due to my sensor being faulty it could be sending feedback to the ECU to increase fuel. I will revert back with my findings.