Historically since I owned the car I had been getting 26 city, 32 highway. Now that the car has over 250K miles on it, my mileage has decreased to about 19/21. Why the decrease over this time? I've been doing all of the normal maintenance things and have replaced just about everything in the engine compartment except for the motor itself over the past 18 - 20 months. Does anyone have any ideas as to what I should check or have checked? With the price of fuel going up daily here I'd sure like to get my fuel economy back up again to where it used to be.

Previous posts here have said many auto parts stores have the wrong 850 O2 sensor in their data base. Should be Bosch 12077864B00

If this as a N/A car (based on your previous mileage I am going to guess that it is not a turbo) check the airbox thermostat. The thermostats go bad and force hot air into the intake full time and it kills mileage and performance.

On my N/A cars I just force it open to the cold air side and forget about it.

A thread on the subject with some pictures is here:

http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?t=8830

...Lee

I'm waiting for the airbox t-stat to come in. I'm going to replace it anyways, especially with 258K on the vehicle.

Now I just have to figure out why my front oxygen sensor comes on every couple of days. Then I clear the 212 code and it works fine for another couple of days. Does this mean that the unit is going bad?

You have undoubtedly replaced the vacuum elbows around the intake and inspected others under the fan shroud. They tend to degrade over time an allow small amounts of air to get in and drive the front O2 sensor crazy.

The other culprit is the front O2 sensor getting dirty and giving false electrical readlings. Some people on other sites have removed the front O2 sensor and cleaned the area where it sits, usually full of dirty stuff. Or even replaced the sensor.

Checking suspected vacuum leaks is easy with a butane cigarette lighter. Turn on the engine and let it idle, then push the lever down on the lighter (without a flame) and direct the gas to the suspected leak area. If it leaks, the engine idle will increase slightly.

Is your throttle body clean?

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

I have replaced all of the vacuum hoses as of yesterday morning. They were starting to look preety bad. Since yesterday morning I've put another 250 or so miles on the car, but the fuel economy is not improving. 19.9 mpg currently.
The Front O2 sensor was replaced again this past spring time and the rear O2 Sensor was done in the fall time of last year.
I can possibly take the O2 out snd see if it needs a cleaning and the internal area of it as well. Anything special needed for doing this?
Throttle body is clean as a whisle..
Any other ideas here?
Currently the only error codes that come up on the system are 212 & 153. Everything else is error free.
Someone mentioned in passing that I should check the mass air flow. I was unsure of this, so I have not touched it.

Your 2 error codes are for the O2 sensors. The ECU is not getting a signal from them!! That will destroy MPG in a hurry.
Find out why the "new" O2 sensors don't work. It could be bad plugs, poor wiring, or just not compatable.

Cleaning the O2 sensor holes with carb cleaner only works if the O2 sensors are sending weak readings. In your case, they are not sending anything, which causes an overrich condition.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

Air filter, synthetic oil, coolant temp sensor, mass air, and tire pressure.

I use Amsoil products from top to bottom in this car. The only thing that has not been replaced in the engine compartment besides the motor itself is the mass air unit. Everything else is new within the past 18 months. Tires are brand new as well!

Go for a quick spin in the car. After, put the back of your hand against each brake disc and feel if it is excessively hot. If it is, you have binding brakes which is the cause of your poor MPG. Also keep your tyres inflated to the correct pressure. I am currently getting 39MPG due to leaving 5 minutes early and keeping my speed down and the use of some "creative coasting downhills". Cost of petrol here in the UK is currently £1.18 a LITRE.....which is over $10 a gallon. UNREAL.
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Tek

Complete brake job done by a trusted Volvo mechanic. No problem there.
We're over $4 a gallon now in the states and it is bring the economy down along with all the other garbage as well.

.....also, I forgot to mention the coolant thermostat. Is your temp guage sitting at the 3 o'clock position? If it stuck open, it will seriously kill your MPG.
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Tek

No problem here, the guage moves constantly pending driving conditions.

"the gauge moves constantly pending driving conditions"

The gauge should rise to 3 O'clock and stay there thoughout your journey. If it moves down at all after warming up, your thermostat is faulty!
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Tek