Possible Lambda sensor failure ?
 

Over the last few days I have been noticing on the Lambda display (on the SECS unit) that I've been getting lower than normal readings when crusing on the motorway.

Instead of reading 1.0 or .99 I consistantly now get .89 or thereabouts when doing around 50+ mph.

And when I apply a small amount of throttle I get the reading of .76 which I used to get only on full throttle.

So, I'm running rich.

Is this something as simple as fitting a new Lambda sensor as I've been reading that they can start failing at 50k miles or higher.

I've now done 65k miles.

Do you think this is the problem, or do you think it may be something more sinister.

Car is MY02 STI.

Thanks.

I'd expect the standard ECU to be in closed loop mode when you are cruising at 50. If it is then the lambda value should continually cycle up and down as it adjusts the fuelling.

For my car (MY95) the lambda output drops to 0 on overrun, cycles when cruising, and rises to a constant value (I can't remember the exact value but it's about 0.9) when accelerating hard.

It may be that you haven't travelled far enough for the sensor to get hot - it can take 5 minutes or more for it to start working, depending on how you are driving.

Matthew

What you read on the SECS and what the sensor actually does is two different things. And needs to be taken into account
A lambda sensor works like a switch, it is either on or off: it rapidly switches from 0 to 1 volts (well, about 200mV to 800mV) in the space of roughly 1 second.

What you see as the result on a digital voltmeter is an averaged voltage (avround 450 to 500mv). (or on your SECs is calculated readout of AFR )

What very often occurs with a failing lambda sensor is it starts to switch slower and slower, thus this affects the readings taken by the SECs. And the overall mixture.

As above though, it does take a few mins to start working, and tests should be performed after doing a fast idle of around 3000rpm for a few minutes with a hot engine, to assertain correct switching. However it could also indicate the heater circuit has failed (the sensor is preheated by a heater coil.)

ALi-B

OK, I didn't realise the SECS averaged the readings.

You are wrong about the sensor though, it doesn't work like a switch, it just has a steep voltage slope about the stoichiometric point, depending on the strength of the mixture. I'm monitoring the voltage value every few mS and logging the output so I can see exactly what is happening.

As far as I can remember stoich is 0.7v so marklemac is now running leaner than before (1.0v is rich). Closed loop running should average to about 0.7v as that's what it is trying to attain, but to get maximum power you need to run richer so under hard acceleration it should be higher (0.8v+).

Light throttle below about 4k revs is still likely to be closed loop so 0.7v is a good value.

Matthew

thanks guys.

I'm going to be 'trying' a new sensor next week so will report back as to what the outcome is.

In the meantime, I might try and log it with DD.

Not wrong........"LIKE a switch".....

http://www.nataz.pwp.blueyonder.co.uk/pics/osgraph.jpg


UNless you can think of a better layman's term ;) :Suspiciou (think of digital waveforms)

Averaged isn't the best of words, more of a different interpretation of live data compared to what is physically happening measured directly off the sensor with an oscilloscope (although I'd guess the live data reading of secs is at a slower sampling rate to what the ECU sees)

Actually this is not true, Marks car is a new age which has a wide band in the front headers, on these cars the reading will remain relatively continuous, he is also reading the air fuel ratio in lambda while you guys are relating things back to narrow band sensors and voltages, so Mark please this is not really helpful and will confuse the issue.

What he also needs to know is what is the closed loop short term fueling doing at this time, if his lambda reading is 0.89 on cruise then something has happened to overcome the ecu's ability to correct, his ecu has plus aor minus 25% closed loop adjustment in addition to plus or minus 15% long term closed loop trim adjustment, for that reading to remain means something is definately not right, however it could also be that the PSi 3 monitor is faulty.

Mark you need to get the car looked at either by a dealer who can use his Select monitor to check out several other readings or by someone with Deltadash who can do the same thing, as things are you could replace the lambda (which are very expensive) ad find its not that at all.

The SECS doesn't average the readings in this respect btw it shows real time output.

Bob

Thanks Bob.

I can plug DD in today and go for a run.

What readings should I 'tick' and record ?

Mark

Sorry for the duff info, I didn't know about the difference with the newage cars until Bob posted.

Matthew

Did some more driving today and noted:

Problem doens't occur until around 60mph (not 50 as previously stated) and continues up to around 90 where is seems to go back to 1.0 reading.

The .89 reading comes into play from 60-90

Plugged in DD tonight and went out and did some runs but when I parked up I saw that the laptop said it had been disconnected so ended up recording nothing ! (typical)

Bob, Triton are going to fit a new Lambda sensor for me next week to 'try before I buy :)' so hopefully we can get to the bottom of it then.

One more thing, where exactly is the Lambda sensor on the Newage ? Is it in the headers ?

Also, what purpose is the sensor in the downpipe ?

Hi, you needed to have ticked the air fuel correction and air fuel learning boxes, also maf voiltage and the rear lambda sensor. Make sure you get coolant temp, engine load and tumble valves (if wrx) otherwise cam angles if STi.

Good for Andy, he's one of the most helpful guys going.

The rear sensor in the down pipe is only there are a cat detector, if decatted and the limit settings are not adjusted you get a check engine, it has no effect on fueling at all though. Your lambda is in the headers at the collector which is drivers side front. You should also have an egt sensor in the up pipe on a WRX, not on STi though.

bob

yep its an STI. 2002

Mods are:

• Milltek 3” exhaust system inc sports cat
• GT Spec Gen II UnEqual Length Headers
• GT Spec Gen II Up-Pipe
• Walbro uprated fuel pump
• SX Fuel Regulator
• TEK3 Remap (Powerstation) 382bhp/360lbs
• TD05/06-20G Turbo
• Power Enterprise 650cc injectors
• APS 725DR Front Mount Intercooler
• RCM Direct Air Box Feed
• RCM / K&N Cone Air Filter
• 3 port boost solenoid

I'll have another go at DD logging it tomorrow as I'm on the motorway again :)

Let us know, Andy will be able to take a look around under the bonnet to see that everything looks ok as Triton are more than familier with your mod setup.

cheers

bob

Crikey when did they switch to the five wire wide band sensors for OE use (not after market ECUs) BOB?

I'm getting old...I must start tinkering with newer cars. :D:D

Aplogies for any confusion, but to add for future reference, anything I type does still apply to older models with the older industry standard (at the time ;) ) three wire narrowband sensor.

Hey Bob,

Went to Triton today and Andy popped the bonnet, looked around, listened, looked around some more, listened a bit more, and then said 'FOUND IT' :D

LOL ;)

Turns out the pipe from the boost solenoid to the turbo had split big time, so he replaced it. (which is also why I was boosting more than usual!!!)

I drove off and the SECS was still showing weird results. I pulled over, reset the ECU and now its all perfect and back to normal :D :D

Thanks for your input.

Mark.

Excellant, Andy's the man.

Glad its sorted

Bob