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Discussion Starter · #1 ·
I hope someone can help me by throwing some ideas as to why my car has a high co, I have owned it for five years and this is the first time it fails on high co emissions. It is a 1997 vvc. So far I have changed the cat, had the lambda sensor checked, it is okay, new air and oil filters along with oil change, new plugs. The co is not too far out, the car runs and idles good. Is there anything else that may upset it, I have heard and read about the water temperature sensor for the ECU, this appears to be working as it should as the radiator fan comes on after a long time and goes off again. I am waiting on a Pscan diagnostic.
Any ideas would be very much appreciated.
Dean, Shetland.
 

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mg_tf
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High CO indicates the engine is running rich. There's not enough oxygen for the amount of fuel being injected to burn that last bit of CO into CO2. The cat can't help get CO down if there's not enough excess oxygen to burn CO.

It's allowed to exceed CO limits under certain circumstances, when cold for instance. For some reason the ECU is probably being fooled into overfueling it. The water temperature sensor could well be the cause if it sends the wrong info to the ECU, telling it the engine is cold (ECU will adopt a rich fuel mix when the engine is cold). However, you seem to have looked into that (but it would be better to directly measure the actual sensor voltage it's sending back or get the PSCAN results for it).

Your new air filter rules out it being starved of air. You've changed the cat (by the way the aftermarket ones seem to be not very good from reports I've heard from posters here, but presumably it was failing on the original cat too). Sounds like you've ruled out the lambda sensor too.

It could however be related the measurement of air intake via the MAP (Manifold Air Pressure) sensor or even the Throttle Position Sensor (TPS). I think they're all used by the ECU for fuelling info under normal circumstances.

There was a thread on setting up the TPS recently. Not sure about how to set up/test the MAP. Be aware that leads and connectors to sensors can also fail/go high resistance so check the connectors too and clean them. CJJ on this forum did a post with all the engine sensors shown and identified. It will be interesting to read the PSCAN results.

The other advice often given in this situation to get it through the MOT is the 'Italian Tune-up' where you take it out for a good hard run for half-an-hour before the MOT.
 

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There can be (usually are?) two temperature sensors, one controlling the gauge - which I suspect you are eliminating as a suspect since you say the fan cuts in. The other signals the ECU to control the mixture dependent on sensing the coolant temperature. This could be at fault, since it works on a varying resistance according to the temperature it senses.
 

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Discussion Starter · #4 ·
High CO indicates the engine is running rich. There's not enough oxygen for the amount of fuel being injected to burn that last bit of CO into CO2. The cat can't help get CO down if there's not enough excess oxygen to burn CO.

It's allowed to exceed CO limits under certain circumstances, when cold for instance. For some reason the ECU is probably being fooled into overfueling it. The water temperature sensor could well be the cause if it sends the wrong info to the ECU, telling it the engine is cold (ECU will adopt a rich fuel mix when the engine is cold). However, you seem to have looked into that (but it would be better to directly measure the actual sensor voltage it's sending back or get the PSCAN results for it).

Your new air filter rules out it being starved of air. You've changed the cat (by the way the aftermarket ones seem to be not very good from reports I've heard from posters here, but presumably it was failing on the original cat too). Sounds like you've ruled out the lambda sensor too.

It could however be related the measurement of air intake via the MAP (Manifold Air Pressure) sensor or even the Throttle Position Sensor (TPS). I think they're all used by the ECU for fuelling info under normal circumstances.

There was a thread on setting up the TPS recently. Not sure about how to set up/test the MAP. Be aware that leads and connectors to sensors can also fail/go high resistance so check the connectors too and clean them. CJJ on this forum did a post with all the engine sensors shown and identified. It will be interesting to read the PSCAN results.

The other advice often given in this situation to get it through the MOT is the 'Italian Tune-up' where you take it out for a good hard run for half-an-hour before the MOT.
 

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Discussion Starter · #5 ·
Many thanks for your thoughts on the high co , Austin I thought the brown sensor controlled the ECU and the radiator fan, which would tell the ECU that I have a cold engine, the one next to it is black which I thought operated the water temperature gauge. I have tried the Italian tune up made no difference, I have tried 3 after market cats. As nocturnblue says be interesting to read the codes when the Pscan arrives, I hope it can help as been struggling with this since April. Garages up here in Shetland don’t have diagnostic equipment to read mems2j. The car passed its mot except for its emissions.
 

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As I understand it, the gauge sensor controls the fan which cuts in when the coolant temperature rises to the unacceptable.
The second sensor signals the ECU according to the variable resistance effect as the coolant temperature varies.
In simple terms, don’t assume that is serviceable because the fan comes on.
 

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Apologies. So now you need to check that the CTS is outputting correct resistance values as the coolant temperature changes.
 

