So I just got my xj back on the road after sitting for about 3 years and I am currently get 10 miles to the gallon. Does anyone know why that could be?

 

Check the tire pressure and tire condition. Add a little dry gas to the tank. If you like, give it a tuneup and change the oil, check the coolant level and condition. I would clean the engine grounds as they can rust just enough to upset the PCM. If the battery has removeable caps, pop them off and check the level of each cell. If any are low, use distilled water only. Do not get any of the "water" already in the battery in your eyes. Once it goes thru the charging cycle, it turns to acid.

 

O2 sensor is a prime suspect. Been sitting? Yeah, vacuum leak is also a good possibility. Both will cause the PCM to dump fuel.

How sooty is your exhaust?

 

What have you done to it recently ? Or even in the recent past?
Have you clean the grounds?
Have you done a tune up?
Have you taken it off roading?
More info helps a diagnosis instead of pissing in the dark...

 

I have done a tune up. I have not cleaned the grounds. I took it off-road a few times.

 

I have brand new tires on it and a brand new battery. Coolant looks good.

 

Have you had to put any gas in it yet?

Did you do a fuel filter, or maybe even throw some injector cleaner in it.

After that, then maybe look into the O2 sensor.

 

If it's been sitting for 3 years. Change the fuel filter. Wait a few weeks and change it again. Then wait a few months and change it again.

Really it would be good to drop the tank and clean it.

As long as it had antifreeze in it while it sit then your probably OK not doing a flush.

 

It’s got a new fuel tank and fuel filter.

 

Oh well if the tank and filter are new then that should be fine. Just didn't want you to burn up pumps like me.....haha.

I got one that had sit. Put in a new filter and pump. Stupidly didn't clean the tank (I've learned since then). Burnt the pump out. Thought it was airtex piece of junk. Switched it. Burned it out. Switched stores burned it out. Thought WTH?

Turms out the tank was pretty dirty so it immediately clogged my new filter and that was the cause. Didn't think about it because, hey, it was new. Dropped the tank and cleaned it. Got another pump and so far it's doing OK.

I will say if I burn another pump out though I am getting an original sending unit and putting a Bosch pump in. They don't make the whole Bosch assemblies for 89's now. Stupid.