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Thread: Fuel trims what makes them max out at idle?

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    Davezj's Avatar

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    Fuel trims what makes them max out at idle?

    What i am after is a bit of discussion as to what could possibly make the fuel trims max out.

    a bit of back ground into fuel trims.
    fuel trims add and remove fuel based on the O2 sensor going rich or lean.
    fuel trims only work in closed loop operation with the O2 sensor.
    you can think of long term fuel trim and short term fuel trim as 2 columns of numbers, Tens and Units LTFT are the tens and STFT are the units.
    STFT goes from -16.8 to +16.8
    LTFT goes from -4.7 to +4.7
    when STFT climbs higher and high the LTFT will increase a bit to say 0.4 So as the LTFT has add more fuel to the required level the STFT does not have to add more fuel so it will reduce to about zero and fuel trims will stablise until the load on the engine changes.

    my car has the following fuel trim at idle while running petrol.
    fuel trim low (LTFT) 4.7
    fuel trim in use (LTFT) 4.7
    oxygen feedback trim (STFT) 16.8 then after its maxed out for a while it defaults 5.1 constant.

    the long term fuel trim should sit at zero 0, then the short term fuel trim should wobble up and down above and below zero depending on driving conditions, but generally it should average out to zero.

    my car is adding as much fuel as possible when the car is sat at idle.

    if you disconnect the battery it resets all the fuel trim to zero. if i reset the fuel trim to zero and go for a drive immediately keeping the car in closed loop fuel operation. The long term fuel trim stays at zero and the short term fuel trim fluctuates around the zero point which is as it should be.

    So my question is why do the trims max out at idle. this maxing out at idle is a real issue, because once the fuel trims have maxed out they will not reduce when driving the car in closed loop operation. so the car is running very rich when pootling about road town which wastes so much more fuel than i actually need to waste by driving a VR4.

    i know this is a lot to get your head round from my description but it is worth knowing about.
    all help very welcome.

    this video is long but does explain what fuel trims do and how O2 sensors influence the fuel trim it is very good.

    https://www.youtube.com/watch?v=oRX2V6_a3dc


    can some of you do me a favour and check your fuel trims at idle to see what yours are doing. just as a comparison so i can gauge if my car is really bad or just all VR4 max out the fuel trim for some reason.
    Last edited by Davezj; 30-07-2015 at 12:46 AM.

    Bye for Now!

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    One of main function of LTFT and STFT to adjust fuelling to wear and tear, so I would say its max out in most vr4 due to they age, simple injectors flow 1000s gallons of petrol through them over they life (especially in vr4s ) and they just not as new any more hence ECU max them out to even things up. That's IMO anyway.


    of course you must have ZERO boost/vacuum leaks for engine to run correctly in first place, not to mention things like MAF, lambda, temp sensors ect must be 'as new'
    Last edited by adaxo; 30-07-2015 at 07:32 AM.
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    Fuel trims what makes them max out at idle?

    From what I can gather the following are the contributing factors,
    Air leak
    O2 sensor faulty
    Fuel delivery issue
    Temp sensor
    MAF sensor issue

    I have a know good set of injectors fitted. Cleaned flow tested and from a car that does not max out its fuel trims.
    Standard O2 sensor that changes state regularly as monitored with evoscan
    Standard fuel pressure regulator.
    Ido have a gss342 fuel pump fitted (255LPH) so that could be over pressurising the standard FPR.
    I can swap a standard fuel pump back in to check.

    I will do a boost leak test for piece of mind.
    Last edited by Davezj; 30-07-2015 at 09:43 AM.

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    ok so i have done a but of digging tonight. this is what i have done.

    done a boost leak test, turbo outlet of Y pipe to inlet manifold, only one leak and we all know where that is the idle adjustment screw but there is not a lot i can do about that. but all other leaks upto 20psi are fixed and onlt very very slight leak at 25psi.
    reset all the fuel tims to zero,
    ran the engine and the fuel trims maxed out.

    reset all the fuel tims to zero, plugged idle screw leak
    ran the engine and the fuel trims maxed out.

    Swapped maf to a spare, reset fuel trims to zero
    ran the engine and fuel trims maxed out.

    so i think the following,
    injectors ok
    maf is OK
    i can see the O2 sensor going laen and rich sensor ok
    no air leaks
    temp sensor is working as seen in evosacn.

    any other contenders?

