Since I've done a tune up, replaced 02 sensors, upgraded the whole exhaust (manifold included), and cleaned the cooling system, the XJ seems to be getting better gas mileage. Still, the largest fuel loss (as noted from the gauge dropping) appears to be when I start the vehicle. Does an XJ just dump gas when cranking? I've been trying to limit how often I start and shut off the truck because of this.

 

How can you tell this??? If you see the fuel gauge drop when starting you have more problems then you think.
No the fule pump kicks on for only a couple of seconds to pressurize the system and shut off until it need to re-pressurize.

 

How are you measuring this drop? Of course a vehicle will burn more gas on a cold start as the mixture is rich.. But it will not cause a noticeable drop on the fuel gauge.

 

It will run a little rich until it reaches a certain temp. So, it does use more gas when it's cold than after it warms up.

 

Try a locking gas cap.

 

What Turbo said. Although last time they drilled a hole in the bottom of my tank. Beyond that maybe you aren't getting a good enough seal with your current gas cap.

 

haha, right.....

I just mean that I can drive for 20 minutes and barely notice the gauge move, but when I shut the car off and start it back up the gauge has dropped like a quarter of a tick mark.

 

Since you say this started after doing a tune up and other maintenance items is it possible that you are just paying too close of attention to the gauge? If you know what you averaged in miles per tank of gas then check the trip odometer the next couple of times you fill up and see if those numbers are in the ballpark of what you used to get.