A leaky/cracked manifold or faulty O2 sensors will likely cause a 30-40% drop in gas mpg's.
Tim, what is the proper way to test the coil?
The quickest and simplest test is easy, and very definitive.
Remove a plug wire, hold it about 1/4" from ground and have someone crank the engine or even start it. You'll need insulted pliers to hold it.
Check the COLOR of the spark it's a great indication of the secondary voltage. If the insulation inside the coil is breaking down, it limits the voltage the coil can provide.
It should be a SNAPPING BRIGHT BLUE SPARK - any other color indicates a lower voltage. White, yellow, or red is not near enough voltage.
If it's not the desired blue, check the color right at the coil tower. If it's still not blue, replace coil.
If it's blue at the coil tower but not at a plug - replace coil wire!
Secondary insulation breakdown is very common on the Jeep coils.
Aftermarket coils or crappy “Lifetime Warranty” coils from China don't last long.
Test the coil with your volts/ohms meter. You should have about 1.0 OHMS here
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and about 10,000-12,000 OHMS here
.