Stop in for a cup of coffee

Rainy Day Auto said

I did an exhaust system on an F250 with a 460 and now the customer is saying the gas mileage is in the toilet.

How does a truck go from 13 miles to the gallon with a plugged up, piece of trash, rusted exhaust to 7 miles to the gallon with a shiny new unrestricted flowmaster exhaust?????

Something wong

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Better flow,more power,tempted to drive a little harder. Efi or carb?
If carb, i would check choke/heat riser and air cleaner.

I know with efi,if headers are installed the flow will carry out unburned fuel as its injected before exhaust valve closes. But 1/2 the mileage? That’s a big change. May have to flatten exhaust pipe so its as restricted as it was before. Kidding.
And some older fords took a while to relearn after dramatic changes were made. A battery disconnect would help on some.

I cleaned an iac on a 5.0 a couple weeks ago, went from stalling cold to a very high idle. A few days of driving and it adapted.