Recently I've been making horrible mgp...13 in the city and 20 hwy. This began shortly after the 50k tune up. I've replaced the spark plugs again thinking that after market vs. volvo plugs would make a difference but to no avail. The OBD-II system is not pulling any codes, but I suspect faulty HO2S's. Before I begin on a wild goose chase any insight would be greatly appreciated.

Thanks everyone for the info...hopefully I'll be able to get things squared away today.

My mpg has been slipping some over the last month...not as bad as yours but some and it shouldn't be as I've replaced most hoses with silicone. So...please post back as to what you find. I'm very curious and would like to know.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

This might be a little far fetched however my friends 1998 S70 had diminished brake pad life poor gas mileage and eventually lugging in the transmission, His TRACS system was in default "on" mode and the car is literally fighting it's own braking system. Ford sourced integrated 4 channel ABS, $69X.00

Turn off your TRACS manually after each start, Mechanic Nmeumonic Click the belt, click the TRACS.

If you do not have TRACS I appologize for the Goose

AFLAC!! oh that's a duck.

Dave

Probable vacuum leak. Check all of the vacuum elbows first for cracks or spongyness. Make sure one of the lines didn't come apart during the "tune up".

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

Try looking at all of your hoses and see if there are any leaks. Bad air filter, gas or fuel filter and pump as well as relay for that. Last of all, although you replaced spark plugs, how are your wires and rotor? Check these things before suspecting 02sensor(s) which are expensive.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.