How does "Fuel Trim" work in the ECU?
Sep 28, 2006, 05:16 AM
#16
Originally Posted by l2r99gst
In the DSM ECU, there was a range from +12.5% to -12.5% for the LTFT.
Oct 1, 2006, 05:47 AM
#18
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Originally Posted by MalibuJack
If you find your car idles fine for some time, and your trims are okay, then every so often the idle goes real lean, or real rich, its due to increased load (Alternator load due to fans kicking in, etc) This would be one of the places you look, since there is a target idle, and the ISCV value makes assumptions of a good starting point for that value.. If its wacked, it will occasionally reset to that value...
Oct 1, 2006, 05:53 AM
#19
Look up a post about ICSV (Idle speed control valve) The valve resets to different settings under an assumption that its a stock car.. So sometimes it may be "in the wrong place" on a modified car, and can do something similar..
you need to monitor your trims when your driving (log the data) and when you come to a stop.. You'll find that the long term trims will settle on a value, if its outside +/- 10% then it needs to be adjusted a little and will help also..
you need to monitor your trims when your driving (log the data) and when you come to a stop.. You'll find that the long term trims will settle on a value, if its outside +/- 10% then it needs to be adjusted a little and will help also..
Oct 1, 2006, 07:29 AM
#20
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Whats weird about my setup is that my 02 trims are usually never more than 10-20 % above or below 100 when I'm cruising or idling, yet the short and mid fuel trims in evoscan are sitting in the 60s. The car does not crank up overly rich on a cold start, and in fact my AFRs are leaner than the fuel map numbers state when I'm at WOT. My only niggle is that every now and then I'll get a funky warm start, but most of the time the car starts and drives like stock.
Oct 1, 2006, 07:54 AM
#21
Originally Posted by AutoXer
I thought my ECU would throw a code at 12.5. Jack, are you sure about the +-25 values? I need to verify it but I think it's a range of 25 with values from + to - 12.5. I'll try to verify it this evening.
I correlated the idle LTFT in my OBDII logger and EvoScan. Seems that the range in EvoScan is from -25 to +25. However, I've seen my OBDII logger max out at 12.5. I know this because I was throwing a fuel trm code due to a loose BOV connection.
Oct 1, 2006, 08:14 AM
#22
I think every OBD-II logger is slightly different.. I'm using an ELM based "Elmscan" tool as my backup, or the ECU+ OBD-II tool, and both show +/-25% I suspect everyone has a different way of expressing the value.. However I chose to keep the numbers where I"m familiar.. I'll alter it as I feel I need to at some point once I get my tools stable.
Oct 1, 2006, 10:29 AM
#23
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I think I'm starting to get an understanding of fuel trims and how they effect things. I did some logging and adjustments and here is what I came up with, if there is something I'm saying wrong please correct me.
My o2 feedback in EVOScan was showing 199, which I figured out meant the ECU couldn't add enough fuel to bring the A/F at idle to make 14.7 so I tried adjusting 3 things to correct the situation.
First was my injector scaling which was originally set to 860cc for the RC1000cc injectors. After a couple of tries I have it scaled down to 790cc. Does this seem to low for a 1000cc injector?
I also adjusted the voltage latency in the 14v area. Originally I had it set to 0.360ms @ 14.04v per the AEM wizard as recommended by MJ. After a couple of tweaks I scaled this value up to .432ms @ 14.04v
Last I adjusted the 10-60% areas between 500-1000RPM on my high octain fuel map. Originally these blocks were set to 13.3 & 14.7 respectivley. I changed the 500RPM section to 13.0 and the 1000RPM section to 13.8.
I made these adjustments in small increments over the course of about 10 flashes. The result is that now instead of my o2 feedback value in EVOScan maxing out at 199 it will vary between 60-140. However I am still getting an occasional real rich to real lean condition at idle but I'm afraid to adjust my scaling/latency values much more...
Does anyone have any input they could provide.
My o2 feedback in EVOScan was showing 199, which I figured out meant the ECU couldn't add enough fuel to bring the A/F at idle to make 14.7 so I tried adjusting 3 things to correct the situation.
First was my injector scaling which was originally set to 860cc for the RC1000cc injectors. After a couple of tries I have it scaled down to 790cc. Does this seem to low for a 1000cc injector?
I also adjusted the voltage latency in the 14v area. Originally I had it set to 0.360ms @ 14.04v per the AEM wizard as recommended by MJ. After a couple of tweaks I scaled this value up to .432ms @ 14.04v
Last I adjusted the 10-60% areas between 500-1000RPM on my high octain fuel map. Originally these blocks were set to 13.3 & 14.7 respectivley. I changed the 500RPM section to 13.0 and the 1000RPM section to 13.8.
