Terrible mileage after battery replacement?

First... Battery died. I replaced... resulted in very rough idle. Cleaned out throttle body and my idle RPM came back up. I then started on some maintenance. Replaced plugs & found one of them full of oil (also found oil level low). Pulled valve cover and replaced gaskets/orings/etc.. Also addressed a coolant leak from the water pump (replaced seal). Truck now seems to idle a tad high sometimes but runs fines otherwise. Problem is that my city driving is now under 3.5mpg.... and I can smell fuel in my exhaust. Any thoughts to get started? I did go back over my work, didnt find any sensors or vacuum lines off.

Thanks all

 

Run the entire tank of current gas out of it and then refill and run that tank. Let all those sensors get reaquainted with their surroundings.

 

That’s a little more involved that just battery replacement.

Hand calculate the mileage and see if it’s accurate or the truck it wrong. I’d fill the tank, drive 20-30 miles, and fill again. If it too 10 gallons then yeah something if wrong.

 

So no CEL on dash? I'd read it anyway, may have something Pending.

 

I'm not a mechanic, but I seriously doubt your replacement battery is causing that! Seriously, it sound like you know your way around an engine bay. Good luck finding what is still out of kilter!

 

The only time I've had something like this happen was in my old 82 Trans Am (gutless 305 v8). Filled up and drove it 30 miles, used 3/4 tank of gas. That turned out to be the mechanical fuel pump leaking quite profusely while it was running.

So I'd start there. Maybe you accidentally bumped something loose, or maybe a leak has appeared coincidentally after you did that work. Surprised the truck seems to be running good if it's actually pumping that much fuel through it.