MOT EMISSIONS FAILURE - ADVICE PLEASE

Hi All

My girlfriend and I have just bought a 1990 Eunos Roadster MK1 as her main car. She loves it…

We just put it in for the MOT and it failed on emissions. The garage did the correct test for a car pre 1992 but the numbers came back high on CO i.e. 10% against a target of 3.5%.

The garage think it may be a leak in the exhaust manifold which I will check out this weekend.

When we bought the car, it was doing the tick-over hunting thing and stalling at roundabouts. I did the base idle reset thing which has improved the stalling, but the hunting still happens. It drops below idle comes back up over idle and does it again about three or four times before settling down.

Would an exhaust manifold issue explain this? I would imagine the O2 sensor in the down pipe might be getting confused and adding more fuel if the manifold is sucking air in at tick over.

Any advice and help would be really useful

I have checked previous post but couldn’t find one that matches this exactly.

Thanks

Rob

https://www.mx5oc.co.uk/forum/yaf_postsm676482_MOT-Emissions-fail–CO-high-only.aspx                    This of any use ?

After the sensors, Check the AFM, we had a similar issue and re calibrated the AFM which fixed it.

Also invest in a Fault Code reader and see if you can see any fault codes.  

You can get a basic one from Autolink: https://www.autolinkmx5.com/diagnostic-fault-code-reader-led-mazda-mx-5-mk1--mk2-6665-p.asp

The hunting may be caused by a weak battery, put a voltmeter across it and check you are getting approx. 14v, if not might have an issue with alternator or simply belt tightness.

 

 

 

 

That could be something as really simple as a vacuum pipe between 2 points causing that. Have a look around for any corroded pipes and replace if you need to. 

Once everything else is checked and car running smoothly have the car Terracleaned.

It will reduce emissions and improve performance.

Around £100, discount to MX5 Owners Club members.

Hi All

 

Thanks for all the suggestions. 

Just to close the loop on this one, I bought myself one of those basic fault code readers and it said that the O2 sensor wasn’t changing reading, so fitted a new O2 sensor. Took it back to the garage and it still failed with no change in reading.

Next step, I contacted the guy we had just bought the car off and he suggested that it might be the garage being dicks and suggested taking it to the place it had been MOTed the year before. I did this and it passed first time with a reading a tenth of what it was getting at the other garage. Their comment was that the other garage probably didn’t have their analyser set on the right setting ( I had asked them and the assured me they had) or that they hadn’t got it hot enough before testing.

My advice and learning from this.

The simple fault code reader is probably worth having for the money and did point to a failed part.

If you find a garage that is sympathetic to older cars and is willing to go beyond “computer says no”, stick with them and spread the word, so

I DO recommend

Heathcote Auto Services

425 Tachbrook Rd

Leamington Spa

CV31 3DQ

 

I DO NOT Recommend

Comptons Garage

77 Rugby Rd

Leamington Spa

CV32 7HY

Glad your car passed but out of interest what was the CO2 readings on the second failed test and then on the test that passed? I know there is a debate about which limits to apply to the early cars, especially if they are Eunos imports but I believe 3.5 percent is the max. You say there was no change so was it still 10% on the second fail? If there is a difference in reading between the two garages when nothing has been done to the car that would be interesting. TSTL
D

Some years ago I got caught out by an MOT tester who applied the wrong limits to a Volvo I owned.

I couldn’t understand why the car had failed so miserably because I’d only just serviced it (as I always did), I knew that the engine was in vgc and it had no problems the year before. After the car failed the test I went straight down to my local Volvo dealer and bought an expensive carb overhaul kit (It had a complicated Stromberg carb with a lot of stuff relating to cleaner emissions) and I fitted it the same evening, no small job on that car, which took me up to midnight to fit. Everything else I could think of had already been renewed when I serviced the car before the test. By chance, later that night I read an article in a car magazine which had a table showing the emissions limits. I checked the readings printout for my car and realised that the readings were well withing the required limits and it should have already been given a pass.

I went back to the MOT station first thing the next morning and showed them what they’d done. They were adamant that they hadn’t made a mistake and tried to tell me the engine needed an overhaul! I then asked the manager to show me what limits the car had been tested to, which he did - wrong limits! I pointed out their mistake - they had misread the registration number of the car; they had looked at the first letter of the registration and tested it as if it was a “prefix” registration. It was in fact an older, suffix plate. They said I would now need to rebook the car in and pay for another entire test. That was despite a fail on emissions being a free retest item!

As soon as I mentioned the word “appeal” they had it straight on the emissions tester and obviously it passed again.

They went out of business not too long after - good riddance - robbers!

Reading at the first MOT place was 10.5% on the second fast idle test on all three attempts. The car is A 1990 and isn’t subject to a second fast idle test according to the regulations on the .GOV website.

Back to the garage that tested it the year before and registered 1.28%.

I think that that is a bit more than just getting it warmed up.