Looking for some assistance with a long standing fuelling issue I have with my 98 MGF which is causing the CO to be sky high.

Apologies for the length of this post but I want to share all the checks I've performed in the hope that someone can help narrow down what is wrong.

Parts replaced:
- Cat replaced for MGOC standard i.e not an ebay special
- New O2 sensor
- New Coolant temp sensor fitted (brown one).

Despite the above the CO remained unchanged.

However, noticed that whilst at the testing station the cooling fans didn't kick in at any point. The temp gauge remained at 'normal' but my spider senses were alerted. When I next got an opportunity I tested by unplugging the coolant sensor (brown) and the fans kicked in immediately.

Plugging the PSCAN in I could see that the coolant temperature was being reported but the O2 sensor voltage remains at a constant 0.49v regardless so I went hunting.

I have tested the individual conductors from the O2 sensor back to the ECU (pins 27 & 28 on red connector) and these are good.
I have tested the heating element in the O2 sensor and this looks good using Dieter's pages as a reference.
Placing a multimeter across the O2 sensor I can see the voltage steadily rise as the device warms to peak at 0.9v. From this I consider the sensor to be good?
Engine earthing point has been cleaned to ensure a good contact with ground.

I have attached a couple of images as I'm confused by the behaviour of the ECU. My understanding is that the ECU should operate in open state until the coolant temp reaches 'normal' running temp (approx 90 degrees). However, if I'm interpreting the data correctly, mine appears to operate in closed mode when idling regardless of temperature. When I then pick up the revs the ECU drops to open mode - returning to closed mode when the revs return to normal.

Am I correct in thinking that the O2 value of 0.49v is not necessarily coming from the sensor but is actually a default value from the ECU itself?

My thinking is that the ECU is at fault here as it appears to be ignoring the coolant temperature despite the fact it is reporting what I believe to be true values. 

I do have another ECU for a VVC but unsure whether I can simply swap them over or whether that will cause other issues?

Any advice would be greatly appreciated.
 

Read More...