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Old 10-08-2012, 02:43 AM   #1
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Interpreting OBD data - What does it Mean?

Sorry if this is a bit long but.....

I am trying to interpret some data to track down a misfire and need some help to interpret live OBD data on my 1998 2.5 Boxster (UK spec).

Can anyone tell me what the following means?

STFT (Short Term Fuel Trim) and
LTFT (Long Term Fuel Trim)

At times I have seen STFT vary from 0 to 25%. Presumably the readings for B1 and B2 should mirror each other.

I also have readings for:

What do they mean? What is the difference between STFT and SHRTFT?

The readings obviously vary depending on driving conditions but a snapshot I got was:

STFT B1 18.75%
LTFT B1 0.78%
STFT B2 18.75%
LTFT B2 0.78%

SHRTFT 12 = 99.21%
SHRTFT 22 = 99.21%

I also have readings for O2S which I assume is the voltage of the O2 sensors.

What does the following mean?

O2SL 11 12 21 22
O2S11 = 0
O2S12 = 0.435
O2S 21 = 0
O2S 22 = 0.435

Grateful for any help.
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Old 10-08-2012, 06:38 AM   #2
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While he is asking, is there a guide to the readings and ranges.

A book on how to use these readings to debug issues would be awesome.

I bought one of these Durametric Cables but without a frame of reference or definitions its hard to tell what all these readings really mean and even what sensors they are related to.
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Old 10-08-2012, 07:00 AM   #3
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i can help a lil bit. ecu works something like this:

- air flows past maf
- ecu converts maf signal into air quantity
- ecu uses engine load to select pre-programmed table from memory
- ecu uses air quantity and rpm to look-up amount of fuel to administer from table
- ecu then looks at o2 sensor to determine how much uncombusted o2 remains in exhaust
- ecu modifies amount of fuel administered based on o2 sensor data; this is short term fuel trim
- if stft remains constant, it gets programmed into long term fuel trim; this is your ecu learning
- ecu has ability to manage trims up to 25%; this determines how many modifications you can make to your intake and exhaust.

uncertain what the other trim readings mean, but i *assume* they are trims for different engine load states.
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Old 10-09-2012, 09:16 AM   #4
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Fuel Trim
Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions. Long term fuel trim is used to compensate for issues that seem to be present over a much longer period. Fuel trims are expressed in percentages; positive values indicate lean (add fuel) and negative values indicate rich (subtract fuel). Fuel trim banks refer to the cylinder banks in a V style engine. Cylinder #1 is always in bank 1. Fuel trim is generally calculated by using a wide set of data values, including front O2 sensors, intake air temperature/pressure (or the more elegent air mass sensor), engine (coolant) temp, anti-knock sensors, engine load, throttle position (and change in throttle position), and even battery voltage can effect fuel trim. Long term fuel trim generally should not exceed +- 10%.

from here: Interpreting OBDII Data
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Old 10-10-2012, 06:53 AM   #5
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That is very helpful and I am starting to understand.

However, Things have changed!

I now have both Fuel Sys 1 and 2 showing OL (open loop i believe).
STFT and LTFT all read 0% and do not change with load.
O2 sensors (pre-cat, my car does not have post cat) read 0.435v and do not change with load.

No fault codes are showing.

It was suggested to me that the car is now running on its basic mapping without any input from sensors. I have disconnected that battery for 24 hours to see it anything re-sets but without any result.

Any suggestions?
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