1996 F250 7.3 Power Stroke Diesel w/130,000 on odometer

I have been having fluctuating mileage past few months. I have 36gal tank and thru one tank of fuel I get 14-18 mpg, and then on another tank I may get 10-12 mpg under same driving conditions. Mileage to a tank varies from 360 to 400. I have changed air filter, fuel filters, and religiously change oil every 3,000 miles. I bought a maunal and checked all the things that it had listed above, is there maybe something else that I am missing, or has anyone else had this problem. Any help would be greatly appreciated.

Exhaust back pressure valve might be sticking (not opening fully). That WILL definately cause excessive fuel consumption.

Exhaust back pressure valve might be sticking (not opening fully). That WILL definately cause excessive fuel consumption.

Is that the same as a EPR regulator? Where can I find out where it is located? Or how to test it to see if it is bad?

There is no such thing as an "EPR" regulator, whomever told you there was had their terms mixed up.

There is a EBP sensor (Exh backpressure), ICP sensor (inj. control Press), IPR (Inj control Press Reg.), EBP solenoid (Exh Back Press).

It is posible that they were thinking Exhaust Pressure Regulator (EPR), but it is the wrong term for it.

There is a valve that actuates a butterfly in the exhaust near the turbo, that is the EBP solenoid, the butterfly itself is the EBP valve. Solenoid failure is not very common, and neither is the butterfly sticking.

The easiest way to check which it is, is to remove tha actuator rod from the butterfly, and see if it moves FREELY from full stop to full stop and back. If that checks out ok, then it is fairly likely that the solenoid (EBP) is sticking.

Opps I may have said it wrong then, I got the information from here about it. http://www.thedieselstop.com/contents/getitems.php3?Exhaust%20Back%20Pressure%20Valve
Thanks for the input on how to check it, I will try that this weekend.