The check engine light is on all the time. When it idles, it kinda revs, like it want to go and go--I don't even have to give it gas and it moves. Also, only get about 25 mpg, mostly highway driving. Something is wrong--a vacuum leak?

Check the fuel pressure at idle. Did you replace the fuel filter? They can be restricted,mimicking a bad fuel pump. Does it have any codes? don-ohio

Could be so many things. First thing I'd check is fuel pressure. Is your check engine light flashing?

TPS.....Throttle Position Sensor will normally cause this problem.....IAC....Idle Air Control sensor will also have an effect on the idling.....However I would say it is the TPS....IT is rare that both have failed but it is possible.......You should have a check engine light on...?...

What is the trouble code stored in the computer, and have you checked the idle air control valve ?

Running a gas tank lower than 1/4 level is one of the worst things you can do, since gas acts as a coolant for the fuel pump that is in your gas tank. In addition, if the gas is a dirty as you say, the cumulative deposits have become a sediment in the bottom of the tank and at low levels is no longer suspended in a volume of liquid-which will clog the 'sock' around the base of the pump, and can even clog your fuel filter. I suspect your fuel system will need a good going over-have the fuel pressure checked (simple procedure) and take it from there-good luck

i would start with some simple things like fuel filter /plugs /plug wires

It sounds like the gas control lever under the hood may be sticking a little and not retracting fully to idle.Open your hood and have someone start your car.When they give it some gas you should see the actuator move which has a cable attached,probably located at the end of the fuel injectors on the driver side.It should be pretty noticeable as about the only moving mechanism when the gas pedal is pushed.Usually pushing the gas pedal makes the lever move toward the driver.Once you have found it,push down on it in the opposite direction as giving gas to see if the revs decrease back closer to idle.If it does,WD-40 on the hinge where it pivots may help.Spray it,rev it a couple times and check.Even if it works on the first try,do it a couple more times.If this does not work,please reply back and I will get an auto link to see what you posted. Good luck! Greg

Sounds like ignition components may need replaced like the coils and wires.

voltage sounds kinda high for while its running. the voltage regulator in the alternator sounds like it is bad. i would replace the alternator.

The timing belt is one tooth off. Pull the covers back off, set the engine to TDC and recheck your marks, and make sure that the tension is correct.