White spark plugs/ running lean, strange fuel trim results - Toyota yaris 2002.

Hello all,   first post as I have been trying to find a decent forum  where people have great knowledge  and looks like I have found it !

 I made a quick service on the wife's Toyota yaris  1.0 4cyl  car.   In usa it was called vital.  Engine code is 1SZ-FE. 

I noticed the spark plug  ground electrode colour was white. 

checked the vac system and didn't find any leaks, also used carb cleaner  with no change in idle.

I used a Bluetooth OBD adaptor and an app called  "car scanner" to read the fuel trims as live data. At idle STFT  varies between +1.8% to negative -5%,this fluctuation is pretty steady and rhythmic. .   LTFT stays steady at negative -4%.

On the highway at 100kmh /60mph top gear approx 3200rpm  the fuel trims do something strange   STFT  goes between negative 16% to negative 20%, also cyclic but the LTFT is constant at positive 19.53%.

From here on I'm lost  and no idea!!

I looked at the IAT and it's reading 6C / 42F higher when on the highway and in city its 15C / 60F higher, thats a bit suspicious  for me  but it is in the airfilter mounted on the  rocker cover so a hot area.

Any ideas would be greatly appreciated!!

Thank you.

Rob


EDIT:    due to the wildly different readings between the fuel trims  I'm suspecting  one of the oxygen sensors...?  
 

Hey Rob, thanks for joining.
Is there any drivability issue with the car or trouble codes popping up?
Whitish spark plugs isn't really enough for me to want to dive into a fueling issue without some kind of code or other problem.
The trims you posted sound pretty normal. When combined, ideally the trims should equal less than +/- 10%.
So even when you're short term goes -16, it's countering the long term of +19 making for a total trim of +3%.
Another thing to note is loop status.
Your scanner may call it fuel_sys1, or something similar but it will change from open to closed loop during different driving conditions.
My point is, fuel trims mean nothing in open loop. So if you're moving up the highway then let off the go pedal,(fuel cut deceleration),the ECM will enter Open loop, making the fuel trims values null.
Hope that makes sense.

It's been a long time since I've seen the yaris so refresh my memory . Does this have a MAF sensor? MAP sensor? 02s are unlikely since you have different numbers at idle vs highway speed but can easily be tested by graphing the 02s and fuel trims while creating a vacuum leak for 10 seconds,then either adding a fuel source or simply closing the vacuum leak will cause the sensors to change (sensors will go lean initially then fuel trims will catch up to the vacuum leak, then sensors will go rich when you add fuel or plug leak until fuel trims catch up to the change. The idle fuel trims look fine (within 10% is considered normal operation of a good motor) the highway driving is definitely lean and suggests this is not a vacuum leak problem but either a air or fuel metering issue (usual suspect is a dirty MAF sensor if equipped, low fuel pressure, dirty fuel injectors)

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Reading Noah's message I see that long and short countered each other at highway in which case Noah is correct fuel trims are not pointing towards a problem

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Spark plug colour isn't a reliable indicator of a lean mixture any more --- in the old days it was but tight mixture control and changes to fuel composition have changed that.
The rule of thumb is average short term fuel trim + long term  should be with 10% of  zero before any action is needed.

IAT is always higher at idle, less air is being sucked  in so the air in the inlet trunking air filter and manifold  moves more slowly and has a longer time exposed to engine bay heat.  Also when the vehicle is moving the engine bay temperature drops due increased air flowing through the radiator and engine bay.