Poor fuel economy-lambda sensor?

[malc4]

Hello All,
I'm trying to get to the bottom of my poor fuel economy. I'm aiming for around 50mpg like my neighbour with same car/engine (C4 GP DV6C), but I'm about 15-20% short of this. Have done a full service, replaced EGR valve (a long story) but no significant improvement.

I recorded some live data from a drive today and noticed the equivalence ratio lambda (B1-S1) which seems to be around 1.05 at idle (is this too lean?) maxed out at 2.0 for a while as if stuck. Also just had 2 at idle. O2 voltage seems to range from about 2V to 3+V. (too high?)
Any thoughts or advice for further investigation welcome.
Thanks,
Malcolm

[RichardW]

Since it's a diesel it runs lean all the time - I'm not really sure why these have a Lambda sensor, it's something to do with the regen of the DPF. You would need to drive very conservatively to get 50 mpg out of these - I don't even get that from my 3008 with the same engine! It's possible the lambda sensor is used to control the air doser during regen and this could be stuck in a certain position affecting the air flow all the time, but I would have expected this to report a fault code. Intake could be partially blocked - how gummed up was the EGR valve?

[PaulC5]

40 to 45 mpg does not seem too bad, what type of driving/trips are you doing ? Have the brakes been serviced so not binding ? Have you reset the trip counter by pressing on the end of the wiper stalk, then do a long run without stop/start driving and see what you get. They do not go above 9999km (6214 miles) so if at this distance do not trust any values. We've had a few C4s (not Picasso) with the 1.6 hdi and each newer model gave improved fuel economy so if your car is an older one then maybe it will not be that good and tyres make a difference.

[malc4]

Another friends car (308sw) also gets>50mpg- both drive less conservatively than me i reckon. Something's not right and the 02 sensor looks like the prime suspect atm with those readings? Should be an error code with above high readings tho?

[malc4]

Could it be an exhaust leak that causes air/fuel sensor to get stuck lean?

[RichardW]

No, as above, diesel is not mixture controlled like a petrol engine.

You might have a slightly duff injector, or the MAF reading might be off, or the DPF might be a bit more worn, or your tyres might be less efficient, or a brake could be dragging, or, or, or.... You would need some fairly advanced diagnostics and a reference engine to check the details, but TBH I doubt you will find anything and that's just the way it is.
Our 1.6 16V only did 40-45 generally, and our Blue HDi 120 only gets the same around town or country driving - if you keep the speed down to about 50mph on a longer cross country run it will get nearer 50 mpg.

[malc4]

Thanks Richard. Yes I've been trawling the forums and think I've eliminated the MAF and DPF sensors with my OBD dongle (within normal range, increase with revs).
The lambda ratio sticking at 2 can't be normal tho?. Behaving rather like the EGR valve, that turned out to be a connector issue (have noticed some oil residue near the O2 sensor connector.. hmm)

edit sesnor is wideband so above voltages are normal? sensor current ranges from 0.9 to~4mA (lean to very lean?)

instantaneous mpg seems to drop down to 30's with any hint of throttle in highest gears..

[PaulC5]

Maybe you could swap cars with your friends and see what you get driving as you do in your car. How does your mpg compare with your previous car which might be a good long term comparison ?
Instantaneous mpg when using the throttle dropping a lot is quite normal.

[malc4]

Think I'll try same run with sensor disconnected. I suspect anomalous lambda only appearing when temperature gets operational.
here are some more readings:
cold idle lamda(B1-S1) =1.05, 0.13mA, 2V (should be 3+V for wideband sensor?)
erratic/warmed up: lambda=1.34, 0.9mA, 2.3V; lambda=2.0, 2.7-4mA, 2.6-3+V

[malc4]

Yes, it appears that lambda goes haywire approx 2-3 minutes after startup.
Also noticed the coolant temp was 6F below ambient at startup..
Surely some solutions now?;)

[PaulC5]

The coolant temp sensor could be faulty or its wiring/connection. Watch its live data as it warms up, should be ambient (around 15°C) at the first start and get as far as 90ish°C when warmed up and the thermostat opening with the fan coming on around 96°C. If you get another sensor just plug it in to the connector before fitting it to make sure it shows ambient and not a wiring/other fault.

[Rp0thejester]

After reading all that I'd say driver error. Do you accelerate between speeds bumps and then brake and then accelerate again between speed bumps? Also weight in the vehicle, correct tyre pressures can make a big difference.

[malc4]

Well, after disconnecting the lamda sensor I've managed to get 50mpg on a hilly local run (8km)- that's more like it...

[malc4]

The coolant temp sensor could be faulty or its wiring/connection. Watch its live data as it warms up, should be ambient (around 15°C) at the first start and get as far as 90ish°C when warmed up and the thermostat opening with the fan coming on around 96°C. If you get another sensor just plug it in to the connector before fitting it to make sure it shows ambient and not a wiring/other fault.

Thanks Paul. I noticed this morning the coolant temp was again several degress below ambient and jumping about by more than a degree. It does increase tho. Good tip re new sensor. Have recently replaced ambient sensor in wing mirror that was reading off (there was a break in the wire by the hinge- the old sensor was ok)

edit: also noticed the intake air temp was jumping around at idle 59F/60.8F. is this normal?

[PaulC5]

I think the intake air temp is measured in the MAF sensor. It should be at ambient before the first engine start but then tends to go up a bit due to heat from the engine. I would check the MAF air flow live data as well as its temperature in case the MAF or its connection and wiring need checking.

Jumping between 59 and 60.8°F is 15 to 16°C so it might be the temperature is 15.5 +/- 0.01°C and the value is being rounded up/down. Have another look but use centigrade to see if it makes any difference.

It could be the engine block is a bit cooler than ambient if it has not had time to warm up from the sun. If you have another means of checking the area around the coolant temp sensor such as an infra red thermometer it might help.