Constant Open Loop

daiflu

Hey guys,

I've been doing a bit of dataloging/gathering lately.
I've noticed a couple of things that are strange (or I just don't understand fully) about my ECU management.
My engine runs on the lean side right through the rev range. When reading data from an ODII scanner, the short term fuel trim is always negative. My uderstanding is that this means the ecu is pulling fuel, however if I look at the air/fuel ratio on the scanner, it is constantly lean (which correlates to my wideband readings also). Why would the ECU be pulling fuel if it can see the engine is already lean?
And now the strangest thing; when observing the O2 loop status, is is always open, even when cruising and idle! I was under the impression the ECU would get it's information for the AFR and STFT, from the O2 sensor in closed loop. How can it be making these decisons if it's never in closed loop? or is that the reason why it's pulling fuel while lean (because it's not refferencing the O2)?
When I adjust the voltage of the O2 sensor, the AFR's and STFT do not change, but I can monitor the voltage changing on the sensor. This confirms that the sensor values are changing but the ECU doesn't care.
This could be a good thing for tuning, as if you add fuel the ECU may not trim it out if it's not paying attention to the O2.
Has anyone else noticed this?

vocky

possibly either a duff coolant temp sensor or mixture lambda sensor

MikeS

closed loop kicks in about 2 -3 minutes after starting the car once the lambdas have reached operating temperature [readyness]

realistically your fuel trims should not be negative at all, the front should be pulsing positive & close 2 negative and the rear bobbing around a set value

open loop is when the ecu decides best on fueling, closed loop is when the ecu uses the readings from both lambda sensors and know's they are operating correctly

front lambda should pulse .2 - .9 within 3 - 4 minutes of the car starting and the rear should follow suit around .3 - .6

i am surprised u have no DTC codes if the ecu is not going into closed loop concerning your fueling

i am more inclined to say it is your front lambda doing this as the rear will not cause an open loop, u unplug the rear lambda and the ecu will use a set voltage .45 to enable closed loop

it will simply start throwing up p0420 codes and heater element codes after about 30 mins of driving, also dtc codes for inactivity meaning the voltages are not going outside a certain range threshold

if in open loop your front lambda should be pulsing in like a straight line

daiflu

Some visuals:

Initial, after cold start:

After 2min idle:

After a thrash on the freeway:

Open loop all the way.

MikeS

your engine is running quite rich as the rear lambda should settle back down from 800 after a minute or so after thrashing

my software wont show closed loop until i restart it after the car has warmed up, it wont switch from open to closed loop, but usually the graph is a good indication of closed loop

your initial graph shows no reason to stay in open loop, your 2nd graph shows the cat is either quite old or a sports cat

daiflu

Cheers Mike,

Yea it has a sports cat.

I'll try reconnecting the scanner at different times to see if the loop changes.

I'm adding fuel in the high rpm range, this may be why it's rich after a thrash. Without adding any fuel, it's generally lean.

sophos9

Sorry for dragging up an old thread but I have the same testing hardware/software and it displays exactly the same thing on mine. I just replaced the cat and heated o2 sensor and the car every time I test shows open loop but no error codes. Both sensors are working to specification

My guess is a software problem - I've reported it to the software dev's

markv

I would say the graphs are looking pretty good.

The parameter 2 (primary O2) is going between 0 and 1100 (gueass the bottom 1100
is the top value for parameter 3, not a very clear display).
Secondary O2 is not important for mixture, forget about that one.
Your short term fuel trims look pretty ok. You can see the ECU adjusting + and

to get the change in the lamda sensor signal. So that is looking great.

The first parameter, O2 Sensor loop, I would guess it is really in closed loop,
otherwise you would expect the SFT to be 0, because the ECU doesn't use the
SFT in open loop. Did you try and log when you do a full throttle? It should
switch from closed to open loop with a 0 SFT.

Another thing which is very usefull to log is the long term fuel trim. To get
the total adjustment the ECU has to make on top of the base fuel tables, you
need to add the long and short term fuel trims together.

I've tried to do a writeup on this stuff a while back, maybe it is usefull for
what you are trying to do:

http://www.verboom.net/blog/index.html? ... 20100309.0

Mark

sophos9

Strange thing is - if the car was open loop, the LTFT would report 0.0. Ive raised the question to the software dev...

Nice article, good read 🙂

markv

No, in open loop the long term fuel trim will not be 0. The LFT will still be used to correct the base fuel map values, because it has been learned over time. SFT is a direct feedback value, and because the primary O2 sensor is useless when not running an AFR of 14.7, it can't be used to make a SFT..

Mark

sophos9

Why would ECU continue to use LTFT values once a DTC for open loop is shown - the ECU is no longer looking to the O2 sensor and therefore cannot make any adjustments to its fuel maps?

Is this specific to GM?

markv

I have tried to explain this in the article I posted the link to. Basically the long term fuel trim is a table in the z22se ECU, not a single value. The table is a throttle valve position vs. RPM table. The cells in this table are adjusted when the ECU is running closed loop. The ECU tries to determine the adjustment it needs to make to the base fuel table to make a 14.7:1 AFR. It stores these adjustments in the LFT. When injecting fuel, the ECU reads the base fuel table, adds the LFT from the cell it is working in and does an SFT if it is in closed loop. In open loop it reads the base fuel table, adds the LFT and then has an 14.7 AFR. Then it adds the amount of fuel to get the desired target AFR (standard 12.6:1). It can't measure the correctness of the target AFR so O2 and SFT are ignored.

If you want more details on the tables etc, give the link a shot..

Mark

markv

Just opened up a tuning session in OBDTuner and made a graph of the SFT, LFT and the fuel status:

When the fuel status graph (lower graph) is 0, the engine is in closed loop. When it is 4 it is in open loop.

The top graphs has the SFT and LFT. As soon as the fuel status switches to open loop the SFT goes to 0, but
the LFT is still used. In the left most part when the graph goes to openloop for the last time, you see that that part
of the LFT is 0, but just before going back to closed loop a different cell in the LFT table is reached and that contains
a non zero LFT value.

Mark

sophos9

Thanks for the information - interesting to learn about this more. Thanks for taking the time to articulate your response 🙂

sophos9

Confirmed bug by the software developer as thought, will be fixed also we are getting Learn Crank Angle Sensing Sensor function in next version - sweet!