Horrible Gas Mileage (12mpg), Idle issues, Need assistance!

[Rickaytan]

I own a 1990 e30 325i cabrio with roughly 185k miles on it.

Maintenance that has been done so far regarding fuel consumption:
-Timing Belt Changed
- New Fuel rail
- Spark plugs changed
- New O2 sensor
- New spark plug wires

Just recently my idle has slightly been fluctuating between ~1000-1200rpm after a normal warm start. These symptoms have gotten worse after the water hose from the thermostat to the throttle body sprung a large leak. I replaced the hose and inspected the gaskets, so far no further leaks have occurred. It could be a possible vacuum leak, any suggestions on how I might be able to tackle this issue? Any insight on how these issues could be affecting my gas mileage? As for the water hose, how might I be able to prevent future leaks (aside from bypassing it) and test for other issues regarding the coolant?

[jlevie]

12mpg under what conditions (urban or highway cruising)?

Idle problems are most often from intake leaks, which a smoke test will find. A contributor is frequently an aged O2 sensor (replacement interval is 100k). As to the cooling system, the life of the hoses, water pump, thermostat, radiator, fan clutch, expansion tank, and cap is 100k or 10 years.

[digger]

fuel pressure regulator is something i'd be checking as well

[Rickaytan]

I get under 12 mpg under mainly highway conditions.