Spark plug and ignition coil problems.... plugged cat?

Evison: According to DoItYourself.com: " If your vehicle is behaving erratically and you have problems with it running smoothly, it could be showing signs of ignition coil failure. The ignition coil takes power from the generator and relays it to the spark plugs. This ignites the fuel and makes your vehicle run. The severity of the symptoms vary according to which of the ignition coils are failing.
Backfiring If your vehicle is backfiring, this can indicate early symptoms of a failing ignition coil. This occurs when unused fuel is emitted through the exhaust system. If you do not address the issue, you can also do serious damage to your exhaust, resulting in unnecessary and costly repairs. Your exhaust may emit a black smoke and you may even detect the smell of gasoline.
Starting Problems Check your high tension leads (HT Leads). These run between the distributor and spark plugs. Ignition coil failures result in one or more spark plugs not receiving the appropriate amount of charge. If you have trouble starting your vehicle in the cold, this is a good sign of potential ignition coil failure. You can run a simple test on the HT leads to check that there is in fact a spark going from each lead to each spark plug.
Fuel Economy If your vehicle is getting a lot less mileage for the gallon, This can also be a sign of ignition coil failure. When less power is reaching the spark plugs, your vehicle will struggle and use more fuel to compensate for the lack of power transfer.
Engine Misfiring Trying to start your vehicle when the ignition coils fail will result in a rough ride. Your vehicle will cough and splutter regularly and when you drive at higher speeds, your vehicle will jerk and spit. It will also vibrate when you are running idle at a stop sign or intersection.
Vehicle Stalling With ignition coil failure, your vehicle will have irregular sparks emitting to the plugs to keep it running. This will result in stalling. When you bring your vehicle to a stop, it may just shut off totally, thus leaving you needing to restart it.
Coil Failure If your spark plugs are worn out, it forces the ignition coils to operate at a much higher output. Keeping your sparks plug in optimum condition can reduce your chances of having ignition coil problems. Some simple regular maintenance will save you a lot of money in the long run.
If the gap in your spark plug widens due to normal erosion, it leads to an increased voltage being required to create a spark in the chamber. That increased voltage demands more current flow through the primary circuit and this increased current flow can overload the primary transistor. Keeping all these components in good running condition is also a big help towards keeping your coils in a good working condition."

Are you sure the spark plug wires are on the correct spark plug? Recheck the firing order and spark plug wires. Additionally check timing on distrubutor

replace fuel filter, clean fuel injectors.could have bad fuel, do a code scan before buying new parts.faulty egr valve or faulty engine coolant temperature sensor cause engine run poorly.replace spark plugs and wires because if spark plug wires is bad or spark plug and ignition coil is bad that will cause rough idling misfiring and stalling you have dead cylinder not firing properly..

You will need to figure out if you have spark to the plugs, and if not- if you have voltage to the coil to proceed.

did you just replace the plugs or replace ignition wires, rotor, and cap (or) ignition coils if applicable? and you check whether its fuel or spark problem by running the engine and taking the plug wire off the spark plug and seeing if there is spark and noting any change in the engine, if the engine runs rough(er) or stalls then you know there is no problem spark related with that cylender, if no change occours when you take the plug wire off the plug then you know there is either a no spark condition (you will be able to see if this is the case) or if there is spark and it is a fuel problem. self explanatory, if everything you test for passes then you probably have internal engine damage. recommended that you use rubber grip pliers when you take the plug wires off while the engine is running, do not ground yourself. Hope this helps! Joseph.

check your catalytic converter for clogged up.
disconnect the exhaust pipe before the cat and run the engine to see if it still cut off.
if it run better, then that's the problem

　
　
Ignition Coil
Removal & Installation
4.2L Engine
1997-2000
To Remove:
Before servicing the vehicle refer to the precautions at the beginning of this section.
Remove or disconnect the following:
The ignition coil electrical connector
The radio ignition interference capacitor connector
The locking tabs
The six ignition wires, marking for reinstallation
The brackets
The spark plug wire set
CAUTION
Spark plug wires must be connected to the proper ignition coil terminal. Mark spark plug wire locations before removing them.
The accelerator cable bracket nut
The accelerator cable bracket bolt
The accelerator cable bracket and position it aside.
The two ignition coil nuts
The ignition coil stud bolt
The ignition coil
To Install:
Install or connect the following:
The ignition coil
The two ignition coil nuts
The ignition coil stud bolt
Torque to: 71-98 inch-lbs (8-11 Nm)
The accelerator cable bracket
The accelerator cable bracket nut
The accelerator cable bracket bolt
Torque to: 71-98 inch-lbs (8-11 Nm)
The ignition coil electrical connector
The radio capacitor connector


CAUTION
The Spark plug wires must be connected to the proper ignition coil terminal.
Squeeze the locking tabs and connect the six ignition cables.
2001-2003
To Remove:
Before servicing the vehicle refer to the precautions at the beginning of this section.
Remove or disconnect the following:
The ignition coil electrical connector
The spark plug wires, marking for reinstallation
CAUTION
The sparkplug wires must be connected to their correct ignition coil terminal.
The ignition coil
The bolts
Inspect the ignition coil for carbon tracks or damage.
To Install:
Inspect the ignition coil for carbon tracks or damage.
Install or connect the following:
The ignition coil
The bolts
Torque to: 53 inch-lbs (6 Nm)
The ignition coil electrical connector
The spark plug wires, in their original positions
4.6L Engine
1997-1999
To Remove:
NOTE: This procedure is for either RH or LH ignition coil.
Before servicing the vehicle refer to the precautions at the beginning of this section.
Remove or disconnect the following:
The ignition coil electrical connector
The radio capacitor electrical connector
The spark plug wires by squeezing the locking tabs (four each side)
CAUTION
The spark plug wires must be connected to their proper ignition coil terminal. Mark spark plug wire locations.
The bolts
The ignition coil
To Install:
NOTE: The procedure is for either LH or RH ignition coil
Install or connect the following:
The ignition coil
The bolts
Torque to: 44-62 inch-lbs (5-7 Nm)
The ignition coil electrical connector
The radio capacitor electrical connector
The spark plug wires (four each side), into their original positions by squeezing the tabs


CAUTION
The spark plug wires must be connected to their proper ignition coil terminal.

　
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I've got a 2002 Expedition that experienced the same kind of problem. Turned out that the individual load coils on the spark plugs tend to act up when they get wet (either from rain, or possibly from fluid puddling on top of the plugs). Anyways, the fix for me was to replace the load coil. The #4 and #7 plug coils seemed to cause me the most grief (seperately on two different occasions). The wierd thing is unless it gets real bad, no computer code is generated. I took it to the dealer, thinking they had some sure fire way to find the problem. After my third trip back, they were able to figure it out through elimination. The next time it happened, I just did it myself.

Make sure heads are torqued (tight) don't over tighten aluminum heads can get warped, heat up re -torque use lock tight on head bolt treads.