Advice needed car running rich

[Yang]

Hi,
2007 BMW 320i e92 2 litre petrol 106K miles
check engine light
codes suggest nitro sensor signal rich, post cat o2 sensor signal stuck rich
Check engine light came on after driving on the motor way for a couple hours.

previously an MOT test revealed there is a minor leak of exhaust gas front, so could that be the issue?
There is a rough idle when engine is cold.

Plus: replaced nitro sensor at 50K, a post cat o2 sensor at 90K. replaced 2 front o2 sensors last month, but didn't help.

Does anyone know what I should look into?
Thanks.

 

[thomp1983]

Wouldn't be surprised if you've another failed nox sensor, they don't last huge mileage unfortunately especially if things aren't running right.

Any leak in the exhaust system before the sensors needs fixing you can't expect them to work properly if unmetered air is getting in

 

[Yang]

ok thanks, will look into the leaks first.

 

[Avocet]

I'm not sure what a "nitro sensor" is, in this context? Does it check for NOx? I didn't think they had those in 2007?

As for running rich, it might be a coolant temperature sensor that has failed. If the engine "thinks" it is cold, it will richen the mixture. Normally, as it warms up, the coolant temperature sensor tells the engine management computer to weaken it off, but if the sensor has failed, it won't do that. You can usually put them in a pan of hot water with a toffee thermometer and measure the resistance at different temperatures. You might be able to find the correct values of resistance for your car online.

Alternatively, if an O2 sensor is "stuck rich", it could mean that the engine is running lean and that the sensor is working correctly, but has gone right to the "richen" end of its range, trying to tell the engine management system to richen the mixture because it constantly thinks it is running lean. That does sometimes happen with an exhaust leak, because it sucks air in through the leak, and gets the wrong idea of mixture strength. Id definitely try fixing the exhaust leaks before you spend any money.

Lastly, there are some really rubbish, cheap O2 sensors available online. It's worth buying a trusted brand like Bosch or NTK.

 

[Yang]

Thanks for the advice, I will take the car to garage next week to check for leaks in exhaust. And hopefully they can find out if the air fuel mixer is wrong or not.

 

[Agile]

My first thought would be check the air filter.

When driving through dust clouds I always virtually depower the engine to prevent clogging the air filter but see other drivers who charge off oblivious to the potential problems.

 

[freddiemercurystwin]

They're the same ones who blast through really deep puddles without a care in the world.