Failing MOT emissions test
Discussion
As the title describes, the old girl has failed her MOT on emissions. Idle test is fine, but the fast idle is causing problems. The CO2 and Lambda are very marginally over, but the HC reading is extremely high, from just shy of 300 up to 513 (the limit is 200). Thus far the car has been set up on the laptop, and had a replacement throttle pot and lambda sensor. Both exhaust pipes have been probed (ooh matron!) Two sets of cats have been tried.
Can anybody advise? There is a chance the cats are buggered but I really don't want to fork out for a set of cats for five minutes work. Thanks in advance all, Happy Cerbing!
Can anybody advise? There is a chance the cats are buggered but I really don't want to fork out for a set of cats for five minutes work. Thanks in advance all, Happy Cerbing!
GT6k said:
Mine passed last week at my local garage, the CO was 0.06 and HC 76. The only prep I do is to take the car for an Italian tune immediately prior to the test so that it goes in well warmed up.
plus each year I put DAE Fuel System Conditioner in the tank, I have had good results with it, had a mini with lots of fault codes showing, ran it through and they all cleared, normal disclaimers apply.... it will not fix a mechanical fault, but it does run better after putting it in.http://www.dae.co.uk/engine_clear.htm
I also keep it running and up to temperature till they are ready to test it.
Exhaust manifold air leak?
Mine scraped through but only by revving it to 3000 rpm. Never been an issue before.
I am pretty sure I do have an exhaust manifold leak (typical ticking sound). I assume that this would result in the lamdas effectively correcting for what they see as a lean mixture by chucking more fuel in.
My MIL light is occaisionally flashing as well so I definitely have a prob somewhere.
Mine scraped through but only by revving it to 3000 rpm. Never been an issue before.
I am pretty sure I do have an exhaust manifold leak (typical ticking sound). I assume that this would result in the lamdas effectively correcting for what they see as a lean mixture by chucking more fuel in.
My MIL light is occaisionally flashing as well so I definitely have a prob somewhere.
The HC reading being so high could be a faulty coolant temperature sensor i.e. reading colder than it actually is. Or as the above post says, a cracked manifold will allow oxygen into the exhaust and be detected by the lambda sensor which will cause the ECU to inject more fuel.
Or an air leak anywhere on the inlet side causing the same as the cracked exhaust.
A misfire will allow more oxygen into the exhaust as it is not being burned.
If you have had it remapped, then perhaps this is where the problem is? Do you have the software on your laptop to check the readings?
The cats may be fuel contaminated due to overfuelling but before replacing them, might be worth putting some Cataclean (Halfords Eurocarparts etc) into the fuel and see if this makes the cats more efficient but only after the HCs have been reduced.
Or an air leak anywhere on the inlet side causing the same as the cracked exhaust.
A misfire will allow more oxygen into the exhaust as it is not being burned.
If you have had it remapped, then perhaps this is where the problem is? Do you have the software on your laptop to check the readings?
The cats may be fuel contaminated due to overfuelling but before replacing them, might be worth putting some Cataclean (Halfords Eurocarparts etc) into the fuel and see if this makes the cats more efficient but only after the HCs have been reduced.
Thanks for the responses. The car is a standard 4.5 without remap, hence I imagine it will be running a little rich anyway due to the nature of TVR's mapping. I don't personally have the software, but it has just been set up and is running excellently I must say. What is the consensus on fuel system cleaners etc? I heard they aren't advisable.
robsco said:
Thanks for the responses. The car is a standard 4.5 without remap, hence I imagine it will be running a little rich anyway due to the nature of TVR's mapping. I don't personally have the software, but it has just been set up and is running excellently I must say. What is the consensus on fuel system cleaners etc? I heard they aren't advisable.
Are your cats new and did you get the same (or very similar readings) from both banks? My 4.5 recently passed an emissions test with the cats in but one bank was inconsistent in the readings which the tester put down to the cat needing replacement.You could just need some new cats.
ETA. I see you have tried two sets of cats, were both old sets and do you know that one of the cats tested has been used successfully in an emissions test recently?
For example - If another 4.5 cerb passed a test using the cats you also used only a few weeks ago then you know you have a problem elsewhere.
Edited by TimJM on Wednesday 25th June 15:15
TimJM said:
Are your cats new and did you get the same (or very similar readings) from both banks? My 4.5 recently passed an emissions test with the cats in but one bank was inconsistent in the readings which the tester put down to the cat needing replacement.
You could just need some new cats.
ETA. I see you have tried two sets of cats, were both old sets and do you know that one of the cats tested has been used successfully in an emissions test recently?
For example - If another 4.5 cerb passed a test using the cats you also used only a few weeks ago then you know you have a problem elsewhere.
Hi Tim. Both banks have been tried and reading high, and neither sets of cats have been used for a good few years... So there is a chance that a new set of cats would cure the problem. This said, I don't want to spend money on cats to use for a 10 minute emission test, and even then, not a guaranteed fix! I guess until a new set of cats have been tried, we'd be guessing.You could just need some new cats.
ETA. I see you have tried two sets of cats, were both old sets and do you know that one of the cats tested has been used successfully in an emissions test recently?
For example - If another 4.5 cerb passed a test using the cats you also used only a few weeks ago then you know you have a problem elsewhere.
Edited by TimJM on Wednesday 25th June 15:15
If the HC is way too high and the CO and lambda are out then the fault is not just the cats.
If the HC is good and the CO is around 0.60 - 0.90% (approx) on the second fast idle and the lambda is either within limit or very, very close then the cat(s) has failed. This is usually the reading you might get with no cats fitted on a properly tuned engine.
If the lambda is way out but the HC and CO are good then lambda failure.
This is my experience anyway.
If the HC is good and the CO is around 0.60 - 0.90% (approx) on the second fast idle and the lambda is either within limit or very, very close then the cat(s) has failed. This is usually the reading you might get with no cats fitted on a properly tuned engine.
If the lambda is way out but the HC and CO are good then lambda failure.
This is my experience anyway.
robsco said:
Hi Tim. Both banks have been tried and reading high, and neither sets of cats have been used for a good few years... So there is a chance that a new set of cats would cure the problem. This said, I don't want to spend money on cats to use for a 10 minute emission test, and even then, not a guaranteed fix! I guess until a new set of cats have been tried, we'd be guessing.
Where are you based? I'm sure someone will lend you some cats for the MOT if you ask nicely 
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