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Old 05-22-2023, 06:49 PM   #1
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More Fuel Trim Questions

Had a "lugging" problem last year that was resolved with new plugs, magnum wires and MAF sensor.
Started paying attention (Scan Gage II) to Long Term trims and on first trip this year, had long term trims bouncing around between 6 and up to 14 as I ran down the road. Seemed to hover most time at 10 to 12 at highway speed and 15 at idle. Both banks performed the same.
SFT was from 0 to 4 but mostly 2 to 3.
Do the numbers point to a problem? If yes, what should I be looking at?
Thanks, Tim
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Old 05-22-2023, 06:51 PM   #2
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Additional Question

Is there a way to reset the LFT?
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Old 05-26-2023, 12:20 AM   #3
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Those numbers arenít that far off and not in the range that will cause damage. Keep an eye on them and if they continue to drift toward 16-18-20 then you definitely have an issue that needs to be addressed.


I think the LTFT resets every time you start the engine. Its not like weeks of trim data is stored and you need to reset the data logger to get accurate info. The LTFT data should be pretty accurate within a few minutes of restarting the engine.
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Old 05-26-2023, 09:08 PM   #4
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Check your fuel pressure at the fuel rail, there is a shraider valve there that you can screw a diagnostic fuel pressure test gauge on to, most auto parts stores will loan you one for free.
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Old 05-27-2023, 05:49 PM   #5
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Thanks for the input

I ended up disconnecting the chassis battery and the fuel trim readings all went back to zero at idle. Heading out in a few weeks and will see how it does on the road.
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Old 06-05-2023, 06:20 PM   #6
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Took ride to nearby campground about 45 minutes away mixed driving. On highway at steady 62 mph, LFT's stayed between 7 and 10 with both sides being within 1 point. There were times it would jump around but always seemed to go below 10.
At idle, would settle at 14-15 but upon acceleration always came back to 10 or below.
Not sure if I have a problem to be addressed or just watched.
Any thoughts?
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Old 06-05-2023, 08:49 PM   #7
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In my experience high LFT may be caused by a combination of a number of factors, though certain ones like a dirty MAF or bad O2 sensor can be a major contributor. Mine had been creeping up into the 10-14 range while under load even after installing a new MAF, so prior to our latest 2,800 mile trip last month, I replaced the coil packs, as I was also having an occasional misfire under load problem, but only after the engine was hot after a couple of hundred miles of driving that might be explained by a failing coil pack, along the way I found the EGR valve was leaking externally, so I replaced it, and due to age while I was there I replaced the fuel pressure regulator even though the old one seemed to be fine (the 8.1 had a fuel rail mounted pressure regulator from 2001-2003).


I don't know what did it, but on this most recent trips I typically saw LFT running between 0-2 while under load on the entire trip, going higher at idle, which may be an as yet un-diagnosed vacuum or exhaust leak.
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Old 06-06-2023, 05:34 PM   #8
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Appreciate the Info

Have 6 weeks off with no trips so will do a little investigative work.
Will pull the engine cover and review the vacuum hoses and some of the other areas mentioned.
One thing I need to address is the wiring going to the MAf sensor. It is quite stressed and barely can be plugged in. Going to order an extension cable once I figure out which is right.
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Old 06-07-2023, 01:53 PM   #9
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Tim,
Have you replaced the fuel filter?
I had a similar problem with LFT running above 10 all the time. Replaced the filter and it has dropped to the 8 to 10 range.

Martin
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Old 06-07-2023, 06:23 PM   #10
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Fuel Filter

Did the fuel filter last year, have put a few hundred miles on since then.
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Old 06-07-2023, 07:38 PM   #11
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Quote:
Originally Posted by AKIQPilot View Post
Those numbers arenít that far off and not in the range that will cause damage. Keep an eye on them and if they continue to drift toward 16-18-20 then you definitely have an issue that needs to be addressed.


I think the LTFT resets every time you start the engine. Its not like weeks of trim data is stored and you need to reset the data logger to get accurate info. The LTFT data should be pretty accurate within a few minutes of restarting the engine.
It's been awhile, But my understanding is, long term trim is the average trim for the last 300 or so engine starts that the engine ran for at least minimum of a predefined time.
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Old 06-07-2023, 07:46 PM   #12
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Quote:
Originally Posted by AKIQPilot View Post
Those numbers arenít that far off and not in the range that will cause damage. Keep an eye on them and if they continue to drift toward 16-18-20 then you definitely have an issue that needs to be addressed.


I think the LTFT resets every time you start the engine. Its not like weeks of trim data is stored and you need to reset the data logger to get accurate info. The LTFT data should be pretty accurate within a few minutes of restarting the engine.
It's been awhile, But my understanding is, long term trim is the average trim for the last 300 or so engine starts that the engine ran for at least minimum of a predefined time.
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Old 06-07-2023, 07:51 PM   #13
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Quote:
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It's been awhile, But my understanding is, long term trim is the average trim for the last 300 or so engine starts that the engine ran for at least minimum of a predefined time.
Long term is the parameter that short term works within. Long term's goal is to adjust itself to keep short term as close to zero as possible. For example if short trim consistently runs at -10 for a pre-determined time (usually minutes) then long term will cut fuel to bring short term back to zero, if short trim runs consistently at +10 then long trim will add fuel to bring short term back to zero, then at that point LT will be at +10 and ST will be zero. Since short term is what makes the immediate adjustments, it needs to have leeway in both directions, lets say short trim maxes out at + or -20, if it is allowed to max out it won't be able to make necessary quick adjustments so long term makes adjustments to keep short trim in the middle of it's range instead of close to or pegged at it's min or max so short trim can do it's job.
Total fuel trim is always the sum of ST and LT. On an engine with a big problem it is possible for long and short trim to both max out in either direction.

It's kind of like LT has ST's back.
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