MoT Failure - emissions - HELP

[duncan]

As the title; MoT failure and a request for help.
The culprit, emissions, at fast idle;
Lambda, limits 0.970-1.030 - recorded value 0.999 so OK pass,
HC, limits --- to 200 - recorded value 127 so OK pass but last year 48 so big change
CO, limits --- to 0.300 - recorded value 0.730 so FAIL, last year was 0.048 so massive change
The above plus an advisory of a minor blow on one of the exhaust flexi joints.

Where to go? the car is a UK 1995 Manual, almost 123,000 miles with up to date service history, standard everything, manifolds, cats, silencer, etc. etc.
No dash warning lights, but, I haven't done a code check, always assuming that hidden codes exist.
Prime suggests cleaning IAT, done 7000 miles ago, will do again.
Prime half suggests plugs, changed 20,000 miles ago, will check, can change.
Lambda O2 sensors replaced (F)60,000 and (R)24,000 miles ago.
Garage advisory on flexi, assuming to use a universal repair section.
Garage suggested replacement of Cats, OEM Honda at £1,350ish each, would like to be confident that all other options have been exhausted.
Thoughts and thanks in advance.
Duncan

[goldnsx]

Hmmm, not easy.

You can lower HC when you do the test with fresh engine oil but your values are ok.

High CO points to inefficient cats, that's true. But as your car is a 1995 you have two sensors per exhaust pipe. The downside one should act up if the cat is bad but maybe it's not bad enough.

What about CO2?

Before spending that amount of money on new cats I'd have a look in the US for used or aftermarket ones. There are even companies out there who replace the cat core with a new one for much less than the rip off dealer asks.

[NSXGB]

How hot was the car when tested? Had it been sitting for a while? With mine, I take it for a blast up the motorway en route to the test centre, then it passes....if I don't do that it will also fail the emissions.

[britlude]

prime reports cleaning the blow-by oil and crud off the air intake temp sensor (up around the fuel-rail input banjo) .... easy and free, so worth a punt

http://i301.photobucket.com/albums/nn41/thisYup/nsx%20stuff/air%20temp%20sensor/z2012-05-17-521800x600.jpg

oops.... just spotted that on yr list!

a decent blast to get the cats nice and hot helps too

[Hagasan]

Hmmm, not easy.

You can lower HC when you do the test with fresh engine oil but your values are ok.

High CO points to inefficient cats, that's true. But as your car is a 1995 you have two sensors per exhaust pipe. The downside one should act up if the cat is bad but maybe it's not bad enough.




A UK 95 car only has pre-cat o2 sensors. OBDI. I do the hot engine trick at mot time... Cool engine/cats fails.. You might just need that and an understanding/patient tester you can talk to ;)

[Silver Surfer]

Hi Duncan

I have a solution to your MoT issue for road worthiness certification.....it's based in Colchester if you are willing to travel. You can then take your time to work out your emissions issue at leisure.
My solution passes my decat SC rich running car.����

[NSX100]

How hot was the car when tested? Had it been sitting for a while? With mine, I take it for a blast up the motorway en route to the test centre, then it passes....if I don't do that it will also fail the emissions.

Same here! I had the MOT done on my car after its winter "break" having basically just driven it to the garage. It failed the emissions test. The garage called me up and suggested I take the car for a 10 mile drive with "reasonable speed and acceleration" (!). I did so, returned it to the garage and it passed with flying colours.

[goldnsx]

Every emission test should be done with a completely warmed up engine, warm oil and hot cats. When I arrive to the station I don't shut the engine down for the wait. I'll let it idle for the cats to remain hot right until the sniffer test is done.

After an extended winter storage where you don't start the car every 1 month or so you'll be facing injector problems which can lead to poor idle and moreover emission problems.

During the certification of my CTSC my car had to undergo a NEFZ emission test (20 minutes or so) in a test laboratory. The engineer told me that it's not necessary to drive like an id*ot to free the cats. You'd better drive with high revs on the highway (AFR 14.7) for half an hour or so.

[duncan]

Thanks for all the suggestions, all gratefully received.
My [cunning] plan had been to repair the front flexi followed by a simple re-test. With the system 'air tight', then at least every thing should be normal.
What transpires is that my garage was generous in its assessment of my Advisory, not only was the front flexi blowing but once the heat shields were removed from the main silencer that was completely shot, probably not the cause of the failure - but - in definite need of replacement.
22+ years, 123,000 miles of use in all weathers revealed itself.
So look away now if you're of a nervous disposition.

1303713038130391304013041

What a weight the OEM silencer is, if bricks could rust, I'd say built like a brick ****house.
Anyway, onwards and upwards.
Once i've made up my mind what to do I'll be posting in the wanted section.

[Silver Surfer]

Have PM'd you Duncan.

SS

[goldnsx]

The OEM silencer is pretty light regarding the low noise it emits. I've seen lightly louder exhausts with 5-6 kg more weight.

[darock]

Clean the AIT Sensor. That helps a lot.

and then have the injectors cleaned. this also helps.

and in the end, its the cats. They age and need a lot of heat to work. You can test that by accelerating hard and immediately put in the measuring device. If its ok then, the cats are bad.

Bernhard

[duncan]

Chaps,
Thanks to those who made a range of suggestions and to those who sent PMs, all very gratifying.
Third bash at MoT and a pass, the resolved issues were,
Split flexi on front down pipe, not really definable until the pipe was removed and the external braiding cut off. The braiding disguised the extent and nature of the air leak. If you can feel a waft of air or see discolouration on yours be aware.
Once sorted, the O2 readings were even worse - hence fail #2, so both O2 sensors replaced (no warning codes or lights) and now an exhaust system that stops the planet's tears.
Be aware also, Denso Sensors DOX-1459 have a wire length of 350mm rather than the 450mm Denso list them as having so are too short. Once returned it was cheaper to use Denso universals and reuse the OEM plug and support sleeves.
In the course of the above, every hammer-in stud has been heated & un-hammered, every heat shield nut & bolt replaced, every knuckle skinned and every expletive uttered, I'm sure my tinkering won't last a further 22 years but at least should be easier next time around.
My thanks to the forum for it's support.
Duncan

[WhyOne?]

Pleased to hear that you are finally sorted Duncan.