Hi there,

My L reg 1.6 Primera has just failed the MOT on high lambda by the
narrowest of margins.
1st attempt 1.048, 2nd attempt 1.032 (vs limit 1.030)
I understand high lambda means too much oxygen in the exhaust.

CO is incrediby low at 0.005 (vs limit of 0.300)
HC is also extremely low at 3 (vs limit of 200)

The tester said he thought there was the tiniest air leak between the
middle and back exhaust sections, which seems to fit this problem from
my Googling around. Naturally, I'll try to take care of that.

Please comment on these other ideas I found from Googling that might
just nudge it down a touch.

1. New air filter - I would have thought that would have made it worse
due to leaning the mixture.

2. Clean the plugs and close the gap.

3. Use super unleaded fuel prior to the test. - Can't see why that
might help.

4. Remove and clean the oxygen sensor. I'm a bit frightened of
damaging it if it is seized in.

My nightmare scenario is that on recently fitting a new radiator, the
fitter thought there were some bubbles of combustion gas in the
coolent and that a new head gasket and skim was in order. Naturally, I
hate that idea, especially as it uses no coolent and does not
contaminate the oil. He may of course be full of it.
Could that be related to high lambda?

Thanks in advance.

Fred

> The tester said he thought there was the tiniest air leak between the
> middle and back exhaust sections, which seems to fit this problem from
> my Googling around. Naturally, I'll try to take care of that.


If the tester found a leak in the exhaust system then this should be a fail 
anyway regardless of the emissions.  However, as the exhaust system is under 
positive gas pressure a leak would usually allow the exhaust gases out 
rather than fresh air in (hence why it's a failure), so that shouldn't 
directly affect the emissions test result (I could easily be wrong though).



> 1. New air filter - I would have thought that would have made it worse
> due to leaning the mixture.
>
> 2. Clean the plugs and close the gap.


The above are both standard service procedures so are definitely high on the 
list of things to do if they haven't been done for a while.  Not guaranteed 
to solve your problem though...



> 4. Remove and clean the oxygen sensor. I'm a bit frightened of
> damaging it if it is seized in.


Getting them out can be fun, but even if you do there's not really anything 
you can do without damaging or affecting their operation.  Replacement is 
often the best bet.  My Rover 414i failed on emissions many years back 
(can't remember the exact results) and I asked the tester whether I should 
change the sensor or the cat.  He said that as you should replace the sensor 
at the same time as the cat it would be worth replacing the sensor first to 
see if that alone works - if not then you replace the cat.  I was happy as 
the sensor was much cheaper than the cat and in my case that's all that was 
needed!



> My nightmare scenario is that on recently fitting a new radiator, the
> fitter thought there were some bubbles of combustion gas in the
> coolent and that a new head gasket and skim was in order. Naturally, I
> hate that idea, especially as it uses no coolent and does not
> contaminate the oil. He may of course be full of it.
> Could that be related to high lambda?


More likely air trapped in the cooling system.  If there's no other symptoms 
(e.g. overheating, white smoke, loss of coolant/oil, excessive cooling 
system pressure, etc) then I wouldn't suspect the H/G.  If there is air 
trapped in the system, particularly around the cylinder head, then the 
variations in temperature *might* have an effect on the emissions result but 
I honestly couldn't say.

Darren

"Darren Jarvis"  wrote in message
news:u8Hre.32350$8m5.23692@newsfe5-gui.ntli.net...

>
> > The tester said he thought there was the tiniest air leak between the
> > middle and back exhaust sections, which seems to fit this problem from
> > my Googling around. Naturally, I'll try to take care of that.
>
> If the tester found a leak in the exhaust system then this should be a
fail
> anyway regardless of the emissions.
> Darren
>

 nope.

reason for rejection : a [1] major leak of exhaust gases from any part of
the system.
note: a minor exhaust leak from, for example, a connection joint or a pin
hole , is not a reason for rejection.

[1] note the word major, you'd be surprised what would pass.

Darren Jarvis wrote:


> If the tester found a leak in the exhaust system then this should be a fail 
> anyway regardless of the emissions.  However, as the exhaust system is under 
> positive gas pressure a leak would usually allow the exhaust gases out 
> rather than fresh air in (hence why it's a failure), so that shouldn't 
> directly affect the emissions test result (I could easily be wrong though).


You are ;-) As has been mentioned, a small leak isn't a failure. However
a small leak can very easily push the Lambda up too high. I'd
sort that before attempting anything else.

John

"John Greystrong"  wrote in message 
news:d8okul$lh9$1@helium.hgmp.mrc.ac.uk...

> Darren Jarvis wrote:
>

 However, as the exhaust system is under

>> positive gas pressure a leak would usually allow the exhaust gases out 
>> rather than fresh air in (hence why it's a failure), so that shouldn't 
>> directly affect the emissions test result (I could easily be wrong 
>> though).


Even when pipework is under positive pressure any leak present not only 
allows the pressurised gasses out but also creates a pathway for external 
gasses to come in even if they are at a lower pressure.

Andy

Update !
MOT now passed.
Lambda dropped from 1.032 (fail) to 1.01 (pass) after remaking with
new gasket the joint between middle and back exhaust sections.
Charge ~30 with no retest fee (3 days later) !
I came away happy.

Thanks for the advise.
This is a great newsgroup!