I made these adjustments in small increments over the course of about 10 flashes. The result is that now instead of my o2 feedback value in EVOScan maxing out at 199 it will vary between 60-140. However I am still getting an occasional real rich to real lean condition at idle but I'm afraid to adjust my scaling/latency values much more...
Does anyone have any input they could provide.
Oct 1, 2006, 01:13 PM
#24
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I personally would get everything as close as you can with the fuel maps at 14.7. Try adjusting your 11V settings. On my car that was what I used to kind of "zero in" my injectors a bit better. Also, I don't know if all Evos respond the same, but mine was VERY sensitive to very small changes in latency (especially in the 11V bin). IE, my car would go from cranking at around 15:1, to 12:1 with a change from 1.100 to 1.200 in the 11v bin.
Oct 2, 2006, 09:42 AM
#25
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more updates...
I've got a much better result at idle now. according to my pocketlogger my STFT @ idle are anywhere between (-2% & 2%) is that acceptable, thats the range I see when idling and the A/F reading is 14.6-14.7 most of the time. Sometimes it will get richer @ idle and then the STFT reading will go more negative, as much as -11% but I don't under stand why all of a sudden it gets richer for no reason when it sits at 0% & 14.7 during idle most of the time.
My LTFT are (-9%). during closed loop driving my STFT seems to be on the negative side.
The car is running much better but still not 100%, any ideas?
I've got a much better result at idle now. according to my pocketlogger my STFT @ idle are anywhere between (-2% & 2%) is that acceptable, thats the range I see when idling and the A/F reading is 14.6-14.7 most of the time. Sometimes it will get richer @ idle and then the STFT reading will go more negative, as much as -11% but I don't under stand why all of a sudden it gets richer for no reason when it sits at 0% & 14.7 during idle most of the time.
My LTFT are (-9%). during closed loop driving my STFT seems to be on the negative side.
The car is running much better but still not 100%, any ideas?
Oct 2, 2006, 09:55 AM
#26
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one more thing to add, I don't know if it makes a difference or not but when I go to start the car usually it will crank for a second or two before turning over. Now though with the new injectors installed and these settings it takes just a fraction of a second to startup. All I do is tap the start and bam the engine is running. Like I said I don't know if that has to do with anything but I figured it was worth mentioning.
Oct 2, 2006, 11:28 AM
#27
If you find in the future you have to crank the car a few times to get the engine to start, the first place to look is the voltage latency below 12v.. Thats what its for.. If your cranking you could see voltage lower than 11v to the injectors depending on the battery, in some cases the latency can have a big affect on whether you have to crank the engine for one second, or 5 seconds.. ya know?
Oct 2, 2006, 11:30 AM
#28
Its pretty typical to see -9% cruising around.. This is the reason we have injector dead time adjustments, however we don't have that level of control in the rom yet..
-9% is actually an okay situation, it means that the fuel curve is about 9% rich, so when you go into open loop maps, your actually a bit richer than you need to be..
Anything over 12%-25% will throw Cel codes..
You can now adjust your higher RPM fuel table in the load cells where closed loop runs, it should equalize the trims if it bothers you..
I have this problem all the time due to
1) Aftermarket blowthrough MAF has a slightly different airflow curve
2) Larger BB turbo.. when it spins down it causes the car to run slightly rich for a moment or two, pretty normal, more airflow through the MAF thanks to the extra spinning.. Doubt there's much you can do about that.
3) Injector Voltage Latency is a value to compensate for alternator/battery voltage to the injectors, but not true dead time or latency, therefore you don't have the level of control. (I did find items in the rom related to Latency and Dead time, but I have not yet verified what they actually do)
-9% is actually an okay situation, it means that the fuel curve is about 9% rich, so when you go into open loop maps, your actually a bit richer than you need to be..
Anything over 12%-25% will throw Cel codes..
You can now adjust your higher RPM fuel table in the load cells where closed loop runs, it should equalize the trims if it bothers you..
I have this problem all the time due to
1) Aftermarket blowthrough MAF has a slightly different airflow curve
2) Larger BB turbo.. when it spins down it causes the car to run slightly rich for a moment or two, pretty normal, more airflow through the MAF thanks to the extra spinning.. Doubt there's much you can do about that.
3) Injector Voltage Latency is a value to compensate for alternator/battery voltage to the injectors, but not true dead time or latency, therefore you don't have the level of control. (I did find items in the rom related to Latency and Dead time, but I have not yet verified what they actually do)
Last edited by MalibuJack; Oct 2, 2006 at 11:35 AM.
