NO Heat

1990 Volvo 240DL. After 10 minutes of driving the temp. gauge doesn’t go up and there is no heat what so ever. I have also noticed that I get horrible gas mileage. What could cause this problem?

Most likely you have a bad thermostat. Cheap repair, try it and see what happens.
Not related to bad gas mileage.

Yes, it is related to bad gas mileage. The ECM uses the Coolant Temp sensor to determine if the engine needs enriched fuel mix when cold. If the coolant never gets warm, the engine runs like the choke is left on. This kills the fuel mileage.