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    Come over tomorrow, I may start my vr4 so we can compare logs.

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    If the lambda sensor is switching lazily or spends slightly more time shouting lean than shouting rich, then couldn't that be an issue? Putting a wideband with narrow band emulation on my cars has always improved fuel economy.

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    I know my VR-4 may not be "reference", but for me at idle the Low and InUse trims sit at 4.6875 and the others at 0. That was before I changed the lambda, now I need to take some fresh logs again.

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    i have taken the day off tomorrow to do a little bit of prep on my car for the RR Day on Saturday.

    there is someting wrong with my AFR gauge so i am going to swap it with my other one and see if i can diagnose what the problem is, sensor or gauge itself or grounding wires etc.

    however i have been out tonight and reset the LPG idle fuel trim, so when at idle on LPG the oxygen feedback (STFT) will oscilate above and below zero point with the LTFT hovering around zero point. i know the fuel trim will not effect WOT throttle runs, but i thought i would try and sort it out while i had the chance.

    i might just get the dyno done on LPG, i was going to get it done on petrol only just to get a baseline for the replacement engine.

    i need to cut a hole and weld on bung to my decat so i can fit the AFR sensor as i have fitted a standard O2 sensor to actually drive the ecu input, and just use the AFR gauge to monitor.

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    Quote Originally Posted by Grid View Post
    I know my VR-4 may not be "reference", but for me at idle the Low and InUse trims sit at 4.6875 and the others at 0. That was before I changed the lambda, now I need to take some fresh logs again.
    it is starting to seem quite comon that the fuel trims are maxed out. it will be interesting to get some more info off other people to see what there car is like with respect to fuel trims.

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    Screenshot from 2015-07-31 09:05:47.png

    Fuel trim InUse (equal to Low when idling) when idling after a drive stabilizing @4.688.

    Screenshot from 2015-07-31 09:09:26.png

    Fuel trim Low (again equal to InUse) at a red light during that same drive, earlier.

    It seems to hunt for a while but ends up almost saturated every time.

    Log attached if you want other charts.
    Attached Files Attached Files

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    Oh and the above logs are on a wideband AFR with narrowband emulation point set to 15.2

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    Fuel trims what makes them max out at idle?

    Quote Originally Posted by Nick Mann View Post
    If the lambda sensor is switching lazily or spends slightly more time shouting lean than shouting rich, then couldn't that be an issue? Putting a wideband with narrow band emulation on my cars has always improved fuel economy.
    i have another O2 sensor that i can fit so will do that and test to see if the switching time is any different from the one i have have fitted at present. then i can compare both to a working wideband with narrow band emulation.

    does anyone know off the top of there heads what the switching time should be for a standard O2 sensor


    this is the operation i am talking about after a fuel trim reset to zero.
    please bare in mind it takes a while for the LTFT to react to the STFT as you can see the LTFT stays at zero until later in the log when it rises to 4.7 and stays there.
    yellow line is O2 is switch lean low and rich high
    blue line is STFT is moving with O2 sensor
    orange line is rpm and it going up and down because the air con is starting and stopping, and at this point the fuel trim reacts to the change in load on the engine by following what the O2 is telling it to do.
    purple line is LTFT

    The section in the middle off bit fors happen on a regular basis, it is as if the fuel system drops out of closed loop due to O2 not reading and at that point the fuel trims go I am blind I don't know what is happening for safety max out the fuel.
    But I blip the throttle a couple of times and it all start to work again. I don't know if anyone else experiences this.

    I might try rerunning the test and just unplug the O2 sensor and see if it gives the same response is I may have a bad sensor or bad connection.

    what should be happening is the STFT should be going up and down with the O2 sensor but it should be doing this above and below the zero line not at about 12% this is why the LTFT goes to 4.7% it is trying to allow the STFT to reduce and get to the zero point.

    fuel trims1.jpg


    this is a typical O2 switch on my car the interesting time is the time to rise and time to fall not the over all to rise and fall as the time spent at the top and the bottom is determined by the exhaust gas being rich or lean about the 14.7:1 point.
    so the switch time seems to be about 0.5 second (20%to 80% low to high) however i dont know if this is due to the recoding speed of data point in evoscan or the actual sensor switch time, i had a bouot 30 items ticked to record data on so maybe i should redo the o2 switch test and only record the O2 sensor and see if changes the switch time.
    fuel trims2.jpg
    .
    i have just looked up the switch time for an O2 sensor and less than 100ms (0.1 second) so at the moment mine seems very slow.
    Last edited by Davezj; 31-07-2015 at 11:20 AM.

