1990 V8 Fuel Injection issue

The fuel economy on my car has gone approximately in half. Went from around

22 mpg at 70 mph to around 10 to 11.  I replaced the temp sensor, checked 
the mass air flow sensor, checked the idle speed motor? and have cleaned
the 
throttle body. Also replaced the air filter all to no avail.  No codes in 
computer.
What am I missing??

Many thanks
Bill