Volvo 1988 740 GLE Low gas mileage.

I have a 1988 Volvo 740 GLE Wagon. It runs very rich. It does something
like 18mpg or so in the city and around 23mpg or so on the highway.

I changed the O2 sensor, the spark plugs are relatively new, the gas
pressure regulator, the fuel filter. I have tried additives to clean
injector but all this just went in vein.

Does anyone have any clues as what to investigate? What might be the
problem?
I like the car very much and hope that I can drive more often but I
only try to take it in longer trips because of its bad fuel economy.

The car has 150Kmiles on it.


Any help will be very much appreciated.