MOT PASSED!!! update! (former MOT Failed - advice needed)

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MOT PASSED!!! update! (former MOT Failed - advice needed)

Postby vladman » Thu Dec 07, 2006 2:59 pm

Hi guys,

My car just failed the MOT, on three things.

1. Emissions - Lambda sensor reading outside allowed range (higher - not by much) at fast idle
2. Small leak in exhaust
3. Hand-brake recording little or no pressure at offside rear wheel

I thought number 1 and 2 were connected, and once I fixed the exhaust, the emissions reading would be fine too. But I just took the car to a garage this morning, and the guy there told me that the emissions failure is nothing to do with the leak in the exhaust, and that most likely, my lambda sensor needs replacing (at £100 or more, plus labour), and that even then, it might turn out something else is causing the failure, but that it's impossible to tell, and that the first step is to replace the lambda sensor. Also, he thinks the reason for the failure number 3. is most likely a seized caliper, and that it will need replacing. He said, if I have the lambda sensor replaced, the caliper replaced, and the exhaust fixed, this would cost me in the region of £500. I got a distinct impression from him that he was trying to tell me that the job isn't even worth doing, because of the age of the car (R-reg, exactly 9 years old now, but only 60000 miles).

This is definitely not what I was hoping to hear, and I'm wondering what the more knowledgeable of you here think. Was he correct about telling me that the exhaust leak ISN'T the cause of the emissions failure? Is he correct in assuming the caliper has seized and will need changing?

I still love and enjoy the car, and it drives beautifully, and I don't want to have to get rid of it! I need to have this sorted by the end of next week, because I might get a free MOT re-test if I do it before then.

Help!
Last edited by vladman on Sun Dec 17, 2006 12:56 am, edited 2 times in total.
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MOT PASSED!!! update! (former MOT Failed - advice needed)

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Postby Welly » Thu Dec 07, 2006 4:10 pm

My advice:

A leaky eggsauce wont affect emissions, if anything a leak would make the reading better than it would otherwise be.

A leaky eggsuace fails the MOT because gasses HAVE to exit the vehicle beyond the rearmost opening window, a leak could allow fumes to enter the Cabin.

I used to run a Mazda 2.5V6 and lots of guys on the forum used to fail on emissions, a common fix though was to fill up with half a tank of Optimax and add a bottle of injector cleaner, go for a sensible blast before you arrive at the MOT test by using full revs occasionally and to get the CAT really hot etc. this usually allows the car to pass although it is possible a Lambda sensor is duff.

The handbrake would normally be fixed with a bit of a brake service, strip and clean, with copper grease added to sliding parts etc, or it could be that the handbrake cable outer sheathing itself has broken down making the cable stick on that side.

If you cant have a go yourself, I would fix the eggsauce, get a small old fashioned independant garage to sort the rear brake by fixing it rather than replacing the caliper etc etc, then go for another MOT (after running as above).
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Postby Malachy » Thu Dec 07, 2006 4:17 pm

Ok here we have a classic case of a mechanic not knowing wht the fk he is on about, like 95% of mechanics.

the lambda sensor is a sensor that moniters the air fuel ratio of the exhuast.

now the sensor measures the amount of oxygen in the gasses this allows it to tell the ecu how much petrol is being burnt and thus what the Air fuel ratio is.

once the ecu knows what the reading is it trys to achieve stoichiometric burn by adding (lambda 1.0 or AFR 14.7:1 whatever you prefer) this is the cleanest burn that petrol can achieve and is the most environmently friendly, even though the most powerfull is about 12.5 afr.It does this by adding or reducing fuel until the sensor reads 14.7:1 afr.

andy's bosch fuel injection manuals wrote:stoichiometric mixture
Within narrow limits,a fixed quantity (that is, weight) of air contains a certain specific numberof oxygen molecules,and any given weight of any specific gasoline like wise contains some definite number of hydrocarbon molecules. Because the burning process amounts to individual atoms combining with each other, it follows that there is only one particular ratio of gasoline to-air that can ensure that all of the oxygen molecules mate up with all of the hydrocarbon
molecules. This theoretical ideal is called a stoichiometric mixture.

If there is an excess of oxygen molecules,some of them will fail to find partners. In
terms of the number of oxygen-hydrocarbon pairings, and thus the amount of energy
released, the effect is as if we had started with a smaller quantity of air. At the same
time, if there are too many hydrocarbon molecules in relation to the amount of air,
then some of the hydrocarbons will emerge from the combustion process unburned.

