I recently purchased an OBD-II auto scanner. I understand the basic obd codes, but there is also this freeze frame data that is completely beyond me. It is stuff about short and long term fuel trim systems. What do those percentages mean and how can I use them effectively?
The simple answer is that your Freeze Frame Data is a 'saved' profile of all your emissions/operating parameters (usually the MAP/MAF have greatest fuel authority then the EGR/O2 sensors) at the time the CEL code is generated. In otherwords, fuel trim percentages help troubleshoot drivability issues based on comparing sensor outputs from most of the 5V reference emissions sensors on your vehicle. If your short term fuel percentage shows +28, this means that your computer is adding 28% fuel to compensate for a lean condition (could be a bad O2 sensor, fuel leak, bad plug, EGR....). Looking at both fuel trims levels can help troubleshoot IF- THEN situations a little more clearly.
Fuel Trim ratings simply allow us to measure whether fuel is being added or taken away from baseline fuel duty cycles. Short term is basically adjustments on-the-fly to fuel delivery and long term is factored as a result of 'x' amount of drivability cycles to give you a 'long term' average that might eliminate minor adjustments, etc.. As we all know, this is a half-asses explanation as there are many variables and speciific timing involved in the correct 'technical' answer so you would best be served by looking at something like this:
Take care, Andrew.
in article snipped-for-privacy@q75g2000hsh.googlegroups.com, snipped-for-privacy@hotmail.com at snipped-for-privacy@hotmail.com wrote on 6/20/07
12:30 PM:
Unless you have a specific problem, those scanners are totally useless in my opinion.
The data they display can be compared to the engine's specs, but there is dick all you can do about any of them. There is never a hope of getting then the same all the time due to weather, temp, physical makeup of fuel, and on and on.
They sure must be a money maker for someone though with people tossing parts at scanner 'ghosts'....
Just my $0.02
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
This is the case, though, for EVERY diagnostic tool ever made.
--scott
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.