Sudden drop in MPG

So initially I thought it would be related to the change in fuel for winter but the drop is too significant.

92 fully stock 4.0 Auto. well not quite, bored out TB past the plate.

No CEL or EML Could it be the O2 sensor? Perfromance is really good. Fast off the line and no need to go to the floor in any situation except to downshift.

My thinking is O2 due to rich mixture but I would expect the CEL or EML to come on. My second thought is a stuck AIS although the idle is fine. Third would be ignition related but the plugs have been in for only 30K with an adjustment 15k or so ago.

OK I know I should look before posting but i've been really busy lately and with all the experts on line, maybe I could get a direction to go if mine is wrong.

No torque converter lockup?

o2 sensor is the number one cause, but look for a cheaper and easier fix:
Check all the vacuum lines for cracks/breaks leaks.
Check the manifold bolts for tightness.
is your torque converter locking up properly?
these are all free things to check.
Then I'd try an O2 sensor. It won't always trip a check engine light, it just may be sluggish, or it may just be reading wrong, but still within tolerance for the ECU.

I've replaced two o2 sensors. One on my 1991 XJ and one on my 1993 ZJ, and both needed it. Neither one tripped a CEL, and both passed my local emissions test. (And both were later revealed to need Catalytic Converters!)

Like 87manche said, check the cheap stuff first, but don't be surprised if your o2 sensor is the culprit.

Got to be the O2 sensor for sure. Have you changed the rotor and cap recently?

TC lockup works fine.
Rotor and Cap isn't that old but I will check.
will check vac leak too.
Installed new cat last year about 20K ago with new muffler.
Don't hear any exhaust leak from manifold but will check intake. I know they sometimes leak.
It has 220K without any work that I know of.

And thanks for all the replies!

WOW what a difference a year makes! All the activity on this site is just amazing. I would have been able to get back to my thread 24 hours later a still found it on the first page! Now its 3 pages later!

Well, I think I found the problem. I forgot to mention the Adjustable MAP set up I have. The reason I forgot was that I don't use it. I thought I would disconnect the circuit and see if that makes a difference. In attempting to pull the connectors apart, a wire just fell out of the connector! I could easily have had an intermitant connection taking the system into open loop.

If I remember, I will update this thread.