Some of the gasoline is simply wasted. Not only that, but a shortage of oxygen means
that there is a likelihood of some of the carbon atoms in the hydrocarbon fuel to combine
with just one oxygen atom, rather than two, yielding carbon monoxide (CO) rather
than carbon dioxide (CO2)' While CO2 is one of the" greenhouse gasses" that are partly responsible for global warming, at least i is only immediately harmful to animal life
when its concentration grows so large that it displaces much of the oxygen we need to
breathe. CO, on the other hand, is toxic even in small doses.
It turns out that about 14.7 lbs. of air contains the correct number of oxygen molecules
to pair up with the number of hydrocarbon molecules in 1 lb. of gasoline.


so we have established the sensor works off o2 content, now anyone with half a brain also knows oxygen is in the natural atmosphere (we breathe it for example!) an air leak in an exhaust and reduce or increase this content even if the exhaust is leaking downstream of the sensor as it can create a vacuum effect.

so an example is you have an exhaust leak oxygen is being drawn into the exhaust and passes the o2 sensor, the sensor reads this as the car running lean and the ecu adds extra fuel.
now the sensor reads afr 14.7 but it is in reality running rich as the extra fuel is added just because of the oxygen passing over the sensor.

Image

to work out AFR you can multiply lambda by 14.7
1.0 lambda = 14.7 afr.

as you can see from the graph a normal lambda sensor is very focused around 1 lambda this makes it very hard to tune for power with an aftermakret ecu, this is why you opt for a wideband lambda that gives you a linear graph so you can tune richer/leaner areas accuratly.

Image
something like this :P

anyway back on topic, i would expect the exhaust to be leaking especially if it was in the downpipe/cat area to be the cause of your problem.
i would also take the above advice of getting the car warm before going for an emissions test give it a good thrashing :P this gets the cat working well (cats dont really work until upto some silly temp)and removes the excess hydrocarbons.

And if you do need a new lambda sensor it is an easy replacement part, it screws into the exhaust much like a spark plug screws into the head, its a 10 min job so not high labour at all. as for cost well a pug one will be about £100, however they are all universal (they all work the same way registering 14.7 afr at 450mV sensor output ) as long as they have the right number of wires (pugs are 4 wire) and you just need to solder the conector on, you can buy a universal new lambda for about £30


i doubt you need one though and expect it was down to your exhaust leaking.

if you want some help just grab me on msn at
Andy@megasquirt-uk-ltd.co.uk

regards
Andy
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Postby vladman » Thu Dec 07, 2006 5:25 pm

thanks very much both.

andy, wow. i'm suitably impressed with your technical knowledge. i read through your entire post, but i'm afraid i probably didn't really understand all of it. but i understood the gist, that basically you think the emissions failure IS due to the exhaust leak.

ok, i'll follow your advice guys, i'll get the exhaust and the brake fixed - will probably have to look for another garage :|, and then give it some before taking it for a retest, then sit there with all my fingers and toes crossed.

fyi (should've explained in the original post), there are actually two leaks in the exhaust. one is a tiny pinhole leak on the joint approximately in the middle of the car (underneath it obviously), and another one is around the bracket which joins that middle pipe with the back box. i know my terminology is most likely not correct, but i hope you know what i'm talking about. i don't know where the lambda sensor is located, but i'm sure you do, so the location of the leaks might help you to be more precise when diagnosing my problem... and thanks for the info about being able to use any generic lambda sensor and how relatively easy it should be to fit!

btw, does anyone know or can recommend a decent, honest and not too expensive garage that could undertake these tasks, in or near london (ideally west london). i am yet to find a garage i can trust...

again, very grateful for your advice guys.
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Postby Welly » Thu Dec 07, 2006 5:56 pm

If your 'exhaust' is leaking where you said then there is NO WAY it can affect the Lambda reading, after all the Lambda cannot 'see' the leak.

Malachy, please explain how Oxygen can be drawn 'into' and exhaust as the exhaust will be under positive pressure any leaks will be from the inside-out.

As I understood, if a Lambda gets 'lazy' it defaults if you like to a rich setting, many owners will complain of poor fuel economy and even the presence of additional black soot around the tail pipe.
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Postby xplosiv » Thu Dec 07, 2006 6:21 pm

How does one test if there sensor is working (so to speak) and Welton no sexual referances?

