New MOT. Good emissions. Lambda top end of scale?

[Stephen Edwin]

Originally Posted by Zebster

It's running lean. Could be a dirty MAF sensor?

The stoichiometric air:fuel ratio is 14.7:1 (by weight). By measuring the unburnt fuel and free oxygen your car's ratio can be determined, then compared to the stoichiometric ratio to give the lambda figure. In your case the air:fuel ratio is 1.04 x 14.7:1, so 15.288:1

The calculation is interesting to know.

Thanks for that.

[Stephen Edwin]

Originally Posted by Clan

Air mass sensors do not get dirty , they draw freshly filtered air over them and every time you turn off the ignition they glow white hot to burn off anything anyway … Too many people "clean" them and ruin them , not wise on a £250 sensor , I have seen 2 destroyed like this in my time ...

Thanks and I remember this issue from previous threads. I'll leave my air mass meter alone.

I also remember the burn off procedure. There's a revs and volt meter procedure to test that the burn off signal is sent to the air mass meter. I used that test when I was checking my air mass meter nearly two years ago. At that time I did need a new one.

Originally Posted by Clan

a work out is doing over 4000 rpm under load , say up a hill for a period of time it burns carbon from the exhaust front pipe and lambda sensor ..

The MOT gets the lambda from a mathematical calculation from the 4 gas readings , you might be able to read it out on a diagnostic machine , although the older the car the less likely this is .
However .. as long as your engine light is off you are ok ...

Aha. I er, rarely approach 4,000 .... mea culpa. There is a hill not far away that gets increasingly steep and would be worth a try next MOT day. Maybe occasionally through the year.

And I can't get the four gas readings with DIY equipment so I'll stop thinking so much about this.


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[Zebster]

Originally Posted by Clan

The crankcase fumes go into the air intake down-stream of the Air mass meter .. all that goes through the air mass meter is fresh filtered air , the pipes are always clean when you change an air filter .

Same as the MR2 then, and that MAF sensor has needed cleaning several times due to lean running (causing P0171 and P0174 error codes). It's one of the most common MR2 problems. The incoming 'fresh air' is never absolutely clean, the fumes I referred to were traffic fumes, etc., plus of course the air filter can never remove all airborne particulates.

So I really don't understand how Volvos can never suffer from a dirty MAF sensor, as surely some contaminants will not simply burn off? But happy to take your word for it!

I'm new to Volvos but, in my experience, while you will improve poor MoT emmision figures (HC and CO) by getting the engine and cat really hot, you're unlikely to make improvements to a high lambda figure by doing this. Unless of course this is another weird Volvo thing that I've yet to understand!

[Clifford Pope]

One of the connections on the oxygen sensor is a heating element.
It's fed from fuse 4 on my car, the same as the in-tank fuel pump.

I presume if the element had failed the sensor would still work, but would be slow to heat up and lose it's heat quickly if the engine were left idling, eg at an MOT?

[Stephen Edwin]

Clifford, I've heard of the heating element. If I remember correctly it was introduced part way through 240 production. Was it continued for all 240s or was it phased out? Can one spot it by the number and/or colour of the wires? Would it burn off carbon or does it "only" heat up more quickly to an ordinary operating temperature?

Anyways as Clan advises burn off carbon from the front pipe and the sensor by may I say "an uphill Italian Tune Up" I shall endeavor to try that.

Zebster. So far as I remember, forum experience tends towards letting the LH electronic system do what cleaning is necessary to the air mass meter. A brief spike of voltage applied automagically when the engine is switched off.


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[Zebster]

Originally Posted by Stephen Edwin

...Zebster. So far as I remember, forum experience tends towards letting the LH electronic system do what cleaning is necessary to the air mass meter. A brief spike of voltage applied automagically when the engine is switched off.

Happy to bow down to the collective hive knowledge, although in my book heating up something covered in contamination will just as likely bake it on as burn it off.

You do still need to find the reason for the lean running.

[Stephen Edwin]

Originally Posted by Zebster

Happy to bow down to the collective hive knowledge, although in my book heating up something covered in contamination will just as likely bake it on as burn it off.

You do still need to find the reason for the lean running.

Never bow down! I think it's more that for myself I listen to the totality of what members are saying, and who is saying it, and what other information I may have, then I make a decision, and on my head be it.

So far as I know, Volvo or perhaps Bosch of course provide the pulse of electricity to burn any contamination off the air mass meter wire?

Yes I have "made a note" re lean running.

[Zebster]

Originally Posted by Stephen Edwin

Never bow down! I think it's more that for myself I listen to the totality of what members are saying, and who is saying it, and what other information I may have, then I make a decision, and on my head be it.

So far as I know, Volvo or perhaps Bosch of course provide the pulse of electricity to burn any contamination off the air mass meter wire?

Yes I have "made a note" re lean running.

All MAF sensors I've seen are heated in normal operation, I was unaware that there is an addition degree of heating at switch-off. Every day is a school day!

If we're ruling out the MAF sensor, then maybe you have a leak on the intake side? I'd suggest spraying brake cleaner in small bursts at any potential leakage point to see if the idle surges.

[Stephen Edwin]

Originally Posted by Zebster

All MAF sensors I've seen are heated in normal operation, I was unaware that there is an addition degree of heating at switch-off. Every day is a school day!

If we're ruling out the MAF sensor, then maybe you have a leak on the intake side? I'd suggest spraying brake cleaner in small bursts at any potential leakage point to see if the idle surges.

That's good advice.

At the moment, I was just asking about the MOT calculated Lambda in general.

I'll be looking in to the lean running idea somewhat later.

Life gets on top of me so I go in small steps.

[Zebster]

Originally Posted by Stephen Edwin

That's good advice.

At the moment, I was just asking about the MOT calculated Lambda in general.

I'll be looking in to the lean running idea somewhat later.

Life gets on top of me so I go in small steps.

As previously calculated, your air:fuel ratio appears to be 15.288:1 rather than the stochiometric/ideal 14.7:1, if correct then this is a fairly serious condition where the excess/free oxygen in a hot environment can cause localised burning and subsequent damage (exhaust valve seats are a common point of damage in an engine that's running too lean). Apart from MoT concerns, this problem really does need to be addressed for the sake of your engine. A handy confirmation is to look at the sparkplugs tips and assess the colour... a milky coffee colour is ideal, while grey/white would indicate lean running.

Good luck with it.