'99 V70 2.4 NA - O2 sensors battle

[fszczesniak]

No black smoke, more like white steam-like smoke that has fuel scent and not even always, depends on outside temperature

[misha]

No black smoke-NOT running rich....simple as that.
With front o2 reading at steady 0.98v it should have black smoke from the exhaust.
Steam is from condensation if it's cold outside.

[fszczesniak]

What can cause such crazy readings then?

[misha]

...definitely vacuum hoses coming out from the intake manifold are in need of replacing...

Vacuum hoses,small leak from exhaust manifold to cat...between cat and rear o2.
Everything should be checked.

[fszczesniak]

small update - S1B1 is linear in my case, figured it out only because of Vida (showing Lambda around 1.00 so good, I think?)
yesterday checked spark plugs as well, sockets were clean and dry but it seemed like they were a bit loose. Fixed the gaps as well to 0.030 inch - first test drive and the engine works quieter and smoother. Besides, didn't find any leaks in exhaust and vacuum systems.

[AlexVolvo]

Did you resolve this?

[AlexVolvo]

For reference,the front 02 is q wideband sensor (Linear Air Fuel Ratio) and works quite differently to the rear 02 which is a narrowband sensor, able to output from 0-1v and fluctuate betwen around 0.2v (lean) and up to 0.9v (rich).
The Front wideband does not produce it's own voltage but gets a signal supply voltage on two wires from PCM. The higher V is 3.8V, the lower is 3.4V (just need to confirm this as I need to change an O2 in the next month or so). You cannot see a difference or fluctuate on a volt meter, because I a wideband sensor signal to the PCM is an amperage fluctuation in the very low mA range ... on a Volvo roughly from -12mA (rich - yes widebands read rich in the negative range) to +0.60mAm (lean). You can only see this on a scantool and it often calculated and reported by the PCM to the scantool as a V even though this is not strictly what the PCM is reading. This is because of US OBDII rules requiring O2s to produce a voltage reading. Some scantools will give an Amperage reading. A dead wideband is usually due to bad wiring, open heater circuit, damaged/contaminate probe. The PCM will typically shut of the circuit if it is not reading correctly and log a code. Note that what I'm saying is for a four wide wideband sensor. A 5 wire wideband (on the turbo models) is slightly different again.