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[terry paget]

Y188NFA 2.2 petrol manual saloon
CO read 0.00, HC read between 1 and 6, but Lambda read between 1.12 and 1.5 - machine said must be between .97 and 1.03. Tha lad failed it for this reason. I asked what it meant and how I should rectify it, he has no idea. I said I thought it was oxygen level, he said it could be.
My inclination is to change the oxygen (lambda) sensor. Is this wise?

[Nick W]

I'd be looking at the thermostat first.
you would be better finding some way of checking the live data to see if the lambda sensor is working properly before just changing it. Any recent and cheap OBD reader should do that on a 2.2

[Kevin Wood]

Yep, Lambda reading doesn't mean Lambda sensor! Far from it.

First thing to do is to check if there are any fault codes on the engine ECU, next check the live data for anything odd, as said. Also, check the exhaust manifold and exhaust system for any air leaks as this will screw up the lambda reading. 2.2s are prone to cracked exhaust manifolds and broken studs.

[Marks DTM Calib]

Is it getting to operating temperature?

Lambda closed loop control only operates if the engine is upto temp.

An yes, check for codes (engine light on?)

[terry paget]

Thanks for replies.
I have plugged my MaxiScan MS2309 in. No codes, Oxygen Sens mov ING Oxygen Sens Htr OK. I do not know if it offers live data. Car reached 85 degress within 3 miles, settling between 80 and 85 on the gauge. Doesn't sound like dead thermostat. I had a weary one on my 3.2 for years (changed last year) but it did not affect the emissions test . I can hear no bad exhaust system leaks between engine and lambda sensor. Earlier 2.0 Omegas had cracked manifolds, and studs broken off, but they never suffered emisision problems as a result.

I bow to your wisdom, though I would expect the exhaust system to be at higher pressure that outside, so would not expect a small leak outwards to much affect the exhaust gas composition.

I understand the lambda sensor detects oygen in the exhaust gas. How it does this I do not know. I am perplexed.

[terry paget]

I changed the lambda sensor and resubmitted the car. No change - still high lambda. Tester man said this meant high oxygen. I said I could no see the harm in this, but in vain, he was unmoved. He then sprayed something all over the induction side, looking for a leak, but found none. He then considered an exhaust leak. I pointed iut how sweet it sounded, but admitted there was a slight leak at the flexi section. He said this would do it, it could suck in air and raise the oxygen level. I have a spare car section, I suppose fitting it is the next thing to do. What do you think?

[Marks DTM Calib]

What you have to consider is how gas travels down an exhaust.

Form an engine, its not a continuous flow of gases, its short periods of pressure as each exhaust valve opens. A s the gas travels at speed down the pipe, a slight vaccum is created behind the pulse and its this that can pull the air in.

Having said that, to get such an error you need to fool the mixture O2 sensor and that is way before that part of the exhaust

[Kevin Wood]

Having said that, to get such an error you need to fool the mixture O2 sensor and that is way before that part of the exhaust

That does depend on how the emissions tester calculates the Lambda value. Most emissions testers are based on 4 gas analysers and measure Carbon monoxide, Hydro Carbons, Oxygen and Carbon dioxide. I wouldn't mind betting the Oxygen level is used in the Lambda calculation, and, hence it will be wrong if there's a leak upstream of the probe.

I know my kit car never manages an even remotely plausible Lambda value despite idling closed-loop. I put it down to the short exhaust causing it to suck back air. Last MOT it was the equivalent of about 19:1 AFR, and I know it won't idle properly at that. Fortunately, I was able to remind the MOT tester that it's visible smoke only.

[terry paget]

Thanks again. On the 2.2 the cat section goes manifold-1st sensor-small cat-flexi section-large cat-2nd sensor-main exhaust. So the flexi section is upstream of main cat and main lambda sensor, so air laek at flexi could indeed admit air to exhaust stream. When the engine was cold there was water bubbling out of the flexi and I thought it would fail MOT on that, but after the fast running to heat the cat the flexi was dry and not obviously leaking.

I shall change the cat section over the weekend, resubmit and report back.

[Kevin Wood]

The post-cat sensor isn't involved in mixture control, though. It just monitors cat efficiency. Difficult to say if the exhaust leak would be the cause, as it's hard to know how serious it is. Worth changing anyway, as the flexi will soon fall to bits.

Might be worth considering fixing your current cat section with a weld-in flexi section, as the cat in that section is clearly working quite well.

[Marks DTM Calib]

Having said that, to get such an error you need to fool the mixture O2 sensor and that is way before that part of the exhaust

That does depend on how the emissions tester calculates the Lambda value. Most emissions testers are based on 4 gas analysers and measure Carbon monoxide, Hydro Carbons, Oxygen and Carbon dioxide. I wouldn't mind betting the Oxygen level is used in the Lambda calculation, and, hence it will be wrong if there's a leak upstream of the probe.

I know my kit car never manages an even remotely plausible Lambda value despite idling closed-loop. I put it down to the short exhaust causing it to suck back air. Last MOT it was the equivalent of about 19:1 AFR, and I know it won't idle properly at that. Fortunately, I was able to remind the MOT tester that it's visible smoke only.

Agreed but its SO far out it sounds more like its not closed loop.

Plus when the flexi blows they are very loud usualy.

[Kevin Wood]

Indeed. Could do with looking at the live data, really:

ECU coolant temperature reading nice and hot?
Pre-cat Lambda cycling regularly?
Fuel trim values sensible?
MAF sensor output sensible?

[terry paget]

Thanks.
 When I did my pre-MOT check there was a P170 code showing, but I cancelled it and it has not returned.
I thought the post cat-sensor did most of the work, and the pre-cat sensor only functioned during cold start times. Is this so?
My MaxiScan MS309 does not appear to read live data, though  manual does have a section on Retrieving I/M readiness status. Would a TechII give more information? 

[terry paget]

Correction to earlier post. Thers is no leak at the flexible section. I suppose when i first got under the car I splashed it and some water was retained in the wire mesh around the flexi section. This moring, before changing the cat section, I got car over the pit with cold engine running - no moisture, no steam. Had wife rev engine with me in pit, and could detect no leak.
Possibly aircon condesate dripped on it first time.

Where do I go next? Plugged in my OBD2 reader, says MIL status OFF, Misfire Monitor OK, Fuel System OK, Comp Component OK.

Would a Tech2 tell me more?

Car runs sweetly, no misfre, quiet exhaust note.

[TheBoy]

Thanks.
 When I did my pre-MOT check there was a P170 code showing, but I cancelled it and it has not returned.
I thought the post cat-sensor did most of the work, and the pre-cat sensor only functioned during cold start times. Is this so?
My MaxiScan MS309 does not appear to read live data, though  manual does have a section on Retrieving I/M readiness status. Would a TechII give more information?

No lamda is used for warm up (as lamda needs to be hot to work), front one controls fuelling, hence you should see it constantly flick rich/lean at about 1hz.

Rear lamda only fitted to OBDII cars, and is used purely to monitor cat efficiency.