Poor fuel economy-lambda sensor?

[malc4]

Yes, although i was measuring in morning, the (black) car was in full sun, so coolant temp yet to heat up after previous night.
Another 50+mpg over a hill trip to scrapyard to get air/fuel sensor replacement. They look similar (covered in soot atm- awaiting carb cleaner), but noticed the old one's heating element? resistance was higher (600ohm vs 460ohm). Time will tell.. Still surprised no fault code from fuel trim going haywire..

[malc4]

Oh dear, replacement O2 sensor behaving same as old. Fuel trim (Lambda) starts off at 1.05 and after a few min (presumably the warmup time) it starts to go lower but then flips to 2.
Very annoying. Any ideas what to try now?

[malc4]

Interesting update- have noticed that maf sensor flow sometimes changes significantly at idle (eg 5g/s to 10g/s) but air/fuel ratio (and fuel economy?) remains low after o2 sensor warm up when maf disconnected, so maybe best short term solution? (have replaced maf sensor today and readings slightly lower but still unexplained jumps). hope this makes sense.

[RichardW]

Had some spare time, so plugged mine in (2013 3008 DV6C). From cold start about 1.05 and 3% heating, after a few mins idling it increased to about 5 and 40% heating. If revved it drops, then back at idle it goes to about 3 for a little while then goes back to 5. If you rev it then lift off it goes up to 8 (presumably the max to reflect fuel cut) then it drops back to 3. Just seen your post, didn't look at MAF readings....

[malc4]

by 5 you mean 1.05?;) yes mine drops a little after a few min for a few secs then shoots up to 2.0. can't seem to find short and long term trims on my scanner- could be useful. disconnecting the maf and the lambda ratio seems to keeps around 1.05 even after sensor warmup (open loop settings as with disconnecting the air/fuel sensor?).
i believe that the map sensor is used when no maf data, so disconnecting maf should be better than disconnecting O2 sensor?

[RichardW]

No 5, not 1.05. This is a diesel remember, there is no fuel trim, the air is mostly wide open and it runs lean. I found some info that it uses the lambda to help control smoke, but I'm not sure exactly what strategy it's using.

[PaulC5]

I suspect there is nothing wrong with your MAF and what you are seeing is normal.

On our C5 when first started the MAF is about 11 g/s then as it warms it drops to about 8 but at times will go up to about 10 all on idling rpm. When it does go up a quick press of the accelerator and it drops back to about 8. Possibly the egr valve is doing something. The rpm is fairly steady about 780 rpm all the time. With the engine off the MAF shows 4.9 g/s and also a new one gave this. Our other car gives 0 g/s with the engine off but a search on the C5 engine (2.2 hdi) suggests this happens on other cars and is not a fault.

[malc4]

think i've finally discovered a possible wire coding for my 5 pin NTK air fuel sensor. the only two terminals that were showing resistance (~500ohms) that i thought must be the heater are apparently the signal wire and and the unused terminal (looks to be connected to signal with some sort of fuse to earth inside connector?). The heater wires are given as blue (Vh+) and yellow (Vh-). Measuring resistance across these two gives open loop on the two (possibly faulty) sensors in my growing collection:)

So with engine on sensor disconnected, i'm getting these voltages at the harness connector plug (assuming wire coding correct):

1- no wire from sensor
2- white (lp+) signal wire/lambda voltage? : 2.6V decreasing to 2.3V after a few min.
3- yellow (Vh-) heater negative: ~4.3V
4- grey (Vs+) ?: 4.8V
5- blue (Vh+) heater positive: 14V
6- black (lp/Vs common ground): 2.3V/4.29V (two tests- need to recheck?!)

My armchair observations are that

signal voltage is higher than the initial voltage as recorded by OBD when sensor connected (2.1V in open loop rising to 3-4V in closed)
heater negative maybe normal as wideband sensors have pulsed heater voltages?. also heater circuit ok as a circuit lamp lights between 3 and 5
not sure what Vs+ is but assume it should be around 5V?
the high/varying common ground seems the most problematic though?

[malc4]

Backprobing the common ground also gives 2+V so need to check wire at ECU connector? will it have the same color or do i need to get a wiring diagram off givemeabreak?;) any other sources welcome (delphi BSM R02). haynes? also should all sensors have same common ground or are there different sensor grounds at the ecu?

[malc4]

Sorry for duplicate post- makes we wonder what it's for.

Anyway, have looked at grounds- there are two body grounds (high and low power connections to ecu?) one is very rusty so this is where i'll test next (initial test showed 10V which could explain 2V at sensor common ground?)