MX5Ireland

Hi all,
my 1998 mx5 mk2 1.8 rs (jap import) failed the NCT on CO emissions. the readings were as follows:
Engine/oil temperature 92 c

Low Idle CO 0.14 vol% - Pass
(790 rpm) HC 117 ppm - N/A

High Idle Lambda 1.00 - Pass
(2800 rpm) CO 0.64 vol% - FAIL (above 0.30 % is a fail so mine is over double)
HC 77 ppm - Pass

Don't know much about mechanics but I managed to change the engine oil, filter, air filter, diff and gear oil, spark plugs and rocker cover gasket about 2 weeks ago in advance of test. Car is driving fine, and I gave it a good drive (2-3 miles at 140km/h in 6th gear on motorway to NCT centre). Had to wait about half hour before getting tested. Been driving car daily for last couple of months to work but not at much more than 3000 rpm.

Will try Roberts "dipethane" suggestion 1st as a short term solution to try and get it through. But long term (or if the dipethane doesn't work) I'd like to get to the source of the cause.
So I need some advice on what the probable cause would be in the order of trying the cheapest and simplest solutions 1st. As I said Im no mechanic so the nct readings don't mean anything to me and I don't have diagnostic tools or anything like that.

Any advice greatly appreciated,
All the best.

I had the same issue as this recently, exact same year and model (Although mine is now turbo'd).
I tried the Dipethane, but the emissions were even higher! This has led me to believe the cat has gone, so I will be replacing it hopefully in the next 2-3 weeks, with a retest scheduled on the 15th.
I also adjusted the map on the ecu slightly at those rev points so that it is less rich. Of course, though, this won't matter to you as it's N/A.
I'll let you know if I pass or not.

Sound Randy, just talked to local mechanic here and he has the equipment for testing emissions, so just going to drive it with dipethane for a couple of days, get him to test it then and take it from there.

Well exact same problem as yourself. Emissions now higher and cat is gone. Where did you source your replacement cat and what type is it as I think there are a few different types but if our models are the same (98 mk2 1.8 rs jap) then the cats should be same??

tbh I bought one that came off a civic and it's currently being welded in place.
Looked to be in great condition. Will let you know the results when it goes through the test in 2 weeks time.
You're right though, cats would be the same, but the cat on a mk2 is part of the middle exhaust section, so if you want to replace it you might have to place that part too.
I was just lucky with this, as the guy who owned the civic wasn't going to be driving it on the road, so it's easier for me to buy it and get someone to weld in, rather than go looking for a whole mid section. Will make it easier to de-cat as well if it passes.

Getting one off Robert from an mx5. he has cut it off and says its in great condition so gonna get it welded in next week sometime.

Picked mine up this morning and car feels way more responsive with the leaks fixed also, and no more annoying boost cut either. Hopefully it passes now..

Got 2nd hand cat welded in this morning. Before they put the replacement cat in they did emissions test which had now gone from CO 0.64 vol% (high idle nct test fail) to CO 0.83 vol% (anything above 0.3% is a fail.) After welding in the replacement cat it dropped from CO 0.83 vol% to CO 0.04 vol% so old cat obviously not doin its job. Retest on Friday morning so it should be fine.

Wow. Huge difference. Hope I'm as lucky on Saturday..

Alot of small local garages have those emission testers now and most exhaust centres if ya wanted to check it before the test it only takes a couple of minutes. I'd say your cat replacement should have done the job as your symptoms were the same. Fella in the exhaust place said the readings would improve more after a few drives if the replacement cat had been lying up for a while. Good luck with it anyway.

Jihhny ,add the DIPETHANE as suggested !!

Had NCT retest this morning and it flew the emissions. Thanks for the CAT Robert it was a good one. Results below, compare these to the original above.

Engine/oil temperature 66 c

Low Idle CO 0.01 vol% - Pass (above 0.50% is fail)
(820 rpm) HC 19 ppm - N/A

High Idle Lambda 1.00 - Pass
(2860 rpm) CO 0.02 vol% - Pass (above 0.30% is fail)
HC 17 ppm - Pass

Those are crazy results. I have serious hopes now.

Hi Johnny ,good news .

+1. The 10th AE failed on emission Today.
Mine is slightly different tho as idle is too high as well. 0.72 % idle and 0.75% at 2800 rpm. I always thought the car is running a bit rich but never checked because it was perfect otherwise. No excessive fuel consumption or other issues.
I had the very same problem a few Years ago with My impreza and all that needed was a new MAF sensor. However that one was drinking petrol crazy so it was obvious something was wrong.