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Discussion Starter · #21 ·
295/60r20 said:
make sure your engine is warm before checking trims. they will give erroneous figures during open loop. closed loop (engine warm) then re-check fuel trims
the trims I've been reading are after warming up to normal operating temp and the system is closed loop, but thanks for the tip. I'm pretty sure my issue is my fuel alcohol content being wrong. Now I just need to suck it up and pay a dealer to clear it since I can't get ahold of the right scan tool.

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ForaBot's answer is created from summarizing the responses in this thread.

Based on the information provided by members of the community, it is correct to assume that since both banks are reading the same, the issue is likely something that affects all cylinders and not something like a bad injector. Short term fuel trims should be close to the same numbers as long term fuel trims, and ideally should be 0%. Positive readings mean the engine has to add fuel, indicating a lean condition, while negative readings mean the engine is subtracting fuel indicating a rich condition. Long term fuel trims use the fuel trim table to determine how to control the short term, and can take time to adjust.
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Should short term fuel trims and long term fuel trims be close to the same numbers? I haven't been able to find an answer to this question but aren't the long term fuel trims just an average over a longer period of times and the short term is real time data? Or am I thinking of this completely wrong? Its not making sense that my short term trims are bouncing around from 0 to +7 while my long term trims are +20 to +25. Maybe a reset is in order? I will fully understand these trims someday haha.[/quote]
I don't completely understand them yet either, but what I got from what I've read is the long term trims are staying high one way or the either to keep the STFT at that low number to compensate for whatever the issue may be, but if everything is working correctly then they should all be close to zero.[/quote]

Your short term fuel trim is "Live". It is reacting to the O2 sensor directly. You want it to be 0% ideally. If it is at 0% it means you air fuel ratio is perfect 14.7:1, and not fuel needs to be added or subtracted. Positive readings mean the engine has to add fuel, indicating a lean condition. Negative readings meant the engine is subtracting fuel indicating a rich condition. This happens often and quickly, that is why it looks sinusoidal when graphed. Short term fuel trims can quickly run away approaching 25%, 30%, 35%. That is where the long term fuel trim kicks in. The long term fuel trims job is to keep the short term fuel trims at 0%. Yes they are somewhat of an average of the short term fuel trims but it is more of a strategy. The long term fuel trim will use the fuel trim table to determine how to control the short term. If the strategy it is using is not working then it will change strategy. These tables are learned over time and change over time. They are slow to react and take time to adjust. That same rules apply to long term fuel trim. If it is negative it is subtracting fuel, if it is positive it is adding fuel. Also i should note that it is adding /subtracting fuel to the already predetermined fuel table that is programmed into the computer y the factory.
 

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Hi guys ,been reading about this fuel trim shit,man my 08 threw po121 and in the book I have it says tps,well I bought the hole Cha Cha tb and tps,and had another one sent to replace,the new one ,so then I bought a fox well nt530,and they told me it was the equivalent of tech2 but 300 bones later ,and before that's the only code but now it has a misfire and O2 sensor up an down,,so what the heck is goin on this truck has 64k on it,I'm puzzled cause I clean my old Remy tb and reinstall it,and no codes ,I'm about ready to insure it to the hilt and leave the keys in it,I can figure out why ,and do I reset the trims along with everything else I can ,it's a good scan tool but it won't flash the ecu like tech 2,end of my rope
 

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Hi guys ,been reading about this fuel trim shit,man my 08 threw po121
Did you do this?

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What are the other codes? Sign up for a subscription to the shop manual at AllDataDIY so you can use your scan tool to diagnose this properly.
 
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