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New Jeep...new problems!

So, under the advice of Anak, I ordered a new water pump, coolant, radiator hoses, radiator cap, coolant temperature sensor, and a new thermostat. The thermostat housing looks really new so I am not replacing that.

I will also be replacing the fan clutch and belt.

Today, on a whim, I decided to put in the new radiator cap, coolant temperature sensor and vapor solenoid thingamabob. As I was about to start wrenching on the coolant temperature sensor I went to unplug it and the plug just came off. The clips were completely broken off. The gauge works just fine (or it's doing a helluva job fooling me). I've read that on the 97 and newer XJ's that there is only one sensor and that sensor sends signals to the ECU and to the gauges. Furthermore, the sensor could be bad but still functioning well enough to have the gauge working but not functioning well enough to tell the ECU how much fuel to send. Does that sound like that is happening here?

After installing everything, I went to the gas station and put a little over 5 gallons in. I then drove over 40 miles. The gas gauge moved but not nearly as much as expected. I wonder if this was the reason why it was running so rich resulting in my 10mpg.
I've read that a bad/open connection to the temp sender can read as a very low temp to the ECU, which would definitely think it needs to dump more fuel to get the engine temp up (similar to how the choke on a carburetor enriches the air-fuel mix by closing off some of the air intake, but more refined/better controlled), which would agree with what you're seeing.

As far as the dash gauge, you can always use one of those contactless thermometers to check the radiator or thermostat housing to get a "second opinion" on the coolant temperature and confirm the gauge is reading ok.
Yesterday, I went to start replacing accessories like the fan clutch, water pump, thermostat, and harmonic balancer. The day before yesterday I emptied the contents, as much as the lower radiator hose would allow, and I refilled with water. I drove around for half an hour and returned to empty that. I then filled the coolant with Blue Devil radiator flush and water. Drove around for fifty miles or so and then emptied it. Then, I just refilled with straight water. I used distilled water each time. I drove around for about ten minutes and parked it overnight. Drove around yesterday for about an hour and then drained that. The water was very brown.

Unfortunately, I also smelled gasoline in the coolant. Does this mean the head gasket is blown? Would this also explain the overheating issue that previous owner was talking about?

I checked fuel mileage before I dug into her and I was averaging around 16mpg.

As far as the replacement parts process goes...the lower water pump bolt grew legs and left town. The new belt I had to put on was way too short. The harmonic balancer puller I rented from Autozone didn't work and to top it off the bolt for the belt tensioner broke.

This Jeep hates me.
It doesn't hate you. This is just that Jeep thing that rational folks don't understand.

If you solved all of its problems you would have to worry that something dramatically bad was about to happen.

I wish you were closer so I could lend you a hand.

Gasoline smell in the coolant is an odd one. You could use a block test kit (colored chemical in a special vial that draws vapor at the radiator cap and changes color if combustion gases are present) to confirm a head gasket (or worse) problem.

The junkyard is your friend when it comes to replacement hardware. The tensioner bolt can probably still be had new however.

It also helps to have some containers around for keeping track of hardware as you take it off. I use anything from Cool Whip tubs to tuna cans. A coolant bottle laid on its side and a rectangular cut-out (maybe 6" square) in the top side also makes a good parts tray--fairly large and not likely to lose parts when it gets accidentally kicked.
Yeah. I will start working on it tomorrow. I have to replace the harmonic balancer on my camry first.

Thanks for the advice guys.