Lean combustion?

[Chas54]

Ive just had my Elan MOT done and passed Ok but on the way back i could feel a bit of a stutter when reducing speed from say 50mph to 30 mph to pass through rural villages. The engine fault light came on and is a 4.4 when read off the light in service mode, This indicates a lean exhaust at the O2 sensor.
This is a precat 1991 now converted to closed loop .
The elanscan data shows that at the time the lean O2 sensor error occured (850 secs) , the Stochiometric dropped from 14:7 (pretty much as good as it gets) to 14.0 ( which would be richer?)

The IAC is static at the time although the manifold pressure increases a little but this could be a change in throttle position.

Once back up to speed the fault generally clears after a short period as the light goes out on the dashboard .

The petrol is fresh E5 from Tesco .
The fuel reg valve is relatively new
The timing is spot on
TPS is 40 at idle.
I am hopefully pasting an elanscan file here so if anyone has any insights they would be welcome....... no i'm not , cant remember how to paste an ecu file here

charlie

[Chas54]

My elanscan trace may be here as a zip

back from louth vvg louth vvg.zip

file

[Tuga2112]

i would have expected the TPS (throtle position sensor) to read something very low (near zero) at idle. are we talking about the same data ?

[chrism]

Chas54, like Tuga said I'd expect TPS to be more like 0.35V at idle so not sure where your figure of 40 comes from.

Static scans from cold make more sense to me since the sensors can be seen affecting each other more easily than a 'hot run' and also your idle TPS might be available to verify...

I've overlapped your coolant temp with a/f ratio and noticed that your thermostat seems to be opening around 60 - 70C instead of the more usual 82C and there's a largish dip in your a/f ratio at this point. You'll see that later on, around 800s when at 90ishC, all sorts of things start to happen:-



...coolant temp falls and a/f ratio has a 'hissy fit'...

I think I'd be inclined to suspect or replace the t/stat first and post a (say 10min) static warm-up EScan so's we could then look at the temp sensor and O2 sensor contributions. Another thought; I'm wondering whether there's some kind of a vacuum leak when very warm?

I'd certainly be interested to see what others think?

[Chas54]

Sorry about that, it should be 0.4 volts at idle .

I hadn't spotted the erratic coolant temperature temperature but a few weeks ago i thought it was running too cool and put a cardboard blank over part of the radiator grill and it all felt better. So i took the thermostat out and found a split around the centre of the diaphragm .. I replaced it with a new one at 88c i think , ill have to check that. But i haven't taken the cardboard blank off yet (forgot all about it).

I'll have to do a dataset from cold tomorrow , but i've tried and i don't think the fault shows up until the cars moving .

Only other thing now is it's reluctant to idle initially when cold , could be the learned memory wiped when i disconnected to clear fault codes . I think it should reset itself at 30ish.

Thanks for quick responses though.

[Chas54]

I have to apologise as i have posted the wrong elanscan dataset .. I think the data is from before i changed the thermostat.

I will post a correct set as soon as i find it

sorry

[Chas54]

Sorry about earlier foul up.

I have two elanscan data sets
One before the MOT whilst car at idle from cold . All looks good. This is with the new thermostat but my forgotten cardboard radiator blank in place.

The second is a short run after the MOT at at around 970 seconds a code 44 O2 sensor lean fault is recorded . air fuel ratio drops to 13.9 . I haven't spotted anything else happening yet.
I can feel it just before the ECU light comes on.

Any suggestions welcome,

regards
Charlie

8th august 26 pre mot fine.zip

faults aftr mot 2026.zip

[chrism]

Chas54 I'm not ignoring you but:-

1. imo your before MoT idle isn't 'okay' - eg look again at O2 sensor data. (Your t/stat seems good tho'.)
2. I don't mean to be disrespectful but you've included data eg the short run which, to me, doesn't seem to point towards 'code 44 O2 sensor 13.9 a/f ratio' and in my view can't be connected to the warm-up data.

Sorry but I can't 'see' anything...perhaps others LEC-ers can? Is it me or..?

[KenS]

Interesting plots! I agree with Chris that the Pre MOT plot does show something odd with the O2 sensor voltage, the sensor starts up OK around 50sec but dies at 90 sec and recovers again around 450sec. Plotting the 'Closed Loop' flag indicates that it was still in closed loop throughout.

Pre MOT O2-Closedloop.jpg

Then looking at the After MOT plots, again the O2 Sensor data is erratic and drops out, and it drops out of closed loop. (but there are points where the engine is restarted)

afterMOT O2 Closedloop.jpg

Then comparing the Closed/Open loop periods against the Fuel/air ratio, while in closed loop it looks to be lean, when in Open loop near normal or rich.

afterMOT air-fuel.jpg

So question is if its simply an intermittant O2 Sensor or connections to it or is it correctly reporting fueling issues and defaulting periodically to open loop?
My guess its the O2 sensor since when in open loop it can supply more than enough fuel, but not confident enough to say go buy a new O2 Sensor!

It would be interesting to try to get plot while in Service Mode since I think that would remove the O2 sensor from the equation. But dependant on your ElanScan hardware, I don't think its easy to apply the service mode link while the ElanScan interface plugged in.

Any thoughts Chris? Any other members spot anything hiding in the plots?

[chrism]

Erm, I reckon Ken's right that your O2 sensor might be struggling - and in the first instance I'd do another EScan idling from cold for ~10 min for us to see. Your original O2 scan showed an intermittency (black) and I've overlapped my own (healthy) scan in blue:



If your next O2 scan showed 'struggles' I'd substitute your O2 sensor. I've got a spare that you can borrow - paying homage to Ken's reluctance and careful advice about buying a new one. (Perhaps a pal closer to home has one on the shelf?)

I've tested my own O2 sensor in the dim and distant past ( - and I'd be happy to test yours?)

The lhs slate/green lead is the signal lead back to the ecu and enters the rhs O2 sensor as the black lead in the pic.



The lhs brown/yellow is the heater supply. It only gives a supply when the engine is running or ticking over and the red ign light is OFF. It enters the sensor as the bottom white lead in the pic.

The slightly hidden lhs black (under the slate/green) is the heater 'return', which is an earth, and enters the sensor as another white lead on the rhs of the pic.

I'd check the two whites of the (disconnected rhs) m/plug with the resistance side of your m/meter to see if the heater supply circuit is intact. You should get a reading between ~3ohms and ~15ohms or so if the sensor isn't broken. It's a kind of 'continuity check'.

I'm struggling to remember but I seem to think I checked the supply voltage with the sensor disconnected from the multiplug. The engine should be ticking over with the red ign light OFF. I set the m/meter to read 20Vmax and probed the lhs m/plug part between the brown/yellow supply and a convenient earth - the braid at the bonnet hinge - looking for a (12V-ish) voltage to supply the O2 sensor. I think I also checked that the black sensor m/plug earth really was an earth by connecting it to the bonnet braid and looking for continuity when bolted into place - I think the turbo might provide a path to earth.

Other random thoughts:- the brown/yellow O2 heater supply 'feeds' off the red ignition supply light on the dashboard. Has this been flickering at all giving you an interrupted O2 heater feed(?) - cos this might be a cause of your O2 drop-out. I'm thinking "is the alternator rectifier providing enough 'oomph' to supply the heater current?"

I think, too, that heat from the exhaust gases can restart signals from a broken O2 heater circuit supporting/explaining Ken's open/closed intermittency?

Hmm...is all this something to pick the bones out of, I wonder? Or does it add confusion?