Bad Gas Mileage!

Ok. 1991 Silverado 350TBI

I am gettin 9MPG I have done everything I can think of to improve mileage.

plugs
wires
cap
rotor
fuel filter
air filter
foamed throttle body


Motor is stong, burns under a quart of oil every change.

has 181,000 miles on it.

I cant figure it out. What would help???

bad o2 sensor, plugged exhaust / cat, clogged or broke injector, just a few to add to your list

What size truck and what size tires? Larger tires will sink your gas mileage because they effectively change your gear ratio.

I had a 91 extra cab, 1/2 ton, short bed, 350TBI, with a fiberglass shell on the bed and a Gibson 3" cat-back exhaust. I was getting around 13-15 (city-highway) but after I put 31 inch tires on it I was getting around 11-12 mpg. I now have a 92 regular cab, step side, 350TBI and I'm getting 13-18 (city-highway) on this one with stock tires.

What is the foamed TBI?
Base timing ok?
Any codes?
Is the coolant and air tempt reading good to the PCM?

I had a bad egr solenoid on my 93 350ci and no check engine light--mileage dropped from 17-18 hwy to 10-11

Maybe a pinhole in a fuel line that only leaks under pressure (while running)?
Doesn't always happen where you can easily smell gas.

At hgher mileages (my '90 Suburban is at 419,000) simple yet hard-to-find little things start to happen.

Okay sorry for the delay in my post,

I have dual exhaust w/ no cat.
265/75/16 tires.
I guess I am unsure how to check the base timing. Seems powerful, doesnt miss or anything.
The only things I notice out of the ordinary are, A)The tach bounces around alot like within a few hundred rpms. B) once in awhile when I put it to the floor the oil pressure guage will peg.C)I can smell raw fuel when the truck is parked but it doesnt drip or anything.
I dont no about bad egr or o2 sensors, the o2 sensor is right in the manifold so pulling the cat shouldnt affect it much.

BTW bad gad milage before cat was removed, so thats not the problem.

The foamed intake was this can of intake cleaner I got and while the engine was running you spray it in the intake and then I shut the truck off and let it sit over night, then in the morning I ran it, but it didnt seems to do much as far as I could tell.

Good old basic tune up and a scanner check would be first on my list.
Tach bouncing and gas smell spells problems.
And no cat can sometimes make things worse.

I did a full tune up before I posted, as I mentioned in the first post. My neighbor has a handheld scanner, should I hook it up to that?

I did a full tune up before I posted, as I mentioned in the first post. My neighbor has a handheld scanner, should I hook it up to that?

Reread post 4.
A good full tune up includes good ac delco plugs code check and base timing and all scanner readings compression check and fuel pressure check and many other things.
Watch your tempt sensor readings on scanner.
Is the engine running up to proper tempture?
And you should get a cat back on it.

Has it ever got any better gas mileage?
What is the gear ratio?
4 W drive?
What sise tires?
What are your driving habits?