Here what I have and it's got me stumped.

The other day my xj started showing 12v not 14v. Gas mileage went way down. I was thinking it just wasn't reading the battery. Went thru all battery cables, fuses/busses, grounds and connections at alternator. Please advise. I'm stumped.

 

What do you mean by "went through"? Did you remove and clean all the battery cables? This includes the PDC, alternator, and all grounds. Corrosion can kill a connection and let it look good to the eye.

Check engine light on?

Verified with a multimeter on the battery?

 

Salad has a good point. everything you wrote I would check. What shape is your battery in? Furthermore, you could check the grounds to the block itself.

 

Not sure what PDC stands for.

Outside that yes, replaced 1 ground in the process.

No check engine light and volts across battery are 13.81 and steady

I say yes, is there another ground off motor? I checked the one between the head and firewall.

 

PDC is the Power Distribution Center, it's the fuse box behind the battery.

Yes the important grounds are on two studs on the right side of the block by the dipstick tube.

 

Clean ALL of your grounds. They include both battery terminals, Battery to fender, oil dipstick, 2 on the ignition coil, rear of the head to the fire wall and the mounting area between the starter and the block. Don't just check them. Make sure to actually clean them until they shine with either a piece of sand paper or a wire brush. A bad ground can cause a lot of problems. Also make sure that the terminals are clean on both the starter and the alternator.

 

Got the area at the power distribution, took it apart and cleaned it. The 2 by the coil are all that's left. Took a look at them tonight and there wasn't anything obvious.

My wife is 4 weeks from due date and tonight is date night.

Tomorrow I'll get those 2 cleaned up and post an update. Thanks for all the good advise.

 

One thing I forgot to mention. I know that you said you checked the battery cables. How did you check them? Get a hold of the cables and bend them. If you hear or feel like they have a crunch to them you should change them out. The crunching is all of the corrosion built up on the inside of the cable and will also cause problems.

 

Ok cleaned all grounds and replaced the positive battery cable. Still same issue could it be the voltage regulator in the alternator or battery?