O2 Confused

mpg on my 2001 P38 4.0 petrol is down 20%. Smells to be running rich? Wondered if it could be to do with 02 sensors...I have a Faultmate but dont understand live readings.
Lamda A and Lamda B Output both hover either side and close to 1
Upstream Shunt Voltage is constant at 0.11
Upstream O2 Heater Voltage is constans at 11.7
Upstream 02 Sensor Bank A and B both fluctuate between 0 and 0.88 with no obvious link to drive conditions
Upstream O2 Sensor Max is constant at Zero on both banks
Upstream O2 Sensor Min is constant at Zero on both banks

Help on what these readings mean would be appreciated.
Which side is Bank A.

Thanks,
Steve

think Lamda reading 1 is rich.
Not sure about MIN MAX

Thanks,
This subject is really hard to find good info.

Hi guys,
Still struggling to work this out.
Any info greatly appreciated.

Do you have RAVE?

Part of the RAVE is the Technical Brochure....a section on the EOBD system....

There is some information in there about the Lambda sensors and how they work and what values mean what....

I did try to PDF it and attach, but the attachment file size is to big....

Get a copy of RAVE (if you don't already - do a search on here loads of links to it no doubt) and have a read....

On my copy it is pages 36 to 45....

Would it be too simple to assume that if the sensor says your car is running too rich and your mpg has dropped, that it's just possible....... you have a vacuum leak ??

Thanks Saint,
I have looked at Rave but I cant find what the live reading mean that I got from faultmate. I will check the pages you refer to to see if that helps.

Hi Bromsgrove,
It is my suspicion that it is running rich, strong smell of fuel at exhaust (no leaks) rather than faultmate telling me it is rich.
The thing is I dont understand what the Faultmate readings mean as I do not have such experience. First task is to understand the readings I posted. I want to avoid fitting parts just to see...
Thanks,
Steve

Steve Ashworth said:

Hi Bromsgrove,
It is my suspicion that it is running rich, strong smell of fuel at exhaust (no leaks) rather than faultmate telling me it is rich.
The thing is I dont understand what the Faultmate readings mean as I do not have such experience. First task is to understand the readings I posted. I want to avoid fitting parts just to see...
Thanks,
Steve

Click to expand...

Hello Steve,

Never ever fit parts just to see.... what I would do however is have a look at the pipes to see if I had a slight split, cos I think you'll find your faultmate wont tell you that. Oddly enough it can only tell you what the sensors...... sense.

Does that make sense ??

Just re-read the RAVE info...it tells you how the system works and what different sensor voltages etc are or mean....

But it doesn't answer you Q above....so just ignore my answer really...unless you wanna know how the Lambda system works..!!

Sorry..!!!

Thanks,
I will have a look at the pipes for anything obvious...

I understand why the O2 sensors are there but I dont understand what all the readings should be in any given situation.
Cheers,
Steve

Did not find any leaks on the pipes etc.

Steve Ashworth said:

Did not find any leaks on the pipes etc.

Click to expand...

Hmmm heard that before... How hard did you look cos the little splits are hard to spot. Especially if you haven't taken em off to look.

I take it you still haven't found anything that helps understand the readings you've got?

Just read it in rave.
Listen to the professor.

Looked carefully but did not take apart. Tried spraying where poss with easy start and then water but no change in running detected either way.

Can not find answer to my question in Rave so still uncertain.

If you've got a good quality multimeter that doesn't pull too many microamps you can read the Lambda sensor(s) directly. At the stoichiometric af ratio (14.7:1) the sensor will provide 0.45 volts these days most cars run slightly weaker than that. Higher than .45 (to 1V max) means rich, weak < 0.45V, 0.1V being about the minimum you'll see unless the sensor is duff.

PS For best accuracy use the negative wire on the sensor for your multimeter neg rather than the chassis or sensor housing.

StuckAgainSteve's explaination is correct, but is based on an engine running a 'pure' fuel, Petrol with all its contaminents and additives would give a slightly different Air to Fuel ratio as petrol is not 100% petrol, but a cocktail of additives and not to forget that petrol goes 'stale' over time.

UK Unleaded Petrol at its prime will start to go 'off' after about 5-6 days, when it will lose some of its volatility, in about 2 weeks, it is pretty naff and in a month its total crap...will still run the car, but performance will be down, economy will suffer and running won't be quite as sweet.

Good luck with it....follw Steves advise as it will give a ruddy good place to start.