Long Term Fuel Trims
Thread Starter
Burning Brakes
Joined: Aug 2003
Posts: 1,070
Likes: 9
From: MARRIOTTSVILLE MD
Long Term Fuel Trims
My LTFTs are usually -1.5 to -6 but I noticed after filling up the
gas tank I drove away with LTFTs around -12 then after driving
around 10 miles they came down to around -8 then the next day
when I drove another 20 miles the trims were back to normal.
Is this normal ? Why ?
Thanks >>>>> INTHERED
gas tank I drove away with LTFTs around -12 then after driving
around 10 miles they came down to around -8 then the next day
when I drove another 20 miles the trims were back to normal.
Is this normal ? Why ?
Thanks >>>>> INTHERED
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
when you add 5 or more gallons to the fuel tank, the PCM switches back into "93 octane" mode. Then as you drive it may start tapering off on performance due to readings (mostly) from the knock detect sensors.
That's all I know that would tie it into filling up with gas.
???
That's all I know that would tie it into filling up with gas.
???
Safety Car
Joined: Mar 2002
Posts: 4,928
Likes: 1
Cruise-In V Veteran
St. Jude Donor '09
Originally Posted by Mike Mercury
when you add 5 or more gallons to the fuel tank, the PCM switches back into "93 octane" mode. Then as you drive it may start tapering off on performance due to readings (mostly) from the knock detect sensors.
That's all I know that would tie it into filling up with gas.
???
That's all I know that would tie it into filling up with gas.
???
I believe there is a fuel conservation function in the PCM that reduces injector pulsewidth when fuel levels are drastically low.....so LTFTs would go more negative. I don't remember where I read this ...it could be internet b.s. also.
FWIW.
Melting Slicks
Joined: Feb 2002
Posts: 2,613
Likes: 0
From: Winston Salem NC
Originally Posted by INTHERED
My LTFTs are usually -1.5 to -6 but I noticed after filling up the
gas tank I drove away with LTFTs around -12 then after driving
around 10 miles they came down to around -8 then the next day
when I drove another 20 miles the trims were back to normal.
Is this normal ? Why ?
Thanks >>>>> INTHERED
gas tank I drove away with LTFTs around -12 then after driving
around 10 miles they came down to around -8 then the next day
when I drove another 20 miles the trims were back to normal.
Is this normal ? Why ?
Thanks >>>>> INTHERED
how temperatures affect rpms.
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
Sorry, but the 5 gals of gas is old myth. There is nothing in the PCM code for refueling or actual fuel octane (although Ken Kelly did "label" two scalar Timing tables, used in open throttle function, High Octane and Low Octane). There is absolute proof of this on LS1 Tech.com -> search for posts by Fetumpsh
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
I just read the posts you suggested. here is what I seen that directly relates to this:
It is a constantly changing value, it does NOT get reset to 100% when you fill up with fuel, the only thing that will bring it back to 100% and keep it there is minimal knock activity.
This has been tested on my own 2000 Commodore and emarkay's Camaro.
but has this been verified on the post 2001 Corvette LS1 ? (model year 2001 saw a major rewrite of the PCM software for the Corvette).
It is a constantly changing value, it does NOT get reset to 100% when you fill up with fuel, the only thing that will bring it back to 100% and keep it there is minimal knock activity.
This has been tested on my own 2000 Commodore and emarkay's Camaro.
but has this been verified on the post 2001 Corvette LS1 ? (model year 2001 saw a major rewrite of the PCM software for the Corvette).
Thread Starter
Burning Brakes
Joined: Aug 2003
Posts: 1,070
Likes: 9
From: MARRIOTTSVILLE MD
Originally Posted by gojo
Don't you find your ltrims change as a result of temp changes? I suspect that sitting while filling up may have raised ECT and IAT, causing your trims to do a small relearn. Just a theory from watching
how temperatures affect rpms.
how temperatures affect rpms.
resistor that I used drag racing earlier in the day.The ECT is going to be lower after sitting without the engine running. Any other ideas ?
Melting Slicks
Joined: Feb 2002
Posts: 2,613
Likes: 0
From: Winston Salem NC
Originally Posted by INTHERED
The IAT did not change because I had it fixed at 57 degrees with a
resistor that I used drag racing earlier in the day.The ECT is going to be lower after sitting without the engine running. Any other ideas ?
resistor that I used drag racing earlier in the day.The ECT is going to be lower after sitting without the engine running. Any other ideas ?
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Race Director
Joined: Mar 2000
Posts: 17,602
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From: Palm City Florida
Originally Posted by Mike Mercury
thanks. I've always read about the 5 gallon fill. I guess if you hear something enough.. you tend to just accept it as fact. I'll check out the posts you mentioned.
Going to like -10 and -12..then they settled down after a few miles or so..
So yes it does happen with a fillup.
Why I am not sure.. It seems that if it was only doing it to see if the High Octane table was usable it should not affect the LTFT's .
Melting Slicks
Joined: Feb 2002
Posts: 2,613
Likes: 0
From: Winston Salem NC
Originally Posted by chuckster
It's not a myth and you are correct.. I had the same issue.. I was Autotapping (had like -1 to -3 LTFTs) one morning and I stopped for Gas... Upon starting the car and taking off the LTFTS went "WILD" Going to like -10 and -12..
then they settled down after a few miles or so..
So yes it does happen with a fillup.
Why I am not sure.. It seems that if it was only doing it to see if the High Octane table was usable it should not affect the LTFT's .
Going to like -10 and -12..then they settled down after a few miles or so..
So yes it does happen with a fillup.
Why I am not sure.. It seems that if it was only doing it to see if the High Octane table was usable it should not affect the LTFT's .
That's not enough info to be certain.
If your timing was reset to the higher octane table it should create a more lean condition and more pos. trims. Try copying the tables and see if it happens.