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I think I recall reading that some people with poor emissions at testing found that blocking one of the two exhaust outlets and testing the other worked. Something to do with how the backpressure affects the sensors.
 

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Regarding blocking an exhaust outlet for an MOT, I blocked one of mine with a sock (!) as around 3000 rpm (my MGOC s/s backbox has a resonance which comes and goes depending on engine load at around 3000rpm, which can be a bit fatiguing at motorway cruising speeds) and when I got it back from an MOT they had removed the sock! There is presumably some test (emissions or noise presumably) where they disallow blocking one exhaust pipe...
 

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Discussion Starter · #11 ·
I have also tried blocking one side, at a time, I have a very helpful mot tester, it made no difference, it seems there are so many sensors that can affect the fuelling I will probably need diagnostic help.
 

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I would put money on the coolant temperature sensor being the culprit.
 

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Discussion Starter · #15 ·
But on the paper says you failed on the lambda sensor as well? Oxygen (lambda) sensor might be the one you need to replace with a good new one.
I have a new lambda fitted, new cat , three altogether. Did fit a new coolant sensor for the ECU but the radiator fan just kept running .So cleaned the old one and put it back in. The fan comes on after a period of time. The engine from cold idles about 1100 then drops to 800 on heating up.
 

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If you still have fail on emission test with new lambda fitted and emisson test says you fail on that lambda sensor as well that means you still have problem with it. I would fit new lambda sensor from Bosch or NGK as these two never failed me.
It would not be the first time the new part is duff or not appriopriate for your car.
Also even Bosch and NGK might fail for some other reasons or mishandling, so if test is providing fail then I woul look into this direction first.
 

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Discussion Starter · #17 ·
If you still have fail on emission test with new lambda fitted and emisson test says you fail on that lambda sensor as well that means you still have problem with it. I would fit new lambda sensor from Bosch or NGK as these two never failed me.
It would not be the first time the new part is duff or not appriopriate for your car.
Also even Bosch and NGK might fail for some other reasons or mishandling, so if test is providing fail then I woul look into this direction first.
I had fitted a Bosch one it could be that I damaged it fitting as it was not working changed it for a fleabay one which is working but the co is still on the high side. So it is proving hard to pinpoint . Spent quite a bit on it so far. May have to buy another lambda and water temperature sensor both of which I have bought from Rimmers already, original parts as advertised on their site.
 

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What catalytic converter have you fitted?
 

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Discussion Starter · #19 ·
I had fitted a Bosch one it could be that I damaged it fitting as it was not working changed it for a fleabay one which is working but the co is still on the high side. So it is proving hard to pinpoint . Spent quite a bit on it so far. May have to buy another lambda and water temperature sensor both of which I have bought from Rimmers already, original parts as advertised on their site.
The one I fitted thinking it was an xpart original turns out ,it is what is supplied as xpart original the seller advertised it with photos of an original part. It is a klarius one , I fitted it in the hope that it maybe better than the first two after market ones.
With the two others and my old one maybe could recoup some money by selling them for scrap.
 

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The lambda seems to have failed the first fast idle test but passed the second one, so the MOT says. Hmmm.

I'm no expert on MOT tests but both fast RPM idle tests look to be conducted exactly the same. Maybe things were hotter the second time around...

As I understand it a lambda responds to the O2 concentration in the exhaust and goes high voltage (towards 1V) when the exhaust is fuel rich as there is little O2 available then - they act like little max. ~1V batteries which work on the difference in O2 between their exhaust gas end and their other end which gets fresh air.

A bit of digging around lead me to an HonestJohn article which says that the MOT testers don't actually measure the lambda sensor output voltage of the car under test, they calculate what it would be (in theory) from the measurements from their exhaust gas analyser.


This makes me wonder whether the lambda sensor voltage MOT failure isn't just an effect of the high CO reading, rather than the cause? In other words, fix the high CO reading and the theoretical MOT lambda voltage will apparently come within limits again? Thinking about it, if they calculate a theoretical lambda voltage using the emissions test data, it kind of is assuming the lambda sensor is the sole factor in emissions whereas as we all know the ECU relies on engine sensor data as well as the lambda to gauge stoichiometric fuel delivery conditions.

After all, the OP has already changed the cat, what are the chances it's still the cat causing the high CO reading?

Still sounds like a sensor problem to me.

By the way, check for exhaust leaks around the new cat, apparently that can alter emissions readings too. Here's an excerpt from the MOT tester forums regarding someone who had failed on the lambda but passed after sealing the cat joints better:

'i think im the tester who tested your mr2 on wednsday test and advised that the catalyst joint had a minor leak of exhaust gasses. this usually causes a high lambda reading and certainly wont help it getting through the emissions. '

So, case in point, there was nothing wrong with the lambda in that case, an exhaust leak made it look like there was.
 
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