ok.. Here it is.. i'm having a similar issue and cant post a new thread. Cylinder 4 misfire. Over and over no matter what I fix or replace.. there's no change. Replaced the injector even though it tested fine. Noided for signal.. fine. Changed intake manifold and gasket as well as the lower set.. no change. Changed plugs, wires, coils, ignition control module.. no change. I'm at the end of my rope with this car.. Saw to check for plugged cat. going to run it tonight and check it. Only thing I can think that it could be at this point is maybe a lifter loose but theres no noise.. maybe a stuck valve?? I don't know what else to check.. its got fire and fuel.. still missing in cyl 4 only.. Beyond stressed at this point.. Any thoughts??

 

I had the exact same problem with my O2. IT WAS the Catalytic converter. I had a local muffler shop remove it I went for a quick drive. Car never pulled so good. Went back to the muffler shop had them put in a new one and never had a problem since. So REPLACE your Catalytic converter but do as I done. Have the pipe cut off in front of the converter then go for a short drive. My two cents worth. Good luck.

 

Also I've done everything you guys have done to try and trouble shoot the problem then I went to the muffler shop.. Save your time for some other projects. Change your converter. Also save your old converter their worth about 50 $ at the recyclers.

 

Think I can temporarily punch a hole through the center of the cat? I don't have 300.00 just laying around after the retarded amount of parts I've already replaced. Also, I don't want to ruin the new icm with the misfire as I have another wk and a half to drive it before I can replace it. I'm going to remove the pipe from the front of the cat and see if I've got any change in operation before I do anything.. Im a lil skeptical after all I've done already with no change. Thanks for the info Goodz.. imma tear into the exhaust now.

 

Depending how bad a cat may be, you can sometimes look under the car while it's running, if it's glowing red, it's bad.

I've never done it, but I've read a quick check for a cat issue, if you can't easily unbolt the cat, pull the upstream O2 sensor and leave it out just for a brief test (allows the exhaust to vent somewhere else, but the car won't be getting it's O2 sensor readings for proper fueling).

As for things to do if the cat is bad, first, depends on your emissions testing in your area. I have heard of some people on different cars, removing the element inside the cat (basically making it hollow, like a piece of pipe). But on these cars, if you did that, you would have engine codes popping up.

If it's a cat, odd it's not mis-firing on more cylinders or more random. You may want to remove the #4 plug and wire (even though they are new) and re-inspect. Just to ensure neither are damaged. Stranger things have been known to happen.
Also, have you ruled out a failed injector? If you have a 3800 powered Monte, swapping an injector around is not too big a deal, trade the #4 injector with another ones and see if the problem follows the injector.