Portege 1.6 DOHC low performance on gasoline
 

The car has been converted to LPG several years ago. Recently, a problem developed.

Symptoms:
-Very low engine performance on gasoline- acceleration is not responsive, above 2'500 rpm engine starts to stutter, performance is terrible.
-Engine works fine as soon as LPG kick in. Maybe MPG is a bit low'ish

Diagnostics:
-Check engine light would come up
-Code reader revealed P1170 twice (and missfie in cyl 3 ar cyl 4 once)
-HO2 sensor (P1170) Voltage stays around 0-0.2V on idle, belatedly goes up to 1V on acceleration (I used generic ELM327 OBD2 reader- might be artifact)

In general, it looks like HO2 sensor needs check up / replacement. However, what is confusing is that engine runs fine on LPG- to my understanding, it should still exhibit the same issue.

ANy comments?

i would check your fuel metering. 0-0.2 volts is lean. it should fluctuate from .1-.9V (i mean the upstream sensor). it is normal to go to .9-1v as soon as you rev up the engine and let go the gas because at that point the mixture goes rich ( the engine revs are going down, but the throttle body is closed so you have more fuel than air= rich. for all this to be accurate, the car must be at normal operating temperature (closed loop).
i understand that the fuel metering for lgp and gasoline are not the same. in other words, your car is running lean on gasoline. i have to confess that im not experienced in those conversions. but i would check the fuel pressure (fuel pressure regulator, vacuum line going to the fuel pressure regulator, and fuel pump pressure.) then i would have better info to decide if i have to mess with the o2 sensor or check the whole conversion wiring or controls. again, let the record show that i have no experience in that kind of conversion. the only experience was a conversion from diesel to lpg in a kohler backup power generator. the conversion was to full time lpg. that was about 5 years ago.