Hi, I'm from Singapore driving a Volvo 940 Turbo 2.0 liter engine. The fuel consumption has gone from bad to worst achieving only 6.3km/liter (on 50% highway and 50% city) driving compared to 7.5km/liter prior change of fuel filter and 8km/liter about 1 year ago. The strange thing is that it does not happen overnight but a gradual process. The fuel pump, fuel pressure regulator and in-tank pump have been changed about 3 years ago. From my records the only item that I did not change yet was the oxygen sensor. However, aside from the poor consumption, the car is running alright. Please help. If it is the oxygen sensor, what are the symptoms.

With the current high fuel price, it becomes a real concern. Does a bad fuel pump give rise to poor consumption? According to my mechanics, the fuel pump is slightly noisy. However, I read from the forum, it could be the fuel pressure regulator. Hope you understand how confused I'm now.

Bad O2 sensor results in poor gas mileage due to rich running. Another cause for poor fuel consumption is a bad ECT (Engine Coolant Temperature sensor). A bad ECT will cause the fuel injection ECU to sense a cold engine and run rich just like the O2 sensor. A few more causes I can think of: Leaking fuel injector. Does you motor oil smell of fuel? Check the small push in vent line going out of the fuel pressure regulator to the manifold. One of its failure modes is that fuel will spew out if the seal has failed. CAUTION with this, fuel = fire! I am sure others will chime in with more help.
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Tom F. Couple of Volvos Mods, RainX & cup holders.