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Short term and long term fuel trim?

Old 03-16-2008, 01:34 AM
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Default Short term and long term fuel trim?

Will short term or long term fuel trim mess with open loop operation or are thy only used for closed loop operation.
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Old 03-16-2008, 03:53 AM
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Default Re: Short term and long term fuel trim?

From what I've heard Long terms in the Honda ecu are applied when in closed open loop. EDIT* I meant open. They are definitely applied in closed loop.

Short term fuels trims are never applied in open loop, hence the name "open loop", meaning no feedback in the control system which is what short terms are for.


When ever you are tuning, you should ALWAYS disable the O2 sensors. This will keep the fuel trims from screwing with your wideband readings.



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Old 03-16-2008, 11:58 PM
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Default Re: Short term and long term fuel trim?

Originally Posted by MADMAX
From what I've heard Long terms in the Honda ecu are applied when in closed loop.
Are long term fuel trims applied in open loop? (probably not ? maybe)

Long story short my friend wants to get a a/f vtec control. I told him thy suck. Thy tune in 500 rpm intervals, thy only change the map input nothing else. The vtec is on with the controller but the stock ecm will not know when it is on so it will be in the low vtec map. Adding boost is still mested up due to the stock ecm freaks with about 2 psi of boost. Thy only have 2 tuning solutions for the throttle wide open and cruse no in-between (according to my research).

But I was thinking if long term fuel trims applied in open loop all your tuning would go to ----. The computer would relearn around your a/f controller and set rich or lean codes (another friend has one no codes so maybe not). Just a another thing I can use to talk him out of it. He is a good friend and I donít what to see him loose a lot of money on his build. Any other thing wrong with an a/f control I didnít think of? Any thing I got wrong?

Thank you.
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Old 03-17-2008, 07:56 PM
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Default Re: Short term and long term fuel trim?

Thanks OK thy really suck then. LOL almost useless just a fancy vtec controller.
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Old 03-18-2008, 01:18 AM
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Default Re: Short term and long term fuel trim?


They do work, you just have to make sure to tune the part throttle maps as close as possible to 14.7 or stoich.

To do that though you'd have to log short term fuel trims directly from the ecu, otherwise you'd never know.


AFC's are garbage and a waste of money. If you can solder, you can socket your ecu for $30, buy a datalogging cable for $20, and a Moates Burn1 chip burner for $90 and use CROME free and Freelog. That's less than $150 total. V-AFCs are usually over $200 if you are lucky and they can't do nearly as much as the stock ecu.



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Old 03-18-2008, 09:30 PM
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Default Re: Short term and long term fuel trim?

Ok I think I get what you mean. So in other words you could not make any changes if you are in closed loop (cruising/part throttle) but at wide open throttle you are in open loop so changes do not mater the computer ignores o2 sensor input in open loop. Ok so thy work a bit. Still 500 rpm increments and no ignition timing changes for boost, vtec overlap crap I see why thy use the 2 maps for the throttle now.

I can solder but probably nothing that small. Thanks for all your help you are probably thinking ---- this guy by now. LOL


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Old 03-19-2008, 01:43 AM
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Default Re: Short term and long term fuel trim?

LTFT is supplied by constant STFT application.
STFT does not apply to open loop, however the OBD-II standard does state that LTFT IS APPLIED during open-loop mode.
Older OBD-I ECU's may only be supplied with a STFT, but it depends on the age and ECU.
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