What did I do wrong????

I have a 90 XJ Laredo 4WD 4.0L. I was getting around 19 mpg and changed the plugs and plug wires and now am getting just over 17 mpg. I put Champion truck plugs gapped at .035 and Champion truck wires that are supposed to be for SUV\offroad type driving. I also replaced the cap and rotor. I just cannot figure it out. I did not change anything else at this time. I should have at least gotten the same milage. I sure did not expect a drop. Any help with this would be greatly appreciated. I plan to change out the sensors as time allows but have not gotten into that yet. Is there something I should check at this point??

How far have you driven it since all of these changes?

Roughly 300 miles.

I am curious what the gap was on the old plugs you removed?

It may not be anything you did. Could a part going bad at the same time you worked on it, or the last batch of gas you got may have had more ethanol that usual, or some of both.

I'm thinking its because of the "winter formulation" gas....

I had the exact same thing happen on a 1998 4.0 after doing a tune up with the "accel tune up kit". Plugs wires cap rotor.... It also did not run or start as nicely, though it was OK. Never figured out what it was or why.

gasman1 said:

The old plugs were gapped .043 at removal. Do you think I should open the gap a little even though specs call for .035. Also the old plugs were standard Champion plugs.

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No, they were probably worn so that they opened a little and originally put in at .035.

So the $$$$$$$$$ question now is did the old worn gap plugs get you better mileage, or was it a change in the gas, or something else yet to be found.

If one had the time (?) one could regap the new plugs to 0.44 and see what happens, and aslo retry the old plugs (if you still have them) and see what happens each time. Try a tank of gas each time. Probably need to repeat the tests more than once to confirm the source of the change in in mileage.

My sons '96 Ford taurus is factory gapped at 0.44