First things first, I've tried to research this before posting so just looking for some reassurance at this point :)
My fuel consumption has seemed quite high since I got my 2 so plugged in an OBD2 sensor and have spent some time monitoring the situation. I've got a read out below based on a 15/20 minute drive - black line is the post cat (1x2) sensor and anything in the red box is above 1v. Presumably I need a new O2 sensor?
You could try the old switcharoo trick,
Swap the sensors round and re check to see if fault moves.
However, this comes with a huge caveat.
By disturbing them, you are likely to end with two sensors that need replacing.
Merely looking at one the wrong way can cause it to spit its dummy out.
It's tempting though I've heard the 2 sensors on the manifold are shorter than after the cat so I don't know if it'd be possible? I'm thinking at this stage it's worth a punt on a new sensor. I think it was Shnazzle that said in a previous thread that if it's not the sensor I'm likely to need one at some point anyway so it wouldn't be a waste given the sensitivity of the sensors.
It'd be good if someone could confirm that those spikes in the post-cat sensor are abnormal i.e. you're not supposed to see above 1v? Again from what I've found out sensors shouldn't really go above 0.9/1.0 volts so the main 2 are spot on.
EDIT: Adding a second graph which shows the readings much more clearly.
Apologies, auto assumed talikng about top sensors rather than reading post.
Quote from: MrChris on June 15, 2018, 23:31
It's tempting though I've heard the 2 sensors on the manifold are shorter than after the cat so I don't know if it'd be possible? I'm thinking at this stage it's worth a punt on a new sensor. I think it was Shnazzle that said in a previous thread that if it's not the sensor I'm likely to need one at some point anyway so it wouldn't be a waste given the sensitivity of the sensors.
It'd be good if someone could confirm that those spikes in the post-cat sensor are abnormal i.e. you're not supposed to see above 1v? Again from what I've found out sensors shouldn't really go above 0.9/1.0 volts so the main 2 are spot on.
EDIT: Adding a second graph which shows the readings much more clearly.
Your research is correct. Swapping with other sensors is, also, not an option.
Those high spikes do seem a little odd. I've heard of failed O2s sitting at 1.2ish volts before, but they'd sit there constantly on a pooped sensor.
Most of your printout shows fairly healthy flipping in an expected range. So, perhaps your O2 is throwing the odd wobbler but not yet deceased?
That said, the post-cat sensor doesn't figure in your fuelling equation, so as long as your manifold O2s are doing their thing OK you may as well leave it be.
If you start to get CELs about cat efficiency or frazzled heater circuits in the post-cat sensor, that'd be a good time to look at it again. Until then, I'd suggest getting out there and enjoying the weather! ;)
Thanks for the responses. So the readings are abnormal and it doesn't affect fuelling. The only thing is that during my research i found comments saying it does affect fuel consumption... :-\
Any definitive answer on this? I'm not trying to save money necessarily as i just want the car to be at its best :)
The mr-s ( jap spec) does not even have that sensor.
Top 2 provide fueling info.
Air filter clean?
Maf clean?
All intake plumbing leak free?
Pipe to throttle secure?
Given my known and well-mocked obsession with fuel trims I thought I'd chime in.
Toyota were one of the first manufacturers to employ a two-stage fuel control strategy to further manage emissions.
The conditions in which the manifold o2 sensors live (and the speed at which they have to read) causes inaccuracies which are ever so minute but enough to alter fueling off stoichiometric.
The post-cat sensor was introduced when they went ULEV (ultra low emissions vehicle) as an additional 2nd stage in the control loop to "fix" the inaccuracies.
So, yes, the post-cat does get used for fuel management to a minute level.
The MR-S is not ULEV so does not.
(https://uploads.tapatalk-cdn.com/20180617/6258dc0eb97575d4a8e89e3100182cc8.jpg)
Also, the heater circuit which we like to ignore:
The o2 sensor is absolutely dependent on thermodynamic balance. In order to generate the voltage we see, it uses a mathematical formula which uses temperature as one of the main variables. This is why it has a heater
"it only applies when the engine is cold" you say.
Yes indeed.
BUT.
The fueling adjustment strategy Toyota uses is one where if within its defined cycles it sees a lean/rich average reading, it calculates an amount and applies it globally.
You can see this yourself. Reset the ecu and watch long term fuel trims as you explore various rev ranges. You'll notice that at first, when you see LTFT go (or down) and if you rev down or up, that LTFT will also be there. After a while it starts to fine-tune on a multi-layered table. But, it always uses this blanketing as part of it. So that if say your o2 sensor degrades, it applies the correction globally.
