Running too lean :o

[menguin]

My car has suddenly started running lean according to the air fuel gauge.

I haven't changed anything on the setup but it is always showing lean now

I'm running emanage blue and it was tuned by AFM in July last year. Everything has been absolutely fine up until now! Can the emanage software 'lose' the tune? I'm in the Channel Islands so it is a bit of a trek to find anywhere that has any knowledge of FI - least of all MX5s with FI! So I'd really appreciate anyone's ideas as to what it might be...

Could it be a leak somewhere, causing the AFM to give incorrect readings? I'm not particularly clued up on oily bits or forced induction, but am keen to sort this out as I know running lean isn't good!

I have been driving it for the past couple of days at very low revs - no throttle.. Will this still be causing damage?

Any advice would be much appreciated.

[MX-5 Lazza]

Sounds more like your AFR gauge is faulty. Which gauge/sensor do you have?

[D_G]

If it's reading the lambda sensor output and the sensor has failed it will show a permanent lean mixture, i.e < 0.1v.

[menguin]

It is as far as I know an unbranded sensor... literally nothing has changed since yesterday - I thought it might be a faulty sensor, but am very afraid of this! As I'm at least £500 from any kind of professional help (unless there are some hidden PHers in Guernsey) I'd like to make sure I have checked all I can do here, before spending the money and travelling to the mainland!

[skinny]

how lean is lean?

a little bit lean might be an airleak, a lot lean is more likely sensor.

does the EMB do closed loop control based on the wideband AFR output or is it just used to establish your tuning?

[MX-5 Lazza]

The stock ecu uses the NB sensor to control fuelling out of boost, the EMB only comes in when you go into boost (assuming the O2 clamp is properly set up). EMB only uses the WB for logging, it doesn't use any sort of O2 sensor for fuelling, it just runs off a map. The only sensor it reads is the MAP sensor.

Is it lean all the time or only on acceleration i.e. in boost? If it's only in boost then it might not be getting a MAP signal. Check that the wiring from the sensor to the EMB is ok. Mine used the small 3-pin input on the EMB for the MAP input which was prone to coming loose...

If it shows lean all the time then the WB O2 sensor is more likely. You can get that tested at any MOT station. Just get them to test the exhaust emissions. If they are not the same as your AFR reading then you know the sensor is faulty (the cat if you have one makes very little difference to AFR).

[menguin]

I've not got a wideband sensor so the technical diagram below shows where it is on my gauge:



It is running like this all the time - off throttle, on boost.. Thanks very much for the suggestions, will go and get the O2 sensor checked out

[MX-5 Lazza]

Does it seem to drive normally? If it's really running that lean it will have absolutely no power. I reckon it's the WBO2 sensor that's died. The good news is that it doesn't make any different to your setup - the EMB doesn't use the WB signal at all and the stock ecu will be running from the stock narrow band sensor.

[menguin]

It is driving perfectly - no loss of power at all. So that means it is probably the 02 sensor? Phew! I thought it was strange it could go from a good map with no trouble to completely lean. Cheers for your help, will go and get the sensor checked out!

[MX-5 Lazza]

The sensor itself should be pretty cheap. Can be a bit of a pain to get to but should be DIYable. I've swapped mine twice!