NC MX-5 Emissions

Hi guys, first post on the forum as a proud, new MX-5 owner.

  1. My model of MX-5 is: 2007 2.0 NC GT PRHT

The car is in very good condition aside from bushes in the front wishbones needing doing to pass the NCT. The emissions are slightly out as well and I was hoping you guys would be able to help me.

Engine/Oil Temperature: 97C
Low Idle (760 RPM) CO 0.50 vol% (0.30% maximum)
HC 34ppm
High Idle (2828 RPM) Lambda .98
CO 0.83 vol% (0.20% maximum)
HC 22ppm (200ppm maximum)

The car is stock engine and exhaust wise apart from a cobra sports backbox. It is probably overdue a service by inspecting the oil and air filter, I have yet to check spark plugs/coils/MAF for fouling.

Got the car warm and at idle, the AFR is reading 14.3 with the downstream lambda sensor reading .9V

My first thought is obviously a fluids and filter change, and to check spark plugs + clean MAF. But it seems likely to me that the upstream lambda sensor is failing, would I be right in thinking this? There is a slight bit of smoke from the exhaust when you blip the accelerator at a standstill, and you can smell the petrol from the exhaust so it’s definitely running rich, however the car idles and drives well with loads of power and no hunting from the engine. It also seems to get up to and maintain operating temperature just fine.

This will be my first petrol car so please go easy on me and thanks in advance for any help!

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Where does the coolant temp marker read on the gauge, should be just short of midway?
I ask as the stat can fail at your age of NC leading to the engine no operating at the correct temp.

Had it out for a spirited drive and can confirm it does settle at the halfway mark.

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I was just consideri g the emissions test this morning.
I have the cobalt box, the left pipe gets dirtier quicker than the right.
Visualising where the probe goes at my local mot station, he always pops it in yhe right hand side.
I wondered if this could affect emmissions if say most of the gasses are coming out of the other pipe.

Probably doing the usual trick and burning oil.
Keep an eye on the consumption of that…

Your lamda sensor I believe should read more like .45v not .9 so it does support your upstream lambda sensor theory although the upstream sensor is a pukka AF sensor not a basic Lambda so of course is much more expensive.

I had very similar issues to this a couple of years ago, even down to the low idle speed and it turned out that a new MAF and AF sensor did the trick. Got some Denso items from Opie oils

Yeah, I had a feeling from the research I had done that it is the MAF + upstream Lambda sensor.
Not the worst thing to do, couple of hundred euro all in. Will do a full service, clean the MAF and get a new Lambda sensor and she should be perfect.