Do you have an illuminated Check Engine Light? If so, use the FAQ and pull the OBD codes. If not, make sure your Check Engine light is working when you start the motor...it should come on, then go off.

I second that 150 miles is pretty low. Even your older 700 with the 60 liter tank (15.8 gallons) should easily go 200-250 miles around town and more on the highway. That's going from completely full to nearly empty. As stated by aleekat, you need to do some math to get an exact mpg figure and post back.

Mileage can also be affected by how much, if any, ethanol is in your fuel. On Ethanol-10, high teens is the minimum I would expect in normal city driving. If you're getting 150 miles on anything near 3/4 to a full tank, something is going on. Fuel doesn't disappear.

Malfunctioning lambda sensors or a malfunctioning Air Mass Meter (AMM) are common issues that can negatively affect your economy.

My knowledge is primarily with the single-cam motor, but the twin cams do have a different sound...cams do whine to a degree but it's all relative. I'd make sure all your belts are properly tensioned if cam whine is of concern. The twin cam motors are especially important to keep the belts properly tensioned as this is an interference engine and a belt breaking is a catastrophic event.
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A bottle of oil and some basic tools in the trunk. The highway is calling. Always remember the difference between durability and reliability. Embrace the old Volvo in your driveway. She's eager to please and, with a little TLC (and a few headaches), sh