Hi, My Late 92 Roadster failed its MOT today. It failed on the Lamda part of the emission test. Pass is 0.970 to 1.030. Mine scored 1.046!!! The tester stated that this was somthing to do with the Air/Fuel mixture. Could this be the oxegen sensor that is faulty and will replacing it give me a pass ??? Your guidance would be much appriciated. Cheers Paul.
could be that the car was not up to temperature so was running a rich mixture. did they test the oil temp while they did it?
Yes he did go to the second fast idle test and tested the oil temp. 1st test it was 1.099 2nd test down to 1.046. What is this actually testing?
oil temp, to make sure engine is up to temperature to get accurate emissions test.
emissions test is measuring the emissions lol. so if your catalictic convertor, if fitted is working or if the air/fuel mixture is correct.
if the lamba sensor is faulty then the ecu might be giving the wrong mix.
This reading is a figure of your air to fuel ratio. Lambda 1 is the perfect 14.7:1 air to fuel ratio. Greater than 1 (which yours is) means that your car is running slighty lean. Depending on the year of your car if an oxygen sensor was not working the EML light should come on as its an emissions related fault. Running lean could suggest an air leak. Somewhere to start? Hope that helps
I understand its testing the emmisions. But it appears there are 3 parts to the test. 1- CO which it passed. 2 - HC which it passed. 3 - Lamda which it failed. Obviously I dont want to replace this and pay for a retest only to find it fails again. Could it be somthing else or would you go straight for the Lambda/Oxegen sensor.
i’ll leave that answer to someone with more experience of early mx5s
Has anyone got any more thoughts. Your help would be really appriciated.
CO% is carbon monoxide, partially burnt fuel. Nasty killer stuff should be as low as possible (i.e. an engine in tip top condition will burn the fuel very efficiently). HC is hydrocarbons, unburnt fuel, not as nasty as CO but again the less the better. Lambda is not a gas, but as stated a ratio between the air & fuel being burnt in the engine. A catalyst is most efficient at 14.7 parts air to 1 part fuel. Lambda is controlled by the engine ECU injecting a controlled amount of fuel based on the feedback from the Lambda sensor. Lambda can be affected by many things, for example a faulty lambda sensor (which is quite common on MX-5’s), an air leak in the intake system or an air leak in the exhaust. The sensors output can be tested with a multimeter, but I’d be tempted just to change it.
Your CAT may be buggered? You might need to replace to 02 sensor. Or just give your car a good thrash, get the cobwebs out[;)]
A failed catalyst will not affect lambda, the cat has no effect on the air/fuel being drawn into the engine. I suppose the lambda check is a test that the vehicles engine management system is working correctly.
No check engine light on any Mk.1 Roadster or Mx-5. It could be the lambda sensor, but I agree an air leak in the exhaust is a possibility: I’ve heard of cases where people with twin outlet exhausts have failed due to pulsations drawing fresh air into one of their twin pipes. Stuffing a rag into the other pipe let them pass.
Thanks Gents, Found a loose joint where the back box joins the pipe from the cat. A fair bit of soot around it. Clean it up, reseald and fitted a new clamp. Got the old O2 sensor off, just waiting for MX5 parts to deliver the new one. Will get it re-tested next week. Hopefully it will get a pass. Once again thanks for the steer.