Short term trim vs. Long term vs. O2 calculated trim

[mpolli]

I am curious about what the O2 sensor's "calculated trim" is. My short term fuel trim hovers around 0 as it should and the long term trim is at 19. The front O2 sensor's calculated trim is 64. What does that mean? I don't have any trouble, I am just curious what it means.

By watching the long term trim and the O2 trims can anything be determined about the "health" of the MAF sensor or the O2 sensors?

[mbdoc]

The long term trim is usually a good way to determine the MAF's ability to read volume correctly. Having a "clean" air intake is very important.

Vacuum leaks & fuel pressure can FOOL even good technicians on those readings.

[ctaylor738]

Just for comparison, on my SL500 problem child's most recent reading, the long term trim on the left is .8% and the right -.8%. I did read that an absolute value less than 10% for LTFT is "good."

[mpolli]

Hmmmmm. Well that is interesting. So are you guys saying that since my Long term trim is 19 that means the MAF is getting "old". Is there any point to watching this number?

Now I still don't have an answer to my original question. What is the O2 sensors "calculated trim"? Is that number good for anything?

Thanks

[mbdoc]

O2 sensors react only to "fuel mixture" changes. their "adapation" is of little OR no concern. When an O2 sensor has a problem YOU will see a check engine light quickly!

[ctaylor738]

Where are you seeing the "sensor calculated trim"? That one is new to me, but that's not saying much. The sensor is pretty dumb, it can't calculate anything. It simply sends a voltage to the the ECU indicating the amount of oxygen in the exhaust stream.

[mpolli]

It is part of the data stored by the car's computer relating to the O2 sensor. Technically it is byte "B" of PID 14 of the Mode 1 OBDII commands:

"PID 14 Oxy. sensor voltage bank1 sensor1
This PID returns the Oxygen sensor voltage for the sensor located in Bank 1, Sensor 1. There are 7 other PIDs which are similar, but for other locations. In each case, Two bytes are returned. Data A represents the sensor voltage (defined below), and Data B is the short term fuel trim associated with the sensor, or Hex FF in the case where the sensor is not used in the calculation.
Oxygen Sensor Voltage = .005 * A
Short Term Fuel Trim% = .7812 * ( B-128 )"

Other sites referred to this as the "calculated trim". In any case it is not same as the the Short Term fuel trim associated with the bank or the Long Term fuel trim. Those are PID 6 and 7:

"PID 06 Short term fuel % trim Bank 1
The fuel trim is a value from -100% (lean) to +99.22% (rich)
Fuel Trim% = .7812 * ( A-128 )

PID 07 Long term fuel % trim Bank 1"

This is from http://obddiagnostics.com/obdinfo/pids1-2.html

[ctaylor738]

Well, that helped, a little. I think what you are referring to is the value "Fuel trim or lambda" plotted in blue on the attached graph, which is from my S500 idling. It looks to me like the computer records two data values, the sensor voltage and the fuel trim, which is a measure of the correction needed at that particular instant.

Most the fuel trim values in the plot are between -2 and +2, which I assume is a %, so I don't know how to relate that to your value of 64. Maybe that's the raw value of "B" in your equation. But that doesn't make a lot of sense, since plugging 64 into the equation

Short Term Fuel Trim% = .7812 * ( 64 -128 )
STFT = -50 (rounded)

Even if the 64 is a hex value (100 decimal), plugging 100 in gives a value of -21.9.

Maybe someone who understands this stuff will enlighten us.