Crazy Lambda Readings - Any Ideas?

Hi all,

Just got my car up and running but noticed the AFR is oscillating a significant amount. It is steady at idle but goes nuts when changing the rpm even with a steady TPS. Occurs with and without short-term fuel trims active.

Link to map and log (just driving around the block once up to temp):

The car is a Ford Fiesta XR4 (ST150), running an EMU Black, and a 2.5L Mazda L/MZR engine with coil on plug, VVT and a factory VICS intake manifold. This time around, I have the ECU mounted inside the cabin with a fairly long vac hose running to the onboard MAP sensor.

So far I have swapped in new plugs and different coils but this did not affect the result. I did attempt to check for exhaust leaks around the exhaust manifold gasket and sensor area but could not spot or hear anything obvious.

There are a few things I suspect could be possible causes:

  • The oxygen sensor is failing
  • Something is not set up correctly in the tune
  • Some sort of wiring issue?

Does anyone have any ideas or experience that could point me in the right direction?

Cheers in advance for the help! Just don’t want to waste money on a new sensor if it is possibly something else I have overlooked! :slight_smile:

(PS, for the longest time, I thought I was supplied with an LSU 4.9 sensor. After reading the part number which didn’t match a 4.2 or 4.9, I asked the supplier and was told to set it up as an LSU 4.2. Thus, it still has the 150ohm resistor I installed that would be required for the LSU 4.9, could this be another potential cause? Part number of the sensor is 0258007353)