1999 regal buick using alot of gas terrible gas milege

One is for the fuel delivery, one is for the return and the other one goes to the vapor cannister.

is it at idle or while coasting or while your foot is on the gas if at idle or coast probably idle air valve should be a check eng light on

Change the spark plugs, spark plug wires and air filter.

try replacing you fuel filter

the catylitic converter will do that if it is bad

Sounds like you have a blown headgasket,check the coolant in the radiator and if you find oil in it you do need a headgasket.

if your car has a plastic intake manifold it may be cracked or warped.

i dont know the specifics of a Regal, but usually, when a needle fluctuates too much, is because the float gauge in the gas tank is worn out, or its mount became loose. You cant see that effect on a full tank because the instrument needle and the float are pegged against the stop.

A couple of possibilities. 1st, check to see that the fuel pump is working. When you turn the key to on position (not all the way to crank the engine over), can you hear the pump? You should hear a humming sound from the gas tank . The fuel in the tank actually keeps the fuel pump from overheating. It is possible the pump is shot. It would be a good idea to change the fuel filter, as it may be plugged up from sediment from the bottom of the tank.

Clean The Idle Air Controler