96 Pontiac Sunfire pistons #2 an #4 misfiring?

[quick_5_up]

96 Pontiac Sunfire pistons #2 an #4 misfiring. I took it to my friend that has a computer. He hook it up an took it for a drive an piston 4 was missing like crazy wile piston 2 missed a couple of times. i change out my spark plugs and coil boots (wires) did a oil change. also he checked my EGR valve an discovered it was clogged with black soot. so we unclogged it an now after a month its clogged again and the black soot looked worse. Now a new code has come up to say o2 sensor after the catalytic converter is bad could that be the problem cuzing all the soot if not what is ? could it be the pistons misfiring if so why could they be misfiring?
will gutting the cat or replacing it fix the misfiring?

 

[thinlizzy]

more than likely a blocked cat so all the fumes has no-where else to go thus making the car misfire and it would be common for 2&4 to misfire as they more than likely run together i:e 2 up 2 down when pistons 2&4 are up 1&3 are down, a friend had a similar problem and it turnes out to be a really badly clogged cat it came out like black concrete when he took it apart!!!

 

[thebax2006]

The rear 02 sensor only monitors the converter, it does not have anything to do with fuel mixture or a misfire. To test the 02 sensors you need to have a scanner that can read engine data and read the front and rear 02 sensors voltage and watch the rate at which they change voltage. The front sensor should be changing constantly and the rear sensor should change slowly. If they both change at the same rate, the converter is bad. If this is one of GMs motors with the coil packs mounted on the back of the block I'll bet the coil is bad.
It's either spark, fuel, a vacuum leak, or lack of compression.

 

[*~Car-Girl~*]

Yeah it's your O2 sensor and Cat, i suggest gutting your cat as long as the shell's there is should pass emission check. Then run it with the computer and see if things improve.

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[bucks2743]

Sounds like you have a bad catalytic converter.This is what is throwing the code for your downstream O2 sensor.I seriously doubt you have a problem with a coil pack.Cylinder #2 and #4 should use different coils on your sunfire.The firing order is 1-3-4-2 therefore 1 and 4 should share a coil pack and 3 and 2 should share one.These type of ignitions use what is called a waste spark method.Eventhough only one of the cylinders is on the the compression stroke both fire because because they share the same coil.I dont believe spark is your problem.