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Old 12-11-2012, 08:34 PM   #1 (permalink)
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Default Scan tool/Fuel trims

OK so I got my OBDLink SX today and I'm going to read my fuel trims and whatnot...but how do I do this/interpret them? What values should I be looking for, Max Min Mean etc? Should I do it while cruising or at idle or?

Using OBDwiz.
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Old 12-11-2012, 09:01 PM   #2 (permalink)
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LTFT should be +/- 5% ideally and certainly under +/- 10%, and should be similar for both banks under all driving conditions. LTFT and STFT on each bank at any instant should add up to roughly zero. LTFT values change slowly and STFT values jump around pretty fast. Depending on PCM calibration, LTFT over +20% or so will set a 171/174 code and over -20% or so will set a 172/175 code. Over +/-25 to 30% will set O2 stuck lean or stuck rich codes.
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Old 12-11-2012, 09:03 PM   #3 (permalink)
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Ok. So should I press record for the data, then turn the car on, let it idle for a minute, then drive around a bit?
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Old 12-11-2012, 09:04 PM   #4 (permalink)
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Correct. Then playback the data, as long as you know when it's idling and when it's not idling.
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Old 12-11-2012, 09:08 PM   #5 (permalink)
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Quote:
Originally Posted by Breeves002 View Post
Correct. Then playback the data, as long as you know when it's idling and when it's not idling.
I'll tell it to record speed and rpm as well.
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Old 12-11-2012, 09:12 PM   #6 (permalink)
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There ya go! You can almost tell when you are idling based on the trims.
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Old 12-11-2012, 09:31 PM   #7 (permalink)
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You have to have the key on for it to connect and start recording. So you can turn the key to on without starting, hit record, then start it.

The short term fuel trims will be constantly moving up and down to generate the O2 sensor lean/rich switching. The long term trims will be more stable, but will change with different operating conditions, like stepping on the gas.

Like Jeff said, ideally you want to see 5% trims. To get the total fuel trim, you have to add the short term number to the long term number. So if the long term is +7% and the short term is -3%, the total fuel trim is +4%. If the short trim numbers are moving around a lot try to average them.
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Old 12-11-2012, 09:37 PM   #8 (permalink)
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I already told you this in chat.... but I'll write it here! Thanks to behlinla for some of this help and info!!

First, watch trims at idle - you're looking for them to be between -5% to +5% just like Jeff said above. You are looking that they don't go outside of that much during any driving.

If you see the LTFT much higher than that, then you have a probable vacuum leak. If it's super high (+10 to +25%), especially on both banks, then it's most likely a vacuum leak!

Of course record all that and both banks....

Okay, to test the O2 sensors functioning correctly do this test (thanks behlinla):
Find an empty stretch of road, or somewhere you can do this.... idle for 60 seconds and drive normally around 40mph. We're watching that the O2 sensors are switching correctly. They should be moving from around .1v to around .8v at least every 2 seconds, ideally at least ever second.

Next stop the car, then go ahead and go WOT all the way through first gear. The trims may drop out to very low numbers for a second, but all 4 O2 sensors should jump up to .9v or so the entire time through WOT. This means that the correct amount of fuel is being dumped into the motor at WOT. The duratec really likes to dump a lot of fuel at WOT. This also shows the O2 sensors are working correctly, the switching is really what you need, but this shows how much fuel you're burning by doing this
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Old 12-11-2012, 09:46 PM   #9 (permalink)
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I will do this tomorrow morning, thanks to Sam, behlinla, and Jeff! Good info here
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Old 12-12-2012, 12:57 AM   #10 (permalink)
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What are fuel trims all about?

generally, if there below double digits there in the ball park.
When you see over 10% trims, or one bank radically different from the
other, its time to investigate further.
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