NCT failure - Emissions

Hotblack Desiato

Put SWMBO's car ('98 Corsa 1.0 12v) through the NCT yesterday - sailed through with one exception - emissions

Here's the relevant part of the NCT report:

Exhaust Emissions
Engine/Oil Temperature 86C
Low Idle (880rpm)
CO 1.23vol% (above 0.5%: fail) FAIL/REFUSAL
HC 0ppm (above 0ppm: fail)

High Idle (2950rpm)
Lambda: 0.999 (between 0.97 and 1.03: pass)
CO 0.12vol% (above 0.3%: fail)
HC 28ppm (above 200ppm: fail)

So it's spot on at the high idle, but is well above the CO limit at low idle.
Any ideas on the cause of this?
Car has 45000 miles on the clock and is mostly used on short journeys in town.
Plugs recently replaced, I didn't replace the air filter as it looked fine.

I'm assuming that a major problem i.e. catalyst would affect the high idle emissions as well as low? I did find an old thread with the suggestion to ensure the catalyst is really hot when leaving in the car, and using fuel injector cleaner. Could it be that simple? The drive to the test centre was about 20 minutes and it was only left sitting for 5 minutes or so before testing, so shouldn't it have been hot enough? Aren't they supposed to ensure the car is warmed up before taking the test anyway?

JHMEG

Probably the primary O2 sensor. But could be another faulty sensor causing the ecu to get "stuck" in open loop where it makes no adjustments to the engine's operation. (If not, then the cat)

Hotblack Desiato

Surely it can't be the cat if the high idle emissions are well under the limit.

I'd also imagine a failed oxygen sensor would affect the whole operating range of the engine not just low idle. The lambda at high idle is spot on, so the O2 sensor must be ok.

It had the crankshaft sensor replaced recently but when it failed the engine was a dead duck. The engine light isn't illuminating.

It had the plugs and coil pack (€270 ) replaced the day before the NCT. SWMBO has been instructed not to take up any offers of engine steam cleaning in future :eek:

oh and SWMBO = She Who is My Beloved Other, what were you thinking?

JHMEG

Without going too far into it, it looks like the cat is probably ok (imho).

If the cat is ok then the carbon monoxide reading is so high because there is not enough free oxygen in the exhaust gases to combine with the CO to produce CO2.

This would be caused by a sensor not performing properly. The first finger would point at the primary O2 sensor (they are probably the most common sensor to fail as they're bolted into the exhaust), but others cannot be ruled out.

Also, from experience, O2 sensors go dodgy more often than outright fail. In most cases a dodgy O2S won't cause the CEL to come on.

The easiest thing would be to bring it to someone who has a scan tool (I think that car is probably OBD2). That way you can "see" the readings from all the sensors and check which ones are within spec and find the one(s) which isn't.

What's that about steam cleaning?

Hotblack Desiato

The other half thought she'd get the car valeted as the NCT form says the underbody should be clean.

Not only did they place she took it to not clean the underbody, they asked her if she wanted the engine steam cleaned and, as it looked a bit manky, she said yes. So they blasted a steam hose right into the top of the engine which of course is full of electrics. A few days later (day before the NCT) it developed a severe misfire and the Opel dealer put in new plugs and a new coil pack which cured it.

Now, my other half didn't know any better but the guys doing the steam cleaning should have.

Incidentally, we have the NCT reports from 4 and 2 years ago - it only barely passed on the low idle CO 2 years ago but is fine on everything else.

Looks like it's back to the Opel dealer for us - so long as it's not the cat :eek: then that's not the end of the world.

JHMEG

I wouldn't hesitate to get a couple of opinions on it. Most dealers don't charge for a diagnosis (on the assumption they are going to get the repair job), so essentially the diagnosis should be free.

I got a quote for a Honda cat recently at 700 euro. A generic cat from the motor factors was 150 euro. A cat from a breaker was 30 euro.

Patience, Wd40 and brute force & ignorance (and maybe a hacksaw) is all that is required to change it. Oh and a few new nuts and bolts (the old ones will have rusted to sh1te)

Fey!

Ninja - I've used those fuel cleaner emission things (stp) and filled a couple of times on v-power before the test, and it passed emissions fine (drove the 2 miles to the centre at 4,000rpm too!). Mine's a '98 with 85k miles. Emissions were a bit higher previous to that - tested before and after treatments by a mechanic friend; was curious to see if they worked. Did the treatments about 3 times over 3 weeks.

Pity the car failed on front shocks and no rubber on the clutch pedal (plus the idiot that is me forgetting to change the number plates that were sitting in the boot...).

Also, I clean my engine fairly regularly myself, but I use degreaser and a powerhose to do it, but I don't aim a burst directly at the engine; I use the lowest pressure part of the stream of water. Haven't had any problems - yet (fingers crossed and touching wood)! Then again, I tend to spray away from electrics or cover them.

Finally, I always thought that SWMBO was She Who Must Be Obeyed.....

Hotblack Desiato

Fey! wrote:

Finally, I always thought that SWMBO was She Who Must Be Obeyed.....

Shhhh!

SouperComputer

Are you sure you dont have an exhaust leak?

Hotblack Desiato

SouperComputer wrote:

Are you sure you dont have an exhaust leak?

No, but there are no funny noises either so if there is one it's small.

How is an exhaust leak going to richen the fuel mixture?

thewing

Get the throttle cleaned, and then see how it goes.If emissions return to bad levels at idle, then it's the O2/Lambda sensor.If it was your cat all your readings would be off.Not a big job, about 300 and u'll be golden(if it is the sensor, otherwise the price of a throttle clean.)

Hotblack Desiato

thewing wrote:

If it was your cat all your readings would be off.

Thought so - but still a relief to hear it

Not a big job, about 300 and u'll be golden(if it is the sensor, otherwise the price of a throttle clean.)

The idle emissions were marginal at the last NCT but fine at the one before that.
I doubt a sensor could be gradually getting worse for >2 years but not actually fail!
Hopefully the throttle clean will sort it.

SouperComputer

ninja900 wrote:

How is an exhaust leak going to richen the fuel mixture?

Its possible with a carb. Thats not what Im getting at though. If there is a small leak in the exhaust system it can give a higher CO reading.

Be sure to keep the revs high before the test, stay in a gear lower than normal. Push it hard up through the gears. I dont mean redline it, just keep those hot exhaust gases flowing through the system to get that cat temp up as high as possible.

REDZ

We used to turn down the fuel pump to get it throught the emissions test!

thewing

Agree with SouperComputer, along with getting it checked over, try and arrive as close to the time of your NCT as poss so the cat is still hot.They don't tell you these things.....

Hotblack Desiato

Well the verdict from the dealer is that the Mass Air Flow sensor is faulty, they've stuck a new one in and say the emissions are now fine (though I'll have to let the NCT have the final say on that!)
Cost 284 euro all in - found a website in the UK offering the part for £100STG + VAT and a couple of UK car site threads giving even higher prices so it's not too bad considering. One of those sites said the expected life of the sensor is 40-60,000 miles and the car's done 45,000 now.

JHMEG

So they stuck a scan tool on it then? ;-)

Give a quick phone call to a motor factors: there might be a spurios one available, esp if they fail *that* often.

There are several on ebay too, starting at about £45

Darragh29

Purely academic at this stage then!