So I've been getting some awful gas mileage, like 9-11mpg and a friend suggested checking my maf sensor... Where's that, and how do I check it? haha

 

There isn't one, so you can't. 4.0 doesn't use a MAF sensor.

You do have a MAP sensor, mounted to the throttle body. Could be that, or failing upstream o2 sensor(s), or temp sender, or a lot of other things.
When was the last "tune up"? Plugs, etc? Any other symptoms that accompanied the drop in mileage?

 

Just picked the car up last month. No records were kept. Did the plugs yesterday and they had some pretty aggressive wear... So I guessing a lot of stuff has gone unchanged

 

I would start with the simple stuff first before getting into complicated issues. If the plugs look pretty worn out, that may be an indicator that the car was not particularly well maintained before you got your hands on it last month.
Maybe try hitting it with a Seafoam or MMO treatment and an oil change first to see if that affects your MPGs?

 

What do you consider terrible mileage? My 99 gets 12 mpg and I have tried all the tips to get better, it ain't happening. That's as good as I'm gonna get. Although on a long trip I can get over 20 mpg. Local driving it's 12.

 

14 city and 20 highway is the approximate mpg for a 2000 XJ so a reduction of 3 mpg could all be in your right foot!

 

Drive style is HUGE especially with the XJs. On another note I installed the Spectre Cowl intake last week and my console is showing me 17.5+ mpg average. Before the intake I got 14.5-15.5. Still waitin to run the first tank out and check that milage that way too, but so far so good.

 

Geeze! Hard to believe a 2 mpg increase from the stock airbox but good for you, and let us know for sure after a tank!

 

I was thinking the same thing but ill take it. Doesnt seem much different accelerating around town but the MPG climb up much quicker once I get to cruising speed and the highway mileage is much better. The base of the windshield is a high pressure area so maybe it getting a lil positive pressure into the intake? The air is certainly MUCH cooler! On a hot day the ambient temp sensor mounted right under the front bumper will increase 10 degrees after some driving, it can only be worse under the hood.

 

Last time I filled up, old school math style was 8.5, and no it's not lifted

 

I drive like a granny haha. I got 11.5 once, threw on an aem short ram and Flowmasters and my next fill came up 8.5

 

The short ram still sucks in the hot air from under the hood and there no increased pressure in the intake air either.

 

I've got a stock '89 and over the past four months have averaged 15.7 in town and 21.5 on the highway. I'm careful on the throttle (mostly) and typically cruise at 70 on the highway. I think your best approach for now would be to do a complete tune-up. Air filter, oil, cap and rotor, plugs, plug wires, check all vacuum lines and tire pressure. Also, have the codes checked. I've seen failing O² sensors cause a rich condition that didn't trigger a check engine light.

You should be getting 14-16 mpg city and around 20 on the highway with a stock rig, less-than-aggressive use of the skinny pedal, and properly maintained engine.

 

Did I mention I have 31s and 3" lift?

 

Yeah that seems very low, I have a stock 88. I get 19-22 in the city and 24-26 on the highway, you should be hitting at least mid teens if you are stock. Definitely needs maintenance, I would think that a 2000 would have to throw some code running that rich.