NCT failure - CO2 emmission

macskanadrag

Hi all,
I have a '95 Corsa 1.2l engine and today it just failed at the NCT for the second time due to the high CO2 emmission. The first time it was 0.37 (high idle) and the second time it was 0.4. The mechanic checked the exhaust and found it all right and I used the STP NCT pre-test petrol + injector cleaner thinking that and revving the car up a few times on the motorway should be enough (the limit is 0.3). Low idle is grand.
Any ideas?

Thanks.

DubOnHoliday

had a similar issue with a Corsa on high idle, you need to check the Air Mass Meter, if its not that, it could well be the CAT.

flatpack

i m no expert, do you most of time do short run in car? if so then do few miles on motorway.

macskanadrag wrote: »

Hi all,
I have a '95 Corsa 1.2l engine and today it just failed at the NCT for the second time due to the high CO emmission. The first time it was 0.37 (high idle) and the second time it was 0.4. The mechanic checked the exhaust and found it all right and I used the STP NCT pre-test petrol + injector cleaner thinking that and revving the car up a few times on the motorway should be enough (the limit is 0.3). High idle is grand.
Any ideas?

Thanks.

Muckie

Diagnostic check, nail down the exact problem. I'd guess

its your lamda sensor thats gone.

macskanadrag

Thanks for the help!

I took the car to 2 mechanics today. One of them checked it with the diagnostic equipment which took him nearly 2 hours and came back saying that he's still not sure what's wrong with it and I think he mentioned the air mass meter and because he also said that it's €200+ I didn't think I'd go down that road (and he said he doesn't really recommend it as he's not sure it'd help).

The other mechanic suggested I have the lambda sensor changed but the lambda value is grand so I'm not convinced that changing it would help either.

So I'll rev it up before the test, add some Dipothane (or what's the name) and have my fingers crossed. I'm reluctant to believe that the values would change this much within 2 weeks - low idle: from 0.49 down to 0.4 and high idle: from 0.37 up to 0.4... It seems it's a matter of luck if the car passes or not or it's the test centre guys who modify the results. I'll see how different the 3rd values will be.

The car is in such a great condition though, serviced 2 months ago, usually not a bother with it, the mechanic remarked that the engine runs nice and smooth so I was very surprised when it failed the second time too. And yes, it's mostly on the short run when i use it :-/

Mr Benevolent

Rev the crap out of it on the way to the NCT... drive in a low gear all the way.

macskanadrag

Will do The NCT is on Friday, I'll let youse know how it went.

DubOnHoliday

if it is the MAF, you can pick up an original part here :
http://www.vauxhallworldparts.com/vauxhall.php?partnumber=90543282

much better than the 229 Euro quoted by my local friendly Opel dealer.

Gurgle

Confab wrote: »

Rev the crap out of it on the way to the NCT... drive in a low gear all the way.

erm... its a petrol, how is that going to help?

itarumaa

Gurgle wrote: »

erm... its a petrol, how is that going to help?

I once did that with really old Peugeot 205, the HC value was around 400 but after driving about 20 mins in motorway at 100km/h at 3rd gear, the new HC value was under 200.

So it really does help, only problem with the 205 was that it was running so hot that it actually started to overheat in the test:)

Carawaystick

Is it not a CO emission reading? totally different chemical.

Gurgle

Carawaystick wrote: »

Is it not a CO emission reading? totally different chemical.

Thats what I thought - Hydrocarbons will reduce because you're running the engine hotter and burning out build-ups of unburned fuel in the exhaust pipes.

CO2 emissions from a diesel are due to the raw carbon content of the fuel and can be reduced by running it good and hot, but NCT failing levels from a petrol engine just mean the engine is fcuked.

Or am I misunderstanding stuff?
(answers from chemical engineers only please )

macskanadrag

The HC value is indeed measured with the CO2. In my case that's a good number, way below the limit. The rest, dunno, no chemical engineer here :P

To turn the question back, revving it out for the last 1 or 2 miles won't help? I got this advice from 2 mechanics.

DubOnHoliday

macskanadrag wrote: »

The HC value is indeed measured with the CO2. In my case that's a good number, way below the limit. The rest, dunno, no chemical engineer here :P

To turn the question back, revving it out for the last 1 or 2 miles won't help? I got this advice from 2 mechanics.

plus if there's something wrong, revving it isn't gonna solve anything (tried that, and STP) . Got mine (a 99 Corsa) thru on its 3rd retest, the CAT has disintegrated. But your mechanic said thats fine? The other candidates are as already said, the MAF, the Lambda sensor or the air filter.

macskanadrag

At this stage I was merely interested if I get yet again different results at the third test. I did get different low idle results (0.18 as opposed to 0.49 and 0.4) and asked the mechanic why is that result so different if I didn't change anything. He got a bit aggressive, why am I interested in a result that passed when I failed again on the high idle result, I should be concerned with that. I couldn't explain to him that I don't know if their diagnostics are reliable or not and since he refused to explain I don't think it is, I think it's an easy way to get a car re-tested. It was in Kells by the way but I'd say it's the same everywhere.
Anyways, thanks for all the help lads.

Gurgle

macskanadrag wrote: »

since he refused to explain I don't think it is.

I'd think he refused to explain because he doesn't have a clue. They just put the sensor into the exhaust and press the 'Test' button.

macskanadrag

I'm pretty sure they have an idea what's changing the values and what isn't.
It doesn't matter really, the car needs a new cat or air mass meter, I just don't trust the NCT centre.