Emissions fail - advice please!

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JimW
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Emissions fail - advice please!

Post by JimW »

The BX failed it's emissions test today with a CO reading of 7.86 %vol (a pass is 3.50). This is actually over double the allowed limit, it's always passed before (and it's been through a few MOTs now!).
I doubt it's the air filter as it's only done roughly 10,000 miles on the current one.
Mine is an "early-type" 8v 19 GTi, by the way, and seems to be running well (mostly) and certainly when hot everything is fine, including tickover.
As far as I can remember I've never fiddled with any of the idle adjustments.
However the car has been off the road for a couple of months, until just before the MOT. I did change Oil & Filter (and give the breather hoses, etc. a good clean out) fairly recently, though.
Has anybody any ideas? Anything obvious to check for or replace?
Or is it just a matter of going back and persuading the tester to adjust the idle mixture screw when it's next tested (seems a long way off to simply be adjusted out, though)?
Thanks for any suggestions,
Jim.
jeremy
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Post by jeremy »

If its been standing around for some time, shunted around, and never had a run the silencers are probably full of unburnt fuel which could be the source of your problems.
Have a look at the air cleaner element - its easy - and does not need to be there for an MOT. (Just make sure there's no grit or dirt on the atmosphere side that could get drawn into the engine) off with the filter missing.
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Post by Stinkwheel »

If you are sure you have not adjusted anything, take it for a good run to warm it up and get a nice open bit of road and give it some 'beans' on 2nd and third, nice italian tune up :-)
Also Im pretty sure the 8v gti engine has a nice easy to get at air compensator screw, maybe just tweak it a bit also.
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Post by Dave Bamber »

Take it out and give it a good thrash, plenty of revs in the lower gears to blow all the crud out of the engine and exhaust.
JimW
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Post by JimW »

Thank you, Gentlemen, for your responses!
Jeremy: Are you suggesting that I simply go back for a retest with the Air Filter out (I think I've read this works with Diesels) ?
In fact the BX has been standing some time, and might well have been running on idle for a while, so the theory about unburnt - or partly burnt - fuel in the exhaust system may well be valid. It also isn't far to the testing station - although I think I'll make sure it is next time - so the car, whilst hot, wasn't like it would have been after say 1/2 hour on the motorway.
Re: adjusting the idle mixture screw - does anyone know which way it should go for weaker (hence lower CO)? Out seems to make sense (e.g. more air)...
Thanks again,
Jim.
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Post by fastandfurryous »

Most idle mixture screws operate by opening or closing a small fuel orifice, so actually you have to wind the screw IN to reduce fuelling. You may find that raising the idle speed on the idle stop screw helps the emissions a bit too.
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Post by citroenzx »

If the lamba probe is knackered it would fail on emissions
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Post by PeterMann »

To reduce CO emission may need a richer mixture (CO being a product of lean burn). Catalytic converter (if fitted) may also play a role in completing oxidation of CO to CO2.
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Post by JohnT »

On diesels which failed the emmission tests we put a fuel treatment in before a fast run. The product is called 'Forte' and it is superb, accept for one old knacker it did the job. I all else fails try this, we have never been dissapointed.
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Post by Kowalski »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by PeterMann</i>

To reduce CO emission may need a richer mixture (CO being a product of lean burn). Catalytic converter (if fitted) may also play a role in completing oxidation of CO to CO2.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I have to admit to being rather confused about this one. I though high CO meant too rich a mixture not too lean. My thinking being that CO indicated that there isn't enough oxygen to oxidise the carbon all the way down to CO2.
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fastandfurryous
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Post by fastandfurryous »

You're right. High CO is related to a rich mixture. Weak mixture will give you a very low CO reading, which can be just as bad.
As you adjust away a rich mixture it can be hard to see the CO drop properly without "clearing" the exhaust every 20-30 seconds (running the engine at 2500rpm for 10 seconds clears the exhaust)
Usually, if your working on a carburettor, you just wind the screw in and out, finding the fastest speed the engine will run at, and then wind the screw in (to lean the mix) a little until the engine speed just begins to drop.
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Post by jeremy »

Yes your Gti will probably run perfectly well without the air cleaner element. It doesn't really do much but is nice to have there as a precaution if you drive through some loose sand. After all look at how little its extracted when you throw it away and then remember that most of what it has extracted would have gone through the engine harmelssly.
Look at the aftermarket ones as well - they can't get much besides leaves out as you can see straight through them.
I have always had a filter in my BX TD!
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Post by Kowalski »

The aftermarket ones are wet with oil which in theory makes them sticky so they don't act purely by having holes smaller than the dirt particles.
Whether they filter sufficiently or not is debatable, they're expensive and I think to pay for itself it would have to last 50k miles.
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Post by jeremy »

A hole is a hole is a hole - and while the oil will trap anything touching it and prevent dirt bouncing off the sides it will not help trap anything going straight at the hole.
When oil bath cleaners were used there was often a centrifugal pre cleaner which may have done something and various baffle arrangements which may have helped. Certainlly agricultural machinery seems to last well with these devices.
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Post by Kowalski »

There is some sort of centrifugal effect with an oily cotton filter, every time the air has to change direction you get the centrifugal effect they are much more effective than you would give them credit for but like I said, I don't know how they compare with the OEM paper filter.
My Dad's International 434 is 'F' reg (old F on black numberplates) and it has a rather huge oil bath air cleaner, his old JCB had a smaller filter that operated on similar principles and that was on an old 'P' plate. The filter on the International has a centrifugal bit and a ball of heavy wire coated in oil, the engine starts as knew but it's only done 1200 hours or there abouts.
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