Long term fuel trim

[AlVal]

looking at data through obdII at the moment - on a 4 cylinder engine (Renault K4J Engine - 1.4 litre Megane II) , why does fuel trim short and long term show for banks 1 and 3? Doesn't an inline-4 engine only have one bank?!

Apologies for what I'm sure is a silly question

[blitzracing]

Generic OBD software misinterpreting the results?

[AlVal]

blitzracing said:

Generic OBD software misinterpreting the results?

hmm, it crossed my mind,

http://www.scantool.net/scanxl-pro.html

Awaiting delivery of a renault can-clip, meanwhile will try and find another 4 cylinder vehicle to test on, to see if they all do it

[AlVal]

blitzracing said:

Generic OBD software misinterpreting the results?

very good suggestion turned out quite accurate.

"Palmer ScanXL" "generic" obd software totally misinterpreted , "touchscan" seems to be processing the information a lot better, although like I say, I'm still waiting on the proper renault clip computer to come, the generic obdii information isn't quite enough for me.

Touchscan
PID values
0x06 short term fuel trim 25%
0x07 long term fuel trim 9.38%

The car runs fine, however, it sometimes displays the following symptom:

While driving along the engine sort of 'judders' like when you release your foot from the accelerator and press it again very fast.

This so far has happened while I was very lightly pressing the gas pedal and cruising along.

It's like a hiccup of sorts but doesn't feel like a misfire.

I'm thinking I might try replacing the MAP sensor - could this be contributing to the fuel trim issue?

[Pumaracing]

The fuel trims are there to calibrate the injector pulse durations from what the system thinks they should be in a perfect world with nominally specced components and what the 1st oxygen sensor tells the ecu it really requires to adjust for actual injector output, fuel pressure, MAF or MAP values, air leaks and any other thing which affects the A/F mixture.

The long term trim will change gradually over time as injectors wear, MAF wires become coated etc. Ideally it adjusts to try and keep short term trim below 10% or indeed as close to zero as possible. Normally a fault code will only trigger when the total of long and short term trims rises above about 35% to 40% depending on the system. Long term trim of only 9% is quite normal.

However your short term trim is high. If this is as a result of a new problem then gradually the long term trim will rise to reduce the short term one but of course this will not cure the fault, just mask it.

You may have a recently started air leak, low fuel pressure or some other fault which is causing the engine to not get enough fuel for what the MAF or MAP sensor readings are telling it but as long as the 1st oxygen sensor monitors all this properly it will try and maintain a stoichiometric A/F mixture and the engine should run fine.

If your stumble at cruise is intermittent it would not tend to indicate a steady fault. Maybe just an intermittent poor connection somewhere.

Start with the basic checks of mechanical health before assuming the worst about the electrickery.

http://www.pumaracing.co.uk/diagnose.htm

[anonymous-user]

Also, transient fuelling to correct for intake manifold puddle mass is triggered by the throttle position sensor, often, this potentiometer can wear and get "scratchy", which can result in transient fuelling events at funny times, leading to rich/lean AFR excursions and a matching drop in torque (felt as a flat spot).

Easy to check, get multimeter on a 5v setting onto the throttle pot (need to find the ground and signal wires (will be 5V, Signal, ground in some order or other) and carefully move the throttle shaft and check that the output voltage moves linearly with throttle shaft position. Any steps/jumps or dropsouts are bad!

Some systems also need the TPS to be within a certain "range" to sense idle and off idle threshold, if missadjusted this can also cause issues etc.

As mentioned above, i would not expect long or short trim to be above + or - 10% (the short trim is just a "snapshot" and you really need to log it and plot it over time against various loads to determine if it is no longer centered on zero or of too great a magnitude etc)

[Pumaracing]

Max_Torque said:

As mentioned above, i would not expect long or short trim to be above + or - 10%

Long term trim of 10% or even higher is quite normal which is why fault codes are only triggered at 35% or so. If the fuel pressure regulator has a manufacturing tolerance of +/- 10% that would change injected fuel mass by 5%. Add tolerances to injectors, MAF or MAP sensors, temperature sensors etc and 10% on the fuel trim stacks up easily even before things start to wear.

[anonymous-user]

Pumaracing said:

Max_Torque said:

As mentioned above, i would not expect long or short trim to be above + or - 10%

Long term trim of 10% or even higher is quite normal which is why fault codes are only triggered at 35% or so. If the fuel pressure regulator has a manufacturing tolerance of +/- 10% that would change injected fuel mass by 5%. Add tolerances to injectors, MAF or MAP sensors, temperature sensors etc and 10% on the fuel trim stacks up easily even before things start to wear.

normal?? not on my watch it ain't ;-) Come to me with a cal that shows a long term trim above 10% and i'll send you straight back to the dyno to do it again.......... I would expect a MAXIMUM of 10% on a fully worn/out of tollerance engine !

The "trigger" point for the OBD fault is really up to the manufacturer, as long as it occurs before the fuel shift causes an OBD emissions test fail. We tend to set it up quite "high", but generally between 20 and 30% to minimise false MIL events, as these days fuel systems (and airside sensors) actually tend to run fairly median, until they have a gross failure)