bad Xantia CO readings?

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pseud
Posts: 80
Joined: 21 Jan 2012, 22:10
Location: Leeds, UK
My Cars: L15 (my first car and first profit: bought for $750 aged 16; sold for $1,500 aged 20)
Xantia Mk1 SX 1.8 (gone) (aged 40, oh dear where did they go...)
Xantia Mk1 SX 2.0 (gone)
Xantia Mk1 SX station wagon 2.0 (gone)
C5 Mk1 Ex (dead, and going... gone!)
Xantia Mk2 Ex (going)
x 2
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bad Xantia CO readings?

Post by pseud »

Every year I have a nightmare getting the CO readings down for the MOT - hours (not quite) spent with me on the accelerator and the very tolerant MOT man shouting at me to keep it steady... keep it steady...

Is there a known answer to this annual irritation? Blast out on the M'way; high-octane fuel; Wynns goup, fixing the exhaust... I always get there in the end - I'm going to remove the air filter when I take it back in tomorrow...

But, is there any adjustment anywhere to actually fix it? Perhaps some Lexia parameter? Or is it just the tolerances of a generation ago are not the same as today?
L15 (my first car and first profit: bought for $750 aged 16; sold for $1,500 aged 20)
Xantia Mk1 SX 1.8 (gone) (aged 40, oh dear where did they go...)
Xantia Mk1 SX 2.0 (gone)
Xantia Mk1 SX station wagon 2.0 (gone)
C5 Mk1 Ex (dead, and going... gone!)
Xantia Mk2 Ex (going)
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Re: bad Xantia CO readings?

Post by myglaren »

Not much help but probably the old Italian tune up just before submitting.
The last petrol car I had was a low mileage Accord.
My son in law took it for the MOT and it failed on emissions, so the tester took it out and screamed it around for ten minutes then ran the test again, passed with ease.
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Mandrake
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Re: bad Xantia CO readings?

Post by Mandrake »

What engine ? Hard to tell from your signature as all your cars are listed as gone ?

Is the HC high as well ? How is the lambda figure ? Is it the idle test that you're struggling most with or the test at 2500 rpm ?

High CO and HC readings are usually only one of two things - incomplete combustion/misfire/rich mixture or the catalytic converter is on the way out and is no loneger working properly.

I had high CO and HC readings on my previous Xantia V6 that was coming extremely close to failing the emissions test, it turned out to be caused by intermittent misfires in one or more cylinders due to both a dud coil pack which was causing misfires when the coil pack got hot, and a break in one of the spark plug leads. I changed the coil pack (which on the ES9J4 supplies all 6 cylinders) and replaced the spark plug lead cassette and the following year the emissions test was passed with flying colours with the HC and CO values being about 20x less than the limits.

This was a relief as I had been convinced that the catyalytic converter was stuffed. It is true however that if a car is running rich or is misfiring significantly for long enough that the raw fuel going into the exhaust can cause the catalytic converter to overheat and be permanently damaged - so even after fixing the original problem a damaged or poisioned converter will continue cause the car to almost fail or fail the test.

A bit hard to give specific advice without knowing exactly which engine the car has, but I'd start by checking tha the oxygen sensor is working correctly (lambda value very close to 1.0 in the test, and/or the oxygen sensor voltage cycles between about 0.1 and 0.9 volts every couple of seconds at idle and faster at 2000 rpm) and then check the ignition system (coils, plugs, leads etc) for anything that might cause intermittent misfires - if the spark plugs haven't been changed for a long time I'd at least have a look at them, maybe just replace them. If the engine has traditional spark plug leads and you've never changed them then at that age I would change them!

The insulation in spark plug leads can degrade gradually over time due to heat exposure, causing the insulation to break down. Another thing that can happen is the wire inside actually breaks, but the cylinder continues to fire (after a fashion) because the voltage jumps across the break in the wire, however it significantly reduces the voltage available at the spark plug.

Either of these can cause misfires under load because the required spark voltage increases with the higher cylinder pressures that result from opening the throttle and pumping more fuel and air in. So ignition faults can easily cause the engine to run OK at idle but misfire when you stomp on the throttle or hold it at a high RPM.

If you're constantly struggling to pass the test new spark plugs and spark plug leads would be a good start and at the age Xantia's are now they should have been replaced at least once by now. Spark plug leads can last as little as 10-15 years before they start giving problems.
Simon

1997 Xantia S1 3.0 V6 Auto Exclusive in Silex Grey
2016 Nissan Leaf Tekna 30kWh in White

2011 Peugeot Ion Full Electric in Silver
1977 G Special 1129cc LHD
1978 CX 2400
1997 Xantia S1 2.0i Auto VSX
1998 Xantia S2 3.0 V6 Auto Exclusive
pseud
Posts: 80
Joined: 21 Jan 2012, 22:10
Location: Leeds, UK
My Cars: L15 (my first car and first profit: bought for $750 aged 16; sold for $1,500 aged 20)
Xantia Mk1 SX 1.8 (gone) (aged 40, oh dear where did they go...)
Xantia Mk1 SX 2.0 (gone)
Xantia Mk1 SX station wagon 2.0 (gone)
C5 Mk1 Ex (dead, and going... gone!)
Xantia Mk2 Ex (going)
x 2
Contact:

Re: bad Xantia CO readings?

Post by pseud »

Mandrake wrote: 03 Oct 2019, 20:44 What engine ? Hard to tell from your signature as all your cars are listed as gone ?
Oops, perhaps I should put some lines through my gone ones too - the one that's going hasn't gone yet, it's still going :) But, that only says so much: 2.0i 16V. On the limited information I gave, thank you the very comprehensive discussion.

I replaced the coil pack about 3 years ago while learning the lambda sensor needed replacing (expensive lesson) and would have done the sparks about the same time, iirc.

The HC and Lambda are OK:
#1
- CO 0.89 (max 0.3, I guess the maximums are standard...)
- HC 105 (max 200)
- Lambda 0.997 (max 1.03)
#2
- CO 0.60
- HC 113
- Lambda 1.010

I guess I was hoping there was something in the Lexia I could twiddle - a bit like the mixture screw on a carburettor... :o
L15 (my first car and first profit: bought for $750 aged 16; sold for $1,500 aged 20)
Xantia Mk1 SX 1.8 (gone) (aged 40, oh dear where did they go...)
Xantia Mk1 SX 2.0 (gone)
Xantia Mk1 SX station wagon 2.0 (gone)
C5 Mk1 Ex (dead, and going... gone!)
Xantia Mk2 Ex (going)
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