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Gas mileage problem on Alero

Have a 2004 Olds Alero 4 cylinder. engine code light came on and had it checked. Said fuel vapor system. changed fuel pump, fuel filter, vent solenoid. Code now comes and goes. Not on all the time. runs fine, no loss in power, but gas mileage is less than half what we were getting. Example 120 miles highway driving used a little over half tank of gas. When we first bought the car 2 years ago, 420 miles to a tank of gas. No fuel leaks that we can find, no leak on tank. Any suggestions will be appreciated, daughter and wife are ready to chuck it and buy another.

You have insufficient data to measure your gas mileage by.

We need the codes that it was storing and the ones that it’s currently storing in order to help you. We could make educated guesses, but that’s all they’d be without the codes.

What was the specific code #?
Theses websites might help you:

How many gallons? Tank and half tank is way too vague.

 That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

my daughters 2004 Olds Alero is getting very bad gas mileage I recently changed the fuel pump,purge valve,fuel filter,air filter,gas cap,and spark plugs the CEL off went after I changed each item one at a time it is back on now. After changing the plugs last the mileage dropped. the code is po455

Here’s one site’s explanation of P0455 (zero, not the letter o):
"The Evaporative Emissions system has applied a vacuum to the fuel system and detected a major leak. This is usually a sign that you’ve left the gas cap off, or that the gas cap is not sealing properly against the neck of the filler tube.

First, check the gas cap to make sure it’s tight. If it is, remove the cap and check the condition of the “O” ring seal and the neck of the filler tube. If the gas cap “O” ring is worn or damaged, replace it. Some car makers sell the “O” ring, but most make you buy a new gas cap. If you’re close to a dealer, buy it from them rather than a generic one from the parts store. You’ll have better luck over the long haul with an OEM part."

So I don’t understand all the parts replaced, it’s a leak in the evap system, a ‘smoke test’ is in order to find the leak.

The P0455 is a “gross” leak - that means big. In practical terms, its meaning to me would be to start with a visual inspection of the evap lines. Its the small leaks that can be nearly impossible.

If you have had some drastic change in mpgs the places to look first are the thermostat & coolant temp sensor, tires, and brakes (dragging).

In the end though, how do you know you’re not just dealing with a wacky gas gauge? On top of that “by the tank” is not a good way to figure out mpgs. So as mentioned, get a real number through a real method. Fill up the car. Write down the odometer mileage. Drive the car a good 1000 miles or so. Anytime gas is added just fill it to the first click. Keep the receipts or write down # of gallons. (But not the initial fill-up - that’s from miles you already drove). After the last fill up, do the math - divide the miles driven by the # of gallons used.

I would blow a little air into the gas tank and listen for hissing.
Fashion a bung for the filler neck and use a compressed air nozzle.