I was also wondering that too. And I hope you have a net up. Thatd be a ***** if you rolled over or got into an accident..just some food for thought..

 

Exhaust pulses between pressure to vacuum - that's how headers work.

A leak "sucks in" Oxygen, gets on the O2 sensor telling the computer it's lean - computer makes it run rich - there goes the mileage!

Tie that stuff down hard - unless you think you'll look cool with a screwdriver, wrench, or jack sticking out of the back of your head. Not just a roll, a simple fender bender sends stuff flying.

 

All of my tools, jacks, straps, chains, etc, are either underneath the back seat, or in metal boxes that are tied down. The "screwdriver to the head" had crossed my mind.

 

Oh well good, I'm not trying to be 'that guy'. Just trying to help out. Hahahah

 

There you go.

Check for a cracked exhaust manifold (very common in the XJ), which will suck in extra oxygen, telling the upstream o2 to feed more gas than is actually needed.

 

After reading all of the posts on this thread, I'm noticing that everyone's "estimating" fuel economy based on distance between fillups...

not accurate, folks!

This works for every vehicle, everytime, regardless of tank size, unless your odometer doesn't match your tire size, and even then this is more accurate than guesstimating:

Fill your tank and write down your mileage or reset your trip odometer.
Drive around as normal until you would normally fill up, then go to the same pump, parked the same direction, to do it. Parking at a different angle could throw your fill gallons off by a gallon or more, if you live in hilly country (like I do).

Fill up, write down gallons and trip miles. If you don't have a trip odometer:

second mileage (minus) first mileage = trip miles

then do the math:

trip mileage (divided by) gallons of second fill = mpg

This is the number to work from. When you've made a change, check again. This is the only way to accurately know if you're making improvements.

Who is it that says, "Without facts, all you have is opinion!"?
Too True!

ps. I check mpg with every fill. It's like an early warning system to let me know if something's going wrong...

 

Thats what I was doing. ^ haha execpt for where I said im just guessing on this one. then i post back later with the real thing. haha

 

On both of my last XJ's, (used) I noticed that the o2 sensors were original (but no engine light). on my 1992 XJ, i replaced the o2 sensor and got a gain of 20-40mi. per tank. On my 1999 XJ i replaced BOTH pre and post cat. That gave me a 30-50mi. per tank improvement. None of them sent codes to the computer, but they were definitely bad in my opinion. and a B**CH to get out. But no matter what, XJ's suck up gas!

 

I have a new exhaust manifold, 2 new O2 sensors, new pipe from the manifold to the cat, the plugs, new wires, new distributor and rotor... an I still get 13 MPG average.

 

Maybe next get a compression check.