hi folks.

tinkering with my son's car to replace the inevitable thermostat housing failure (caught the retaining clip heading down into the engine). job done, but while refitting the lambda probe connector noticed that one of the wires from the ecu (white) was broken and looked as though it may have been cut intentionally.

why would anyone do this? only thing I can think of is that the probe was replaced at some point with a later/different model that doesn't need the white wire and leaving it intact affects engine performance adversely.

the car's done nearly 100k so is a bit lumpy and rattly at times anyway, but apart from the tickover being a bit variable and lumpy, reconnecting the wire doesn't seem to have made much difference. it was cut very close to the plug and the wire was too oily to solder, so I have made the connection with a screw terminal, which isn't a long term fix.

anyone got any ideas or suggestions? the wiring diagrams in the book of hayne aren't much help as the colours don't seem to correspond at all.

thanks guys

The original lambda probe (aka Oxygen sensor) has 4 wires: 3 of them are connected to ECU, and the 4th to +12V.

The heating circuit runs between the black and the brown wires, while the measuring circuit runs between the white and the yellow wires. The latter is using shielded wires. 

 

 

Depending on model, oxygen sensors can have 2, 3 or 4 wires.

 

2 wires - only measuring circuit, no heating circuit (older models)

3 wires - measuring circuit and heating circuit share one wire (usually ground)

4 wires - separate measuring and heating circuits

 

It is possible that at one time somebody fitted an aftermarket oxygen sensor that has 2 or 3 wires. Either way, having the white wire cut you didn't have a measuring circuit all that time. I recommend taking the sensor out, finding what model is and assessing if it is fully functional, and eventually fitting an original one. Keep us posted

Thanks, Adurer, I will try and get some more information about the sensor and post again. Your help is much appreciated.

F.

Well, the uneven running seemed to get worse and worse, so I took it to a local garage and they connected a diagnostic device and found that the lambda probe was reading faulty and the fault returned whenever it was cleared, even after the wire was repaired. It was already a 4-wire probe, so not an incorrect replacement.

 

Next they replaced the probe and bingo - problem solved. Lumpy running and cutting out gone and the probe fault moved to a stored fault. The garage advised that the coolant temperature sensor had also reported faulty, which again I assume must be historic, because I replaced the sensor when I changed the complete thermostat and housing assembly, which comes with a new temp sensor.

 

So, that all seems to be sorted out and the car is running as well as a 13-14 year old car with a rattly timing chain can. The garage said they replaced the timing chain on another Felicia not long ago and it's back to rattling just as noisily - and there's no tensioner or other means of adjustment, is there?

 

That's it for now!

previous lambda probe: it was probably a cheap 3rd party non standard one

timing chain: a new chain followed by precise valve gap adjustment will make all the difference

Thanks for the tip, johndoe - might look into that sometime, but prob not worth replacing the timing chain now.