- My model of MX-5 is: NC1
- I’m based near: Gloucester
- I’m looking for technical help or recommendations on: Resolving emissions/Exhaust Replacement
I’ve owned the 2007 NC1 2.0L for 18 months and it won’t go through the emissions this time (Lambda was 1.04 and needed to be between 0.97-1.03). After the replacement of 2 faulty coil packs and the fix of a blowing exhaust joint, and running it hot, it still couldn’t quite get through. I’m after some advice on the best way to proceed…
The previous owner fitted a stainless exhaust and I subsequently found out they had a 4 into 1 header fitted and a remap with a post-cat O2 sensor delete. There is a sports cat in place.
I’ve spoken to a tuning specialist and they are sure it is likely to be the cat that is at fault.
What is the best option here? Would a replacement or additional sports cat solve this issue? Do I try to get an original mid-pipe with cat? An oem cat and fabrication to make this fit (I assume a remap would then be required?), is there another option?
Are there any recommendations for an MX5/exhaust specialist in Gloucester to look a this (I believe a vehicle can be driven without an MOT if going to a place to resolve an MOT failure issue).
FI, the outer diameter of the pipe near the cat is 5.4cm and the pipe on the cat that fits over this is 6.2cm. The cat iteslf is approx 14" long.
I’ve attached some photos for reference.
Thanks
I think the sports cat is probably the main culprit. Many folk are running a remapped catless sports manifold in to the OE centre section & OE cat with out emissions failures from MOT. A chat with Alex at Performance link in Tewkesbury will probably point you in the right direction.
Thanks. I did call performance link originally who pointed me at Mazspec for a potential oem cat replacement, but now I’ve seen the situation I’ll make contact to see if they can just replace the sports cat. I was considering selling it for something more sensible as we’ve a young driver in the family, but i assume it would be more saleable after a new cat and valid MOT?
A valid MOT is always desirable!
This is possibly going to sound odd but I end up blocking the left tail pipe on my exhaust with a rubber bung during the mot.
The lambda numbers were like yours ( 1.04/1.05 ish )
After blocking the left tail pipe whilst the sensor is in the right tail pipe the numbers reduced to exactly 1.0
The only reason the mot guy and myself can think for the change is that a free flow rear box is acting like an exhaust leak with both tail pipes open.
That’s crazy! I would have thought it would be worse as all the exhaust gasses are going down the pipe that is being checked. I’llcheck the mot testers are amenable to this process as it will be significantly cheaper than a replacement cat 
Thanks
The high lambda reading when CO and CO2 are normal is generally due to extra oxygen ( usually caused by a leak )
If the other readings for emission were good - and only the lambda reading was out - give it a try.
Where this is the issue, is the left tail pipe essentally sucking in air which then exists the right pipe, increasing the percieved amount of O2 in the ‘exhaust’ gases?
That was our conclusion - yes
The bung in the left pipe seemed to support it.
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I’ve spoken to the garage and apparently they did block the other pipe when testing, so unfortunately not this, but an exclent shout nevertheless. I’ll look in to getting a replacement cat, probably sourcing a standard mid section with cat and finding an exhaust place to hook it up however required. Can anyone recommend a suitable gloucester-based exhaust place that is mx5-sympathetic?
