Hi,
My car just failed the NCT (irish equivalent of MOT). The lambda value at high idle was 1.11. Having had a look on the web I can see that lambda=1 is the ideal air/fuel ratio and that greater than 1 means the car is running lean.
My question is, what can cause this and is it a big deal to get it fixed?
btw, I’m not a car person, so I will be getting someone to do this for me, just want to know if it is going to cost me much.
Your engine is receiving too little fuel, or too much air, or both. You will have to approach this from both directions. The simple things to check are fuel filter, (the one near the tank), fuel line restrictions, fuel pump output or anything else preventing fuel from flowing freely.
You should also look to the air intake path. Typically there is a sensor that reads the amount of air flowing into the engine. Any air leaks after this sensor can result in a lean combuston. These airleaks can be in the form of loose fittings on the various pieces the air flows through, to a loose intake manifold. Also, loose vacuum fittings/hoses can do this, so look for loose, burned, or cracked lines there. There is a possibility one of the sensors, airflow, air temp, even the oxygen sensor (lambda) can be bad. This requires special tools for testing. If it isn’t one of the simple things, you’ll probably need to have it diagnosed by a pro.
It’s hard to say. The possiblilies range from the fairly inexpensive, e.g. a vacuum hose off or broken, to the mind-bogglingly expensive, e.g. failure of certain fuel injection system components. We’ll hope for the former.
Lambda value greater than one means that your car is enriching the mixture due to a lean condition. Typically an air leak, which can be on either the intake or exhaust side of the engine
Yes, that is correct an air leak in the exhaust near the Lambda sensor can cause a high lambda value.
Another possible cause of this reading is a bad sensor, but an air leak in more likely.