Very Red XJ

Yes, that maintenance can be useful, but often previous owners of our 20+ year old XJ's didn't perform that maintenance. In addition, some manufactures place large fuel filters between the fuel pump and the injectors. However, Chrysler only places a screen in the fuel tank that lets a lot of impurities past in the XJ's they've built with their fuel injection system. My original Chrysler injectors were so full of muck that a spray can of carb cleaner barely phased it. I had to use my smallest screw driver to remove the muck restricting the flow of fuel in the injectors of my Jeep.

Very Red XJ

YES replace them - a correct mixture should produce a light brown patina on the plugs
Does your jeep smoke when you let off the gas or when you press down hard? What color is the smoke?

robsjeep

The plugs look dirty, Id replace. do all six look the same? The color rendering is not too good, so hard to get a read on the color as the two photos of same plug have different color. But I do not see lots of soot, which is good, but it does appear crusty dirty. remove all six, mark each with a sharpie pen as to which cylinder as you remove them, then compare them all to each other. if any looks significantly different, let us know. an intake leak near the port, and failed injector, bad spark wire, or bad plug or worn piston rings/bore, or valve issues may cause a plug to look different than the others. the plugs should look near identical in color, deposits, wear for all six, if not, then you have another clue to this puzzle

good luck

evil_goat

Ok, I will probably replace the spark plugs. Should I also replace the wires? I know they have been replaced before because they are a faded red color.

evil_goat

I got a better photo of the spark plug

evil_goat

Not really, just the normal whitish smoke that happens when I start it.

Very Red XJ

A VOM can measure the resistance in the wires and tell you if you need to replace them. If it doesn't smoke and it's not burning oil, then you're running rich. If all the plugs look the same, then you should use that VOM to test your sensors: idle solenoid, MAP and TPS. Did you test or replace your O2 sensors? The front one is the more critical if you have two of them. Have you checked for frayed wires? Assuming the plugs are the right heat range, you'll have to find out why it's running rich. A good shop manual will cite the procedures to test your sensors.

Brynjaminjones

Have you figured out your actual MPG by recording how much it takes to fill the tank and calculating it that way?

I wouldn't say that the fuel gauge on these is anywhere near accurate enough to estimate actual MPG from, so I'd try calculating the MPG properly to make sure there really is an issue before throwing any more parts at it.

evil_goat

A full tank to running out of gas resulted in 195 miles on the odometer. or about 10 mpg. I know I should do more tests, but I have not been driving much lately. Next time I fill up, I'll use your suggestion to calculate. Also, would a full tank be slightly above 20 gallons? My mom's car says 25 gallons, but can hold up to 27.

robsjeep

I can squeeze in an extra bit more than two gallons beyond what the gas dispensers shut off at. Exactly how many total, dont know. Fill tank until normal dispenser cut off amount. then drive a bunch of miles, then refill the tank to the normal dispenser shut off, preferably using the same dispenser. Now do consider that if your odometer on which you base your miles is off, so will be your mpg calculation. Tire size changes, wear, tearing cable or just plain bad calibration can screw up the odometer, so it is best to check it over a known course or gps on straight road. Remember just cause if the speedometer IS known accurate, that does not mean the odometer is too, so check the odometer if you want reliable mpg results.

Driving style has a HUGE impact on mpg. My 5.5 inch lift, 33x12.5 mud tires, geared 4.11, winch, overlanding gear, tools, roof rack, gas cans, camp gear, etc.. (i.e. heavy) can get 17 to18 mpg*** on a level highway going maybe 60 to 65 mph. Now if I cruise at north of 70 to 75 mph the mpg goes down to 15 mpg or so. Hammer the pedal in the uphills, hit high winds, go fast, all drop mpg significantly in my jeep, more so than any other car I have owned. Around town, which I try to avoid, maybe 12 to14 mpg usually, but around town can vary a lot. Four 1 mile cold morning trips one each morning will burn more fuel than one 4 mile trip all other things equal.

Offroading, I get 8 mpg at best over the course of near 100 to 200 plus miles or so, mix of crawling, rocks, primitive trails, near all in 4x4 low. I carry at least 10 gallons spare gas on these kinds of longer trips, as diversion to a gas station is not an option.

So my mpg is near 8 mpg to 18 mpg




*** 18 mpg on Nevada free state gasoline, 17 mpg on California "gasoline". All other values are for California " gasoline". 🤔

Very Red XJ

A similar thing happened to my Jeep: At a gas station with the gauge on empty, I could only get about 15 gal into my tank before the pump cut off. Then I'd have to nurse more gallons in slowly. Chrysler engineered the filter/pump/regulator/breather into one unit. After I dropped the tank and replaced that unit, I had no further issues.

evil_goat

Here is what the rest of the spark plugs look like:
They look a bit darker than the first one. Would these plugs indicate running rich/bad O2 sensor? Also, is the black band on the first picture normal?

Very Red XJ

Once again - You are running rich. One can only guess why. You need to get a shop manual and a VOM and test the: O2, TPS, MAP and idle solenoid - clean you throttle body and idle port.

evil_goat

Would an analog voltometer work?

Very Red XJ

YES - they aren't as accurate - but they will convey enough info - for example, the meter moving smoothly in proportion to the movement of the throttle when you test the TPS is more important than the voltage reading