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Fuel Trims
Old June 27th, 2019, 04:55 AM   #1
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Fuel Trims

Installed an ROE supercharger and have +20 long terms at idle when first start the engine but after engine warms up they go to +10. On both banks. I have no vacuum leaks. Spark plugs look great according to ROE.
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Old June 27th, 2019, 09:09 AM   #2
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Quote:
Originally Posted by Jeremias06 View Post
Installed an ROE supercharger and have +20 long terms at idle when first start the engine but after engine warms up they go to +10. On both banks. I have no vacuum leaks. Spark plugs look great according to ROE.
I dont have a ROE setup, but with most any obd2 car tuned properly you should only be hovering near zero on LTFT ideally in a perfect world. This obviously isn't always possible but the closer to zero the better. Its more than likely because
you have increased airflow in the intake just at idle and in general due to the blower being on the threshold of building boost almost instantly.

The LTFT being that far out is an major indication that the ecu is obviously correcting for something that isnt what it expects according to the fuel mapping tables that are currently on the ecu and that its been happening constantly.

Basically the ecu wants something and adjusts but the sensors report back data and the ecu compares and finds its not getting what it thinks it should. This drops into STFT and if this continues for a while without improvements there for long enough it gets committed to LTFTs.

I believe.... 20-25 percent on most (not all) obd2 is like the upper limit before it can just go into limp mode on some cars. I cant say the same for the viper because i've not been down that road fully myself so take that bit with a grain of salt.

Honestly, retune if you can to correct this issue, ultimately you want the LTFTs to be there for a safety net for adjustment when things are off slightly and if you are almost already maxed out this isnt going to give you that "cushion" because you're already right up against the edge on LTFTs.

Also you might check into the bypass valve on the blower if it has one (it should) and adjust if possible? This is obviously an overall adjustment across the board, but i would think that you're shoving more air in at startup due to the nature of the setup and bypassing this a bit more might help but i cant say how much that would affect you once fully warmed up. Something to think about maybe.

Last edited by Lunchbox; June 27th, 2019 at 09:16 AM.
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Old June 29th, 2019, 01:41 PM   #3
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Also noticed that when going into boost I get a negtaive 20 ltft. Wierd since more fuel is being put for the boost but computer is saying WOW too much lol.

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Originally Posted by Lunchbox View Post
I dont have a ROE setup, but with most any obd2 car tuned properly you should only be hovering near zero on LTFT ideally in a perfect world. This obviously isn't always possible but the closer to zero the better. Its more than likely because
you have increased airflow in the intake just at idle and in general due to the blower being on the threshold of building boost almost instantly.

The LTFT being that far out is an major indication that the ecu is obviously correcting for something that isnt what it expects according to the fuel mapping tables that are currently on the ecu and that its been happening constantly.

Basically the ecu wants something and adjusts but the sensors report back data and the ecu compares and finds its not getting what it thinks it should. This drops into STFT and if this continues for a while without improvements there for long enough it gets committed to LTFTs.

I believe.... 20-25 percent on most (not all) obd2 is like the upper limit before it can just go into limp mode on some cars. I cant say the same for the viper because i've not been down that road fully myself so take that bit with a grain of salt.

Honestly, retune if you can to correct this issue, ultimately you want the LTFTs to be there for a safety net for adjustment when things are off slightly and if you are almost already maxed out this isnt going to give you that "cushion" because you're already right up against the edge on LTFTs.

Also you might check into the bypass valve on the blower if it has one (it should) and adjust if possible? This is obviously an overall adjustment across the board, but i would think that you're shoving more air in at startup due to the nature of the setup and bypassing this a bit more might help but i cant say how much that would affect you once fully warmed up. Something to think about maybe.
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Fuel Trims
Old June 29th, 2019, 07:21 PM   #4
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Fuel Trims

LTFT don’t typically apply to open loop, which includes WOT. Some tuning architectures allow some application of LTFT on closed loop and I was always taught to turn it off if available.


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Old July 1st, 2019, 05:26 PM   #5
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Acceleration enrichment (both MAP and TPS based) will trigger that big rich condition when you stab the pedal. Can be (should be) tuned out. Also remember that pump gas today is around 10% ethanol mix which will, all things being equal, trigger your fuel trims to the positive side a bit...ergo the ECU will think you are lean and will add fuel via the trims in closed loop. Why do you think the big petro companies stop at a 10% ethanol mix???? Because older ODB-II cars can only trim so much additional fuel before a check engine light will be illuminated. Big oil would love nothing more than to push the ethanol percentage higher and you see indications of a 15% mandate in the news. As our older cars get put to pasture, newer ECMs can handle and tune more precisely for ethanol blended fuels. Its the way of the future. Obviously true Flex Fuel engines can handle this, our cars tuned for non-ethanol can only adjust so much. Just another reason to get the car's fuel and timing tables specifically tuned for your application.
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Old July 1st, 2019, 05:56 PM   #6
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Side bar, but “Big Oil” is not typically the pusher (or even a proponent) of increased ethanol blending. The bio fuel segment of any major oil and gas company is generally dwarfed by the conventional crude distillation capacity. They are subsequently forced to buy RIN credits from ethanol producers as required by ethanol blending standards, or directly purchase and blend the ethanol. Every increase in ethanol percentage at the pump is less fuel ultimately purchased from a refinery. They can get into the side business of buying and selling RIN credits, but it’s far from the core profitability of a conventional refinery. An increase to E15 could also domino to refiners having to produce lower RVP gasoline in order to offset the negative volatility impact of more ethanol in summer months. That means producing more expensive (and less plentiful) octane enabling blend stocks instead of the cheaper blending components.


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