I don't understand why a bad overflow valve causes a decrease in fuel economy, doesn't it just route excess fuel back to the tank? then it can be used again, so someone wanna explain?

 

Just curious...why do you say you will get bad mileage if the overflow valve is bad?

 

every post I read about crappy mileage, the overflow valve is mentioned in there somewhere.

 

The O/F valve controls fuel pressure to the injectors. When it goes bad, you lose fuel pressure which causes a poor injector spray pattern. The fuel isn't atomized properly and therefore, doesn't burn as efficiently. You lose power and mileage.

 

Berner nailed it.
It's the same reason that a clogged up fuel filter causes poor mileage.

 

When the fuel pressure drops in the pump you also get a delay in timing. The injectors are set for a certain "pop off " pressure and if it takes longer to get there, then effectively your timing is late.

So you get poor economy from poor spray pattern and delayed timing.