So, to all of you that don't remember:



"I searched the forums, found some helpful info, but still no answer, so I post this in hopes for some more info.

So, here's the deal.
89 Volvo 740 turbo wagon. 146k Summer mileage (in a hilly, poor road condition state) in town 20-21. Highway 24-26.5 (and has been getting better steadily) Winter mileage (same conditions, no snow until today) in town 12-16. Highway no more than 20.
Work done/mods:
Plugs, wires, cap, rotor button, vaccumed cleaned all four injectors, cleaned throttle body, adjusted TPS correctly, cleaned IAC, engine temp sensor and guage sensor, fuel system check valve, fuel filter, checked both fuel pumps for operation, fuel pressure regulator brand new, k and n drop in filter, synthetic engine oil, brakes-none dragging, four studded snows on steel rims (this may be part of the problem as the stock alloy rims/tires were about 7 pounds lighter each), adjusted ignition timing to 13 BTDC (book says 12, did this mod to try and resolve poor fuel economy) as it was at 10 BTDC, camshaft seal, timing belt/tensioner, thermostat, new flame trap (called something else on a 740) complete assembly and cleaned all hoses, fittings, etc, checked turbo for wear (no axial play at all), battery, camshaft cover gasket, new headliner, cleaned MAF and connector, probably more I can't remember right now.
Reduced the curb weight by about 100# by removing- frame "vibration dampeners", A/C all components, interior floor mats (under carpet mats), stock radio amp, all extra brackets under the hood, and a few other things I can't remember. Air box is intact, thermostat is fully functional.

No codes, or at least no codes that produce a light. Cannot find a module inside the engine bay to check for codes. All books/stuff on the net says it's on the drivers fender well and show photos of theirs; mine is absent.

Outside temp above 50, great fuel economy for a heavy car. In fact, with a 2500# trailer behind it on the highway I still saw 18mpg. Our weather has been intermitent as of late. I check fuel economy everytime I fill up. One week it will be warm and I get great economy. The next week it's cold and I get terrible economy.

I figured winter gas formula, sure. 1-2mpg. But it doesn't fit because I'm using the same gas station, and one week to the next then back again I'm getting good, bad, good, etc. No snow to speak of so no tire spin, or extra drag, etc. No extra ballast in the boot until today.

I was thinking tires/wheels but that would be bad all the time after putting them on. No beans there either, although they probably aren't helping.

Running out of ideas. Never changed the 02 sensor. Who knows how old it is. Car does use some oil, say a quart in 5k miles at most. Still, I would assume this to be bad fuel economy no matter what the temp is outside.

Thinking of picking up an air/fuel guage. Not sure how to install it, but probably worth a shot."





NEW INFO:
Installed new 02 sensor. Also noticed a low voltage condition at full throttle with the headlights on. Read the article on alternators, replaced my regulator/brushes. The existing brushes were less than 1/8". So, that was time.
The good: Car charges better, feels better and a little more powerful. Appears to be smoother too.
The Bad: Fuel economy has not improved much. I have not tested in town fuel economy, but I did highway. I stopped at a gas station with a fully warmed up car. Filled to the top. Set the odo. Set the GPS (to verify the odo) Stopped about 40 miles later. Drove very economically, cruise set to 70mph. Outdoor temp: 5degrees F. MPG: 20.5

The weather here has been cold. It gets to around 15 during the day, and at night it is around 4-5.
So, starting at 24-26, assuming I'm getting winter mix fuel, I should be seeing a loss of at most 2mpg. I'm seeing 4-6.

Now completely out of ideas. Anyone wanna muster a guess? I'd try about anything at this point. Maybe the mobil ONE 10w 30 I'm running? I ran 15 weight in the summer.