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    If your stock O2 sensor is still factory original it could just be covered in soot from all those years and be very lazy / slow to respond. Hence the need to blip the throttle to wake it up. Just a theory worth checking.

    In general if the LTFT is increasing and maxing out it's because for some reason the EFI thinks we are constantly running too lean for its liking. This could be due to several reasons:

    Running too lean – High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.
    Source: http://www.easterncatalytic.com/educ...agnostic-tool/

    Some / most of the above I was suspecting in my VR-4. So far I got rid of the catalytic converter, replaced the O2 sensor. Now focusing on a potential boost leak in the DV (next on the list - injectors, MAF).

    I will run some Evoscan logs focusing on trims vs AFR, airflow, injector duty cycle.

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    Also from the same source as mentioned above:

    Diagnostic Tip:

    For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.

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    well i have been busy today.

    i removed my decat pipe drilled it and welded a bung on it so i can have original O2 sensor fitted and supplying ecu with signal and have afr gauge fitted just reporting the afr value and not using the narrow band output from it. fitted the decat.

    i have had 3 different standard O2 sensors on my car today and all of them have a switching speed of 500ms (0.5 Sec) and all of them have the same fuel trim . can all of them be lazy/faulty or is evo scan just reporting the switching speed incorrectly.

    i had a faulty MTX-L AFR gauge so i used the sensor from my spare set and it proved that it was just the sensor that was faulty so i can order a new one for about £45 and have a spare arf gauge. which is good.

    so i still have the fuel trim issues, but i have been out today and tonight to setup my LPG fueling. so i know that will work for tomorrow's Rolling road day. i will delve deeper into the fuel trims when i get back. to get a good tune it takes so long and a lot of thinking about. but i have a basic tune in the LPG so all is good.

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    What are your fuel trims while on lpg?

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    Quote Originally Posted by Davezj View Post
    So my question is why do the trims max out at idle. this maxing out at idle is a real issue, because once the fuel trims have maxed out they will not reduce when driving the car in closed loop operation. so the car is running very rich when pootling about road town which wastes so much more fuel than i actually need to waste by driving a VR4.
    I did my checking and reading and I can add to what you wrote here.

    The fuel trims CAN and WILL go down during closed-loop driving if the conditions are right. I saw it happen during my logged driving (cruising) sessions. They went down for a while and went back up again (the Low- trim did).

    It does not impact your fuel consumption in all areas though. Thats what the Low- Mid- and High trims are for. Which one is used depends on the current ECU Load and/or airflow. I can see my VR-4 toggling the InUse Trim between 0 and saturation depending on the driving conditions.

    Granted, during cruising like an old lady it mostly looks at the Low- trim, and it's saturated.

    My VR-4 does not seem to be suffering from a boost-leak or a faulty O2 sensor. For me the fuel trims get much worse if I rev the engine at idle to 2000-3000 rpm. The Low- trim stays saturated, but the STFT goes wild and jumps to 3-6% on top of the Low- trim. That would indicate an issue with the fueling system (pump, filter, injectors), MAF or exhaust restriction. If the trims got better while revving it would indicate a boost leak issue.

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    Davezj's Avatar

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    yes this is sound about right as to the background reading i have been doing.
    my initial thought was vacuum leak, but i went over my pipework very carefully, and fixed every little leak i could find. yes there is a possibility there could be a slight leak somewhere but at 20psi pressure i could not find any leaks, so at normal vacumm levels nothing will get in. unless there is an issue with the brake boost vacuum reservoir.

    Oh i have just had a thought, the Positive crank case valve is letting in extra air in vacuum conditions to scavenge the oil vapour and blow by gasses back into the engine to burn to help the emission.
    i can test this be blocking the PCV so no additional air can get into the engine and retest the fuel trims.

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    Seems that this was discussed a couple of years ago with some conclusions about injectors.
    http://www.clubvr4.com/forum/showthr...ghlight=ID1000

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    I have not tried the PCV Blocking yet I will do it tonight I still think this will be the cause of the lean fuelling at idle which is why the fuel trim try and add loads of extra fuel. Which seems like the logical explanation to me.

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