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Postby jameslxdt » Thu Dec 07, 2006 8:02 pm

do you have the lambda reading? most cars fail due to the mot tester being ignorant and not letting the car warm up fully, and as it fails the BET test they cant be arsed to do a CAT test as it takes ages, believe me i know, if its only slightly out its most likely a lazy tester :frown:
and a leaking exhaust WILL cause a high lambda reading if the leak is before the cat as the o2 sensor will see too much oxygen and run rich, if money is tight just disconnect the o2 sensor and it will put the K light on but WILL pass emissions test
as for the handbrake, 406 handbrakes are always shite, does it have drums or discs on the back?
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Postby vladman » Thu Dec 07, 2006 8:35 pm

yes, i think i have the readings at home, will look later and post here.

i'm not sure about drums or discs, i think it's discs all around...

thanks james.
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Postby vladman » Thu Dec 07, 2006 11:00 pm

here are the details:

reasons for failure:
Image

emissions readings:
Image
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Postby vladman » Fri Dec 08, 2006 3:20 pm

umm, so what's the general consensus here guys, the leaks in my exhaust CAN or CAN'T be causing the failed emissions test? i need to have this sorted one way or another by the end of next week, 'cause my car's now untaxed, and i can't tax it without the MOT! :shock:

thanks!
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Postby Welly » Fri Dec 08, 2006 4:10 pm

You said the leaks in the exhaust were in the middle/back of the car, I know this and I know that this WILL NOT affect the Lambda reading, however the others still say it will :|

If I were you:

    Fix exhaust
    Fix parking brake
    Hot run + Optimax + Injector Cleaner
    MOT


Welly.
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Postby Malachy » Fri Dec 08, 2006 4:10 pm

vladman wrote:umm, so what's the general consensus here guys, the leaks in my exhaust CAN or CAN'T be causing the failed emissions test? i need to have this sorted one way or another by the end of next week, 'cause my car's now untaxed, and i can't tax it without the MOT! :shock:

thanks!


ok that shows a very lean reading 16.9 afr you would know if your car was actually running this lean as it would run like a POS and backfire a lot No assuming it runs fine when you drive it then your engine is actually spot on its just the leak in the exhaust is giving the tester a false reading on his machine.

the emissions is tested from the backbox this means that it is tested at the exit of the exhaust thus meaning air is drawn in through your centre section leak making the emissions test probe read the mixture as lean.

Yes welton it does not seems possible to draw air into an exhaust but it does happen i have had it myself and know of many cases of it. Search the internet yourself if you want :) i could give you the physics behinsd it (the passing gasses in the exhaust create a vacuum over the hole) but i cant be assed to drag my uni books out and i cant remember the correct term to search for it.

I honestly would be suprised if this was anything else than just the exhaust leak

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Postby Malachy » Fri Dec 08, 2006 4:13 pm

Welton wrote:You said the leaks in the exhaust were in the middle/back of the car, I know this and I know that this WILL NOT affect the Lambda reading, however the others still say it will :|

If I were you:

    Fix exhaust
    Fix parking brake
    Hot run + Optimax + Injector Cleaner
    MOT

Welly.


optimax all you want but it wont pull a lean 17:1 afr down to the required 14:1 > 15:1 to pass the mot
if your car was truelly running this lean you would get an ecu fault code.
and as said would run like a POS

your car is not running this lean i promise you you would know it

also the fact your HC is spot on a rich or lean mixture will leave Hydrocarbons in the gass, your hc is spot on so shows the combustion is also spot on, thus meaning the mixture is also spot on!
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Postby Welly » Fri Dec 08, 2006 4:18 pm

That told me then..............

Actually I was kind of barking up the wrong tree, I was thinking about the Lambda sensor being affected by Oxygen drawn into the exhaust and affecting the fueling, when really it was the case of what was being read at your tail pipe that was the issue, Sorry.
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Postby Malachy » Fri Dec 08, 2006 4:23 pm

Welton wrote:That told me then..............

Actually I was kind of barking up the wrong tree, I was thinking about the Lambda sensor being affected by Oxygen drawn into the exhaust and affecting the fueling, when really it was the case of what was being read at your tail pipe that was the issue, Sorry.


well yeah this is possible too and is normally the main cause, but you have to be able to get an accurate reading from the emissions machine aswell and the machine works just as a lambda (although a very slow reacting one)

also take it for a damn good blast on the way to thge mot station get everything upto temp and give the exhaust a blow out :P
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