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Chevy express van TUNE-UP and low MPG

Hi, I have 2008 chevy 2500 express van 4.8L with 110,000.
I noticed that my fuel economy was down from 10.5mpg to 8.5mpg in matter of 2 weeks.(500 miles driven )
Every week 1mpg lower.

No changes to driving habits,correct tire pressure, no excessive idling, no repair ,except for 2 news tires and an wheel alignment and state inspection .

Car sounds the same, runs the same.No check engine light.

I changed the air filter , but no tune-up was performed.

Should I do basic tune-up ,wires,rotor+cap,sparkplugs ,possibly a fuel filter or take it to the mechanic to check the sudden drop in mpg. Could worn parts that i’m planning to replace by doing tune up cause such fast drop in mpg? I follow MPG daily , as it is displayed as default on my dash.

Thank you for your help.

When a sudden drop in fuel mileage occurs, the first two things checked are a thermostat that’s stuck open, or a faulty coolant temp sensor for the computer.


In your case it would be plugs, wires and heatshields . . . no cap and rotor, your van doesn’t have a distributor. Rather, it has 1 coil per cylinder. Each plug has its own dedicated ignition wire and heat shield. The wires and heat shield tend to get all buggered up, when you remove them. Especially the heat shield. If you’re going that route, replace everything, or you’ll be sorry

It was a gradual decrease , i would say 0.1MPG a day while driving about 30miles a day.
Could it still be thermostat or faulty coolant temp sensor problem ?

Could be a thermostat . . . is the engine taking an abnormally long time to reach proper operating temperature

Are you living somewhere, and the filling stations have already switch over to the next season’s fuel?

Are you absolutely sure the parking brake isn’t perhaps partially engaged? No offense, but it’s quite possible to have a bad parking brake switch, and the red brake lamp will not come on, when you have the parking brake pedal depressed

I believe your door jamb says 50psi front, 80psi rear . . . that’s what you’re doing, correct?

Are you carrying more cargo than usual?

You don’t have an external fuel filter, BTW

-I wasnt really paying attention if the engine takes too long to reach oper.temp.

-I live in CT , not sure if the gas was changed already , but I would get about the same mileage even winter ,maybe 10MPG

-I check my parking brake all the time, just to be sure.

-Tire pressure is correct at 50/80 and i have TPS sensors.

-No extra cargo

-The fuel filter then cant be changed correct?


Also regarding operating temperature. I took 5min while idling to get to about 1/2 of the usual operating temperature ,so it looks fine.

After 5 minutes of idling, the coolant temperature was only 1/2 of what it should be?

That sounds as if it may be time for a thermostat

BTW . . . I just noticed you have 110K

It’s time for an ignition tune-up . . . plugs, wires and heat shields, as mentioned earlier . . . if you haven’t done so already

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The temperature increase was always like that, 5min to 1/2 and 10 min to full operating temperature.

Thank you for clarifying

thank you everybody for your help. I’ll do tune up soon