I've owned nothing but 4.3 Blazers and S10s for 20 years. I recently got a 4dr 2002 Blazer 2wd with 85k miles that's in almost mint condition. Truck runs great, but for some reason it makes way worse gas mileage than any of my previous trucks by far. It averages 13.9mpg, mix of city and highway miles. I only get a little over 200miles on the tank, where I easily averaged 250-300 on my other trucks. The only engine code I get, intermittently is for the evap system, doubt that has anything to do with it. Engine fan clutch was staying engaged too much, so I thought that was the cause, but replaced it with a new oem fan clutch, it definitely engages a lot less, but mileage didn't change. I put it new Delco plugs, air filter, and fuel injector cleaner. I did the intake gaskets, and while in there, it didn't seem like the fuel pressure regulator was leaking, no raw fuel or clean area back under it. I was thinking maybe the cat is getting clogged, but the truck has plenty of power, doesn't seem choked. All stock exhaust. It has the same rear end highway gears as my other trucks, but does have the g80. Can't remember the gears, but I ran the rpo codes and it's the same gear ratio as my other trucks, just don't remember it off the top of my head. Truck did have the bilstein shocks and beefier springs. I ran a compression test, all cylinders are equal and perfect. Doesn't burn oil. I'm running out of ideas as to why it's only making 13.9mpg or less. Any suggestions??

 

What is operating temp?

Some tires will cost you 1-2 mpg

How did the plugs look?

Take a look at fuel trims and O2 sensor graphs.

Look at all the other sensors on a cold engine, key on engine off and compare them to ambient

Check for dragging breaks

Do the fuel pressure test in the sticky

George

 

Along with what GeorgeLG stated, when you have a check engine light the truck could be going into Open Loop which means it using default air fuel mixture instead of constantly adjusting for optimal Stoch ratio of 14.7:1.

 

As George says, I would definitely look at the O2 sensors. The other common problem George mentioned is the brakes dragging. This could be caused by caliper pins sticking or the caliper piston sticking if you have rear disc brakes.
Do you have 4 wheel disc brakes ? If so , the parking brake cable may be 'frozen' which will cause the parking brake to stay on all time. The parking brake acts on the two rear disc's. The rear rotors have an inner set of small brake shoes that press on the 'hat' portion of the rear rotors. Because these inner shoes are hidden, they are often ignored. So when you check the rear disc brakes be sure that you look at the parking brake system as well as the brake caliper pins and the caliper piston.