Mk1 CO emissions too high

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: __Mk1 1998
  2. I’m based near: North London
  3. I’m looking for technical help or recommendations on: __exhaust CO emissions too high

The car has done 35k miles. Prior to the cat being stolen typical CO figures were 0.04%, HC 20 Lambda 1.00

Only third party Cats are available and with the one we fitted it just passed the mot last year with a CO of .25 (the max limit is .30)
HC and Lambda readings were about the same as before.

This year it failed with CO of .57 (HC and Lambda unchanged)
I changed the O2 sensor and it failed again with a similar high CO

I have checked and there are no exhaust system leaks

I am assuming the cat I fitted is not great quality so can anyone please suggest a brand that approaches oem standard?

Thank you

2

You should get an exemption for having an old car that done nothing miles most years and therefore is not a threat to the environment. But this argument will fall on deaf ears.

3

You are correct, those Cat’s don’t last. I had one on my MK1 and it lasted years…I took it off and put in a silenced pipe after every test…refitted just before the next test

4

Check the AFM, we had a similar issue and adjusting the AFM solved it.

5

I will do as suggested but is it likely that the AFM went bad immediately on fitting the third party cat?

6

Our 98 Mk1 failed on emissions about 10 years ago after scraping through a few years on the trot. At recommendation of garage Cat was then changed from OEM to an aftermarket BM Cats one.

Sailed through emissions test for first few years after cat change, but starting to get a bit borderline on recent tests.

Always give it an oil service, and run through a bottle of cataclean on lead up. Then give it a good high revving run on way to MOT station and straight onto emissions test on arrival ( helpful MOT guy).

So, my aftermarket cat hasn’t done too badly, albeit that it does need to be red hot at the test to get a pass these days.

7

Sounds coincidental, ours was due to change from standard to cone filter if I remember correctly not linked to the cat.

8

An MOT tester with 30 years experience looked at the test results.

With the OEM cat CO 0.04% average over several years then the new 3rd party cat scraping through with .25% then failing the next year.

The tester suggests that no third party cats are made to the same standard as OE ones and the most reliable fix it to get a similar size OEM cat fitted by an exhaust specialist. He suggests that as a sensible alternative to changing third party cats every couple of years.

Maybe that is the way to go, unless forum members can suggest a brand of third party cat that performs well and will last more than a couple of years.