Volvo 850 Made from 1993 to 1997, this Volvo line was available in both a wagon and a sedan, both with were graced with several trim levels.

Sudden mpg drop?

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Old 03-01-2018, 06:08 PM
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Default Sudden mpg drop?

Hi all. I drive 1996 850R. Ive had mpg around 22 for couple years. Now it dropped to 18.8. I have new airfilter. New vacuum liness to turbo. New plugs wires cap and rotor. My fuel trim is perfect zero. One CEL code for downstream 02 only. Any ideas. Only thing i think of is **** tires. Thanks
 
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Old 03-01-2018, 07:00 PM
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my 95 850T would get 21-22 in the summer and 18-19 in the winter - which I attributed to winter fuel blends used in New England. Tire brands won't make that much of a difference - assuming you keep them at a correct pressure (ie 36 PSI)
 
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Old 03-01-2018, 10:36 PM
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Well, downstream O2 is exactly that. When O2 sensors get old, they drift rich. What can the car do except burn more gas?
 
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Old 03-02-2018, 06:36 AM
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I checked data on a 3000$ snap on scan tool. Fuel trim was 0. Which was surprising just the down stream 02 codes. I thought only upstream had effect on air/ fuel. I also live in NH. I didnt know winter fuel was different. I always use 89 octane.
 
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Old 03-02-2018, 06:51 AM
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I bought bosch downstream sensor from FCP. Anyone think i should exchange for the upper bank sensor instead?
 
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Old 03-02-2018, 07:13 PM
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the upstream o2 sensor controls fuel trim, the downstream is used to compare O2 pre/post cat to measure cat efficiency.
 
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Old 03-03-2018, 08:45 AM
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Thatís exactly what Iím saying. My fuel trim is perfect thereís no sense in changing the O2 sensors I donít need them for inspection in my state.
 
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Old 03-03-2018, 10:53 AM
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Often mileage is a perception unless is it checked regularly. If you have the data, graph it and see what it tells you.

Unless the vehicle is running seriously rich, you are not going to see a 20% drop in mileage. The vehicle's computer has enough default memory to not just dump fuel down the tailpipe regardless of the fuel trim. The only way to dump that much fuel is a stuck open injector.

If there are no codes and no irregular data points, then there is no problem.
 
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Old 03-03-2018, 11:01 AM
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The fuel trims are fine. The only codes Iím getting is downstream oxygen sensor. I bought the downstream oxygen sensor Iím just thinking if I put that sensor in itís not gonna make any difference to my gas mileage And Iíll be out $110.The car runs completely fine.
 
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