Faulty Lambda Sensor - Could Somebody Confirm

[martyn]

Hi,

My car has been running rich and fouling up its spark plugs pretty quickly.  I suspect a failed lambda sensor or an issue with the lambda circuit wiring.  I now have an Easimap data log that I think confirms one of these possibilities.

The reason I believe there is a fault is because the ECU is seeing between 2 - 3 volts from the lambda sensor.  My understanding is that this should be between 0 - 1 volts.  Because of this the ECU is treating that as out of range and not entering closed loop.  The lambda reading stays static at 0.87 and doesn't oscillate.  In addition to this, "Oxygen 1 Sensor" appears as a fault in the ECU and the lambda status reads "Bad AFR Reading".

Anyway here is the graph: 

https://www.dropbox.com/s/7q4biqggh8uu7la/graph.bmp?dl=0

We can see the lambda voltage (burgundy red trace) starting at ~3.1v and gradually reducing to ~2.75v

The lambda (pink trace) doesn't change and stays static at 0.87.  Which is also the value it defaults to during the warmup timer.

At 2:30 I switch the engine off and at that point we briefly see some response.  The green trace (difficult to see) maps the lambda status.  All throughout it has been at the bottom, which means "Bad AFR Reading".  It quickly jumps to the top: "All Conditions OK" then settles back to "Waiting for warmup timer"

What do people read into this?  I'll swap the lambda sensor over as soon as I figure out where to buy the right one.  However, is this more likely to be a wiring issue?

Thanks.

Edit:  Engine is a Sigma 150

 

[martyn]

I've just had a poke about with the multimeter, but to be honest I dont really know what I'm looking for.  Should there be a voltage going to the lambda sensor?  I have removed the sensor and I'm probing the connector on the car.  I've got 12v on one pair of wires, which is the heating element so that's expected.  The other pair of wires has 3.45v on it, is that normal?  My understanding is that the lambda sensor generates the voltage on its signal wire, so I'm confused about why there is a voltage there already.

[ChrisC]

Martyn I am going to do the same trace for you (fingers crossed) this weekend.   Its a shame your not closer because my catbypass pipes lambda sensor was removed because the pipe was polished, and currently I have the cat fitted for MOT (which has its own lambda sensor) so its just sitting there not doing anything. 

Once I have my trace you should be able to compair, and give you a definitive answer.   One thing I know is Bad AFR, does not 100% mean lambda sensor.  It just means the ECU can't keep the Air Fuel Ratio in check within given parameters, this could be because the signal coming from the lambda sensor is wrong, and thereore the ECU is not seeing the AFR correctly.   That said the signal coming from the lambda sensor looks wrong to me, I would expect some variation in the voltage, and from (my bad) memory mine drifts around 0.98v to 1.01v. 

[ChrisC]

OK maybe I am wong. 

Have a read of this 

http://www.lambdasensor.com/main/mtesting.htm 

The sensor can be tested for output by connecting to it a digital voltmeter, set to a 2v scale. Connect the +ve test lead to the sensor signal wire [see wire colours] and the -ve test lead to earth/battery -ve. The voltage produced should swing from app 0.2v to 0.8v and back around once per second. If the voltage is stuck high then the engine is probably running rich for some other reason. If stuck low then usually either a faulty Lambda Sensor, air leak or an engine running very weak for some reason.

This and ths backs up your thinking, that the engine is rich for some reason. 

Chris. 

[martyn]

I'm sure this is either a wiring problem or broken lambda sensor.  The high voltage and "Bad AFR Reading" appear even before the engine is started.  This, I think, rules out being rich for some other reason.

I am concerned about the 3.45v already present on the lambda signal wire even with no sensor plugged in.  I really need to know if this is normal or not.  I have bought a new lambda sensor, but I don’t want to plug it in yet because I have read that passing voltage through them kills them.

[Smiffy69]

Bijou hi-jack ......... where do you get a cable from for car to laptop for easymap?  or even a circuit diagram to build one ?

[martyn]

For a sigma engine, you need MBE-MAP-KIT-3-CAN

Not sure about duratec but it is probably the same if it runs an MBE ECU.

[ChrisC]

There is still that possible earth wire I talked about.  Should be simple to check with the multi meter for continuity.

[martyn]

Ok, I took the plunge and plugged in the new sensor and everything works as expected!  I just hope there isn't actually a wiring fault that will fry this new sensor.

So its official, my car did pass its MOT emmisions test without any closed-loop control of fueling.  In fact the mechanic at Millwood said it was some of the lowest results he's seen!

Problem resolved! 

[martyn]

For reference, here is the new Easimap data log showing correct closed-loop control:

https://www.dropbox.com/s/zeikalioyowkqi6/New%20Lambda%20Sensor.bmp?dl=0

It interesting that the lambds status (shown in green) flicks to "No Sensor" a couple of time very briefly. I wont worry about this too much for now as I've already spent too much time on it.

[aerobod - near CYYC]

MBE-MAP-KIT-3-CAN is also the correct cable and adapter for Duratecs (as well as Sigmas) using the MBE 992 or any other MBE ECU with CANbus.