MPG issue

I've been having an issue with my XJ for a while now. (2000, sport, 4.0L, AW4)
Back in Nov '08 I had the engine replaced with an ATK rebuilt long block & head after the original head cracked near the No.6 cyl. I've put almost exactly 4,000 miles on it since then, and after the engine replacement my MPG dropped from 16-18 down to 12-14. I've replaced both O2 sensors, checked the spark plugs(champions, .035" gap), and even resorted to taking it to my mechanic, who didn't find anything wrong with it. It's not throwing any codes, I haven't changed my driving habits, and the T-stat is good. Any suggestions as to what is wrong with it?

EDIT: i forgot to mention that the spark plugs exhibit some fouling, like they are getting too much fuel.

Bump

This could be a vacuum leak, but it could be a lot of things. I would start by tightening the intake/exhaust manifolds to the block. Then make sure all grounds are connected and all sensors are plugged in.

What other problems does it have?

I would get out the FSM and ohm/volt meter and test each sensor used in the closed loop operation.

Have you changed gas, or has your gas changed? Like ethanol content?

What kind of plug fouling?

I'd put it on a scan tool to verify critical inputs are correct. Test sensors as Ecomike mentioned. Exhaust gas analysis can sometimes be helpful.

If it all checks out, all you can do is to hope that your mileage improves as the engine loosens up. There are other Jeep owners that are getting your gas mileage.

Somebody in another thread just said his stayed in open loop without throwing a code because his CTS read -70 F all the time. Killed his mileage. New CTS fixed it, but he found the trouble sensor by testing it, in his case he had a scanner that allowed him to read live sensor values! An FSM for ohms to Temp conversion table, and multi meter will do this too.

OK, I've got a good multimeter, but I gotta find a new FSM-mine got ruined a while back. Any suggestions on where to get one?
As for the plug fouling, I'm getting scale carbon deposits on the circle of metal exposed to the inside of the cyl, and some carbon deposits (no scaling) on the ground electrode. The insulator has some red on it like some sort of additive was run through the engine, but I don't recall ever putting any additives in it. I frequent the same gas station 90% of the time, and have since before this trouble began. I don't think it has changed fuel content.

Is it having any other problems, like low power or running hot? If so the running rich (carbon indicates this) may have damaged the cat converter(s).

Some of the sensor test data is scattered in threads around here Try a search), MAT and CTS are the two I would look at.

The precat O2 sensor should be reading an average of 0.45 volts, oscillating back and forth from about .15 to .8 volt once a second at idle.