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Hi guys, im trying to get to the heart of a lean fuel trim problem with my 2006 mazda 3 2.3 litre. My long term fuel trim is constantly above 10% and my AFR jumps up to 29:1 every few min for some reason. I dont have any vacuum leaks and i just changed my exhaust manifold gasket yesterday which was leaking and spraying gunk on the back of my engine. The car is really down on power but still runs, doesnt stall out and hasnt given any codes. Changing the manifold gasket helped the problem a little but the car still seriously lacks power. I bought a good scan tool today and im starting with the basics checking everything to narrow down the roblem, starting with the o2 sensors. The car has an upstream and a downstream sensor and the scan tool shows one in volts (Bank 1 – sensor 2) which at idle (after the car has been running for some time) gives a reading of around 0.075 volts. The second (Bank 1 – sensor 1) is shown in milliamps fluctuated a little from around 0.016 mA to -0.008 mA but the majority of the time it sat at 0.004 mA. These numbers seem suspect to me but im not an expert. Can anyone chim in with some advice? Thanks 🙂
At the time same time of the O2 sensor readings the fuel trim was at -1.6 short and +13.3 long
also i have a ton of other readinds here if u need anything.
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The one that reads in volts should be fluctuating between 0.1 and 0.9v and will need to be measured using a scope or a scanner with a logging function that can graph the log. The one reading in milliamps isn’t actually an O2 sensor at all, it is a wideband air-fuel ratio sensor and produces a steady currrent output in response to air/fuel conditions. A lamba of 1.0 mA generally indicates perfect stoich (14.7 parts air : 1 part fuel). More than 1.0 indicates a lean condition, ex. 16:1 = 1.09) because the air part of the ratio is higher, and less than 1.0 indicates a rich condition because the air side of the ratio is lower (ex. 14:1 = 0.95). Take a look at these:
I did this test using a scanner that logged about 34 frames and at no time does the voltage sensor reach 0.1 volts although it does hit 0.095 Volts in one frame. Ok so my other question is where does that wide band one reside? is it actually the second more downstream one by the cat or is that one not reading in my scanner for some reason?
The wideband sensor is Bank1 Sensor 1 (upstream, closest to cylinder head). The Zirconia O2 sensor is Bank 1 Sensor 2 (downstream, closest to catalytic converter).
ah ok. so the readings might be ok then and the lean condition is prolly caused by something else. ill try the scope feature tomorrow and see what the frequency looks like. thanks.
Do you have a MAP (manifold absolute pressure) or MAF (mass airflow sensor)? If MAF, it might just needs to be cleaned for better gas mileage although you didn’t mention that. Have you checked your engine mounts? Other than air restriction with a somewhat healthy vehicle, the #1 loss of torque response is worn engine mounts. I just made that up 🙂
That was my problem at least.
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