So if the heater is bust, it spends the entire time when it's cold adjusting LTFT accordingly. And then when it's hot, it no longer makes sense and it has to go and undo it all
So, I think what I'm trying to say is;
- The ECU is very very good at maintaining Lambda 1.0 using the crappy (but cheaper) narrowband o2 sensors
- the ECU will compensate for sensor degradation but only up to a point. It can only apply logic/math to what it reads. If the readings are bogus enough, your fueling will be wrong.
- the car will run absolutely fine due to the above, but it won't be optimal.
I can believe that happens after a reset, but surely once the LTFTs are set it doesn't mess about adjusting them while the O2 sensors are cold?
Quote from: lamcote on June 17, 2018, 09:15
I can believe that happens after a reset, but surely once the LTFTs are set it doesn't mess about adjusting them while the O2 sensors are cold?
How does the car know when the o2s are up to temp?
There's no EGT sensor :)
It uses coolant temp to determine whether to go open or closed loop, but that's it. Crude.
So yes there is an overlap between coolant being hot enough for open loop and EGTs being too low for proper o2 function. Especially on aged sensors
I had just assumed EGT would rise faster than coolant temperature, maybe not with old sensors?
Quote from: lamcote on June 17, 2018, 09:51
I had just assumed EGT would rise faster than coolant temperature, maybe not with old sensors?
That's the key. On fresh sensors, the corrections from the post-cat are next to nothing or nothing at all.
So... wow... !
I did 30ish miles today and used about 2 bars on the fuel gauge - obviously not very scientific but seems a like high fuel consumption to me!
Here's what I've done so far:
- Cleaned MAF
- New air filter last month
- Intake seems fine? Not really sure what to check on this if I'm honest.
- Spirited drive
Professor Shnazzle, your lecture with slides was very impressive and I now know what stoichiometry is after googling it :D. What you appear to be saying is that the post-cat O2 affects the fuelling (but only a little bit) and the car could be compensating for the main 2 sensors and these could need replacing anyway if they're old...? Bit of a blow if I'm going to need all 3 doing but not the end of the world either I suppose.
The car has done just under 50,000 miles by the way if that helps at all?
Got to say, I've learned so much about cars since owning the 2. Learned how to service it, change brake pads, paint calipers, rust treatment. Love it!
Glad you're loving it :)
Actually I think, if anything, if you're seeing a big drop in fuel economy it's unlikely to be the o2 sensors. Especially with 50k on them.
Spark plugs, fresh oil, air filter and maf are bigger causes.
You've done the last 2.
Spark plugs are cheap as chips and a 15 min job tops.
Fresh oil is not to be underestimated either.
You could see whether the car is overfueling by getting a cheap obd2 dongle. Hell, I can send you one for postage that you can use on an android phone.
Done all of those too when i did the air filter. Got an obd2 reader already but thank you for the offer though :)
So i guess i need to look at fuel trim next. Not really sure what I'm looking at though. Is it worth posting another graph as the feedback so far has been great.
Quote from: MrChris on June 17, 2018, 17:37
Done all of those too when i did the air filter. Got an obd2 reader already but thank you for the offer though :)
So i guess i need to look at fuel trim next. Not really sure what I'm looking at though. Is it worth posting another graph as the feedback so far has been great.
Load up the long term fuel trims and if they're much further than -10 of +10 at the very most, something is amiss.
It turns out I was logging LTFT on Bank 1 and 2 in the CSV file I exported from the Torque app. Stuck a graph on the data over time and... nothing appears to be wrong?
At this point I'm wondering if it's worth just buying the post-cat O2 anyway...?
Alternatively, could my cat itself be playing up? :-\
EDIT: Re-reading the thread, Carolyn's comment about my research being correct is leading me to think I should just get on with it (new o2 sensor)!
One thing we are not into on here, is spending unnecessarily.
Based on your comment of how you are measuring fuel. Bars on guage.
Perphaps worth doing an ctual measure. Let the fuel light come on. Put 10 15 litres in. Take milage. Let light come on and do the maths. At least you will have a proper mpg.
Just for ref, fuel goes down quicker below half on dial than above.
This was really useful. What else can impact fuel trims? Throttle position sensor?
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Quote from: Smcknighty on June 18, 2018, 16:23
This was really useful. What else can impact fuel trims? Throttle position sensor?
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Nope. You can drive with it disconnected. Throttle response will just be pants :)
Fuel trims are impacted by an almost endless list:
O2 sensors
MAF sensor
Air filter
Spark plugs
Intake leaks
Exhaust leaks
Cat flow (blockage or lack of)
Exhaust Manifold
Coolant temp
Air temp
Barometric pressure
Throttle position (actual, not sensed)
IACV
That's a small list of things that can impact fuel trims. But it's calculated off a much smaller list:
MAF and o2 sensors.
Basically it reads airflow and air temp, calculates load on the engine, adds fuel, and add timing, then checks with the o2 sensors whether it got it right. If not, trim up or down.
That makes sense and explains why mine are so far out. Very little of that is normal on my car
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