Very Poor Mileage

Hey everybody i love this site and you guys have helped me before.Im haveing a very poor gas mileage issue that has plauged me every since i changed the oil awhile ago it seems.Since then ive changed it several times and still gets about 12 to 13 miles per gallon and about the same on highway. About 200 miles per tank.
Its a 93 S10 Blazer 4x4, CPI vin W Vortec. Tires and psi are all good.Replaced Cap,Rotor,plugs,wires,ignition coil,Fuel pump and Sender unit "went dead on me" New screen and cleaned EGR valve,New Cold air intake filter clean,pvc valve, No vaccum leaks with tester its in the green zone on guage around 20 or so.

Fuel pressure is about 59 at prime and around 57 steady on idle and 3000rpm rev around the same 57 or so.Leak test is hold ing after engine shut off at about 56 for more than ten min.Replaced FPR a couple times no washing in intake. Brand new exhaust stainless steel tail pipes and Cherry bomb vortex muff, original cat though, before that just runnin on cat and still crap mileage. Sorry for the long post but if anyone can offer help i would love it thank you.Good spark at all plugs maybe to low voltage of ignition coil??? thanks guys :icon16:

Any codes or check engine lights?
Did it happen right after oil change?
Any other changes at the time of oil change?
Did you get the air tempt sensor plugged back up.
Doe you cold air deal bypass or use the air tempt sensor?

How is the basic tune up eng?
Good AC-Delco plugs?
Any engine miis or rough running?
Have you check the base time with est wire unpluged/

the only code im getting is a 43 knock sensor code, but it dosent ping or knock or anything and its always been like that,it prety much happened it seemed right after the oil change and i think that was all i did at the time maybe spark plugs and wires but i think just oil change. I hooked back up the air temp sensor in intake tube "brand new" The engine was running rough intill i replaced the ignition coil then that solved that prob, seems to be runnin good, besides poor mpg.

" Doe you cold air deal bypass or use the air tempt sensor?" ?? dont know what you mean i bought an after market air intake kit with cone filter and i hooked back up the Air charge aka air temp sensor back to it. if thats what you mean. And about the est wire and checking timming with timming light that was going to be my next check, but i dont know which one the est wire is and where. And if the timming is wrong or altered then what lol :) :rofl: and im useing ac delco plugs also

I have also replaced the O2 sensor when exhaust work was done.Ill have to wait till this weekend to get a timming light and check it. Anyone else have any ideas that i can try this weekend? Can the single injector not be working corectly even though the p.s.i. is good ??
Will oil really affect gas mielage this bad??
IAC valve checks out
TPS checks out
MAP checks out
anything else to look for??? thanks MT and people :runaround:

Also when i replaced the fuel pump and sender unit and connecter to it while dropping the tank i found that along the middle seam of the tank if you fill it about 3/4 full it will start to weep from the rear seam of tank, it is noticable, i did a hgack job of sealing it back up because after dropping the tank twice due to first faulty AIRTEX pump "which ill never get again" i justed wanted to be done with it.

My question will that affect my mpg like i am stateing??:rolleyes:

Also one last thing i said my mpg went to crap when i changed my oil, welll come to think of it, it was the Vavoline High gas mileage oil that i used, very soon after word mpg started going down hill:screwy::screwy:

Timing disconnect wire is a single black and tan wire.
Sometimes on firewall wiring on right side and sometimes under heater housing/or carpet inside cab.
MT