I have a 86 745T. I has 171,000 miles and runs fine. When I say it runs fine,

starts on first turn,
accelation seems to be good - both low end and high end,
idle is steady,
no smoke billowing blowing out of the tail pipe,
turbo seems to be spooling fine - at the right times
pass Maryland State emissions testing - question this, because it was a real windy day.

The issue I'm having is that it is consuming a lot of fuel - get about 11-13 mpg from 25-27 mpg. I read all most of the messages on Turbo bricks and some of the messages here regarding this issue, but from what I can tell, I do not have the symptoms like most.

I replaced & checked the following:
o2 sensor replaced 4-5 months ago,
amm replaced last year,
recently go new spark plugs/wires,
just replaced the fpr - yesterday,
replaced in tank prepump,
air box seems to be fine - appears to be directing air as it should,
I even had the tires aligned - with new struts and shocks
checked all vacuum hoses and replaced as needed,
removed, cleaned and replaced throttle body.

I a stumped and loosing patience. I've taken the car to two Volvo specialist and they have yet to find any problems. There are a couple of item that I have not checked, which are the ect, fuel injectors, fuel pump. I not sure how to do the test and if I have the equipment to do. As you and can see from the list above, I can dimantle and replace rather than, test the components - I don't have the guages and I figure if the parts are that old they need to be replaced anyway. Visually and audibly there is no indication that there is any problem, except for the fact that when I drive to work, 12-13 city miles - some stop and go, mostly coasting - you can see the fuel guage needle slowly heading west on the dashboard AND there is more soot than usually around the tail pipe. Also the fuel seems to flow out mostly at idle since my commute is waiting at stop lights.

Do these issues seem familar to anyone? And if so, how did you resolve it? If you need additional info please let me know. Thanks