Fuel Trim questions
#1
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Fuel Trim questions
being new to the world of electronic fuel management, I'm trying to catch up on 40 years of engineering advances in a few weeks.
What are the normal numbers for:
1) short term fuel trim
2) long term fuel trim
3) downstream fuel trim
?????
I'm not getting any fault codes, but the numbers are confusing nevertheless.
For short term fuel trim it's consistently under 1% Usually something like -0.79%
for long term fuel trim the numbers are higher, like in the range of -4.50% to -7.0%
But the downstream fuel trim numbers are really high, consistently over -40.0% Do these number have anything to do with the catalytic converter health?
I've looked in the 1999 shop manual I downloaded from this forum (is there a 2002 manual ? I couldn't find it.) And did a few searches here as well. But I think my questions are too basic, and I've come up empty.
thanks,
Z.
What are the normal numbers for:
1) short term fuel trim
2) long term fuel trim
3) downstream fuel trim
?????
I'm not getting any fault codes, but the numbers are confusing nevertheless.
For short term fuel trim it's consistently under 1% Usually something like -0.79%
for long term fuel trim the numbers are higher, like in the range of -4.50% to -7.0%
But the downstream fuel trim numbers are really high, consistently over -40.0% Do these number have anything to do with the catalytic converter health?
I've looked in the 1999 shop manual I downloaded from this forum (is there a 2002 manual ? I couldn't find it.) And did a few searches here as well. But I think my questions are too basic, and I've come up empty.
thanks,
Z.
#2
Join Date: Apr 2012
Location: Summerville, South Carolina
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here is the link to Gus's website which has some good info on Fuel trims. This should also be in the stickies at the top of the XK8/R forum
JagRepair.com - Jaguar Repair Information Resource
JagRepair.com - Jaguar Repair Information Resource
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Orthodixie (08-24-2018),
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#3
A good place to get started on fuel trims is this thread. I've never heard of 'downstream' fuel trim.
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
#4
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A good place to get started on fuel trims is this thread. I've never heard of 'downstream' fuel trim.
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
the code reader I'm using, the icarsoft v1, is a model that is Land Rover / Jaguar specific, and it has "downstream fuel trim" info displayed along with the the other engine module information.
i don't know much about it, hence my questions. But you can goggle downstream fuel trim" and there is some info, but not as complete as I'd like.
Z
#5
A good place to get started on fuel trims is this thread. I've never heard of 'downstream' fuel trim.
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
https://www.jaguarforums.com/forum/g...ed-quiz-49317/
#6
Remember, fuel trims are like a 3D elevation map, they are a function of Load and rpm. It is only a single number for a given Load and rpm, e.g. at idle. Off idle, it will likely be a different number. Also likely a different number between idle with transmission in "N" or "D" as the load varies. So posting fuel trims numbers only makes sense if Load and rpm are posted as well (or a shortcut like "idle"). Fortunately, Load and rpm are available over OBDII as well.
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zray (08-25-2018)
#7
www.onboarddiagnostics.com/page02.htmConsider them with a hot engine (thus also hot cat convs) when parked. At idle ideally they'd all be 0 but that won't happen so somewhere near would be good.
If any LTFT is quite high, rev to about 2500 and see what happens.
With an air leak the LTFT would drop because there's so much more air going in (thus the effect of a leak is far less).
To get a feel for roughly what the PCM (aka ECM) is doing see OnBoardDiagnostics.com - Engine Basics
If any LTFT is quite high, rev to about 2500 and see what happens.
With an air leak the LTFT would drop because there's so much more air going in (thus the effect of a leak is far less).
To get a feel for roughly what the PCM (aka ECM) is doing see OnBoardDiagnostics.com - Engine Basics
Last edited by JagV8; 08-24-2018 at 12:19 PM.
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zray (08-25-2018)
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#8
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#9
#10
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I think 22 mpg is too ambitious for an XKR @ 75-80 mph.. maybe a XK-8 does a little better ?
Have just cleaned the TB, intake pipe & mass air sensor. My best is 20 - 21 mpg @ those speeds and using the cruise control. I find it difficult to use the cruise control however. Am afraid of missing out on that great feeling one gets accelerating out of a long sweeper.
Z
Last edited by zray; 08-26-2018 at 02:57 PM.
#12
Don't bother with the stuff but if you are keen on such try BG44K (think I've got the runes right).
#13
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#14
#15
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I'm just a tad over 125,000 miles, and have been using 91 octane pure gasoline when I could get it. Does the XK-8 not require the 91 fuel as the XKR does ?
In any case, the only way I'll get above 22 mpg, albeit briefly, is by driving off a cliff.
Z
PS will be driving thru your state on the way to Marietta GA in Sept. Any notorious radar traps to avoid ?
Last edited by zray; 08-28-2018 at 06:36 PM.
#16
at higher altitudes regular octane gasoline can be used in a premium only engine. in the high elevations of Colorado, the sell lower octane than even regular for this reason. it is a premium required car (91 US system) and not using such at sea-level conditions is not the best idea unless one drives very flat roads and does not push the vehicle in the mid-range. speed also makes a big difference in the highway fuel economy. using regular octane, the fuel economy will be slightly lower under all conditions for both energy content (there is a very slightly higher energy content to the octane boosting chemicals, in premium) and engine tune. this all differs from racing fuel in that these fuels are not just higher octane but formulated for higher energy content per unit measure of fuel.
#17
regarding Techron Fuel Treatment, very good results if you have the problem of dripping or somewhat clogged fuel injectors. two treatments consecutively made a big difference in performance, fuel economy and idle smoothness. use the larger bottle, add before filing the tank. run down to 1/8 tank and repeat. most additives are a waste of money; this one is definitely worth it.
at 55 with the cruise control, both calculated and dashboard indicated 30 MPG, 28.6 at 70.
at 55 with the cruise control, both calculated and dashboard indicated 30 MPG, 28.6 at 70.
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zray (08-29-2018)
#18
Another vote for Techron here. All of our vehicles get a 20-ounce bottle down the hatch at every oil & filter change interval. It is the only fuel injector cleaner I will purchase. I watch the sales at Advance / Pep Boys / Autozone and grab several bottles when they run their BOGO deals....
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zray (08-29-2018)
#19
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Another vote for Techron here. All of our vehicles get a 20-ounce bottle down the hatch at every oil & filter change interval. It is the only fuel injector cleaner I will purchase. I watch the sales at Advance / Pep Boys / Autozone and grab several bottles when they run their BOGO deals....
around here there are 2 formulations of Techron, the fuel injector cleaner, and one labeled "total fuel system cleaner", which I believe is a little stronger, and it costs $ 0.40 cents more than their fuel injector cleaner product. I buy it at Wal-Mart Unfortunately W-Mart does not carry the larger 20 oz. size. But two bottles of the smaller size is still cheaper than the sale price of one 20 oz. bottle priced at the usual Auto Parts stores, So I just dump in 2 bottles every 3rd or 4th tank of (pure) ethanol-free gas. Probably more often than I have to, but I'm trying to clean up the system that may have been neglected by the previous owners.
Z
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Jon89 (08-29-2018)
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