Misfiring 2006 Pontiac Grand Prix

[Evans7]

I was wondering if anyone would be able to point me in the right direction. I have a 2006 Pontiac Grand Prix NA with 267,800 miles on it. For the past couple weeks it’s been misfiring under load (around 3.5K - 4K RPM). I’m kinda new to fixing stuff like this that isn’t easy (for me atleast) to figure out why it’s doing it. Before the engine was also stalling and misfiring until the Ignition Control Module got replaced and now it just misfires. 

The 2 codes I have are P0420 and P0300. I had taken the car to a private shop previously to have a used transmission put in the car since the original transmission was on its last leg but it didn’t start having these problems till 2 1/2 months later. When I had taken the car back to him this time he replaced the ICM and it fixed the stalling he then told me the crankshaft position sensor seemed to be going bad and the timing chain might be stretching, and I believe he said that cylinder 2 was misfiring (could be wrong), he had originally thought it was the catalytic converter based off of what I had told him, and he told me it wasn’t and that the catalytic converter wouldn’t get that clogged and that it could essentially unclog itself (kinda sounds like bs to me so I’m not sure). The next day I get a Crankshaft position sensor code, so I replaced it and the code went away but still had a misfire.

Talked to a friend he said to pull the plugs to the coil packs to see if the idle will change, did all 6 plugs and to me each one made the idle rougher so the coil packs are fine id assume.

Backtracking to when this all started I had left a friends house going down there long driveway letting the car coast and then it stalls, I honestly can’t remember if it was the same night or the day before or after that I got the code P0303 for Cylinder 3 Misfire but I cleared it and it never came back. I didn’t feel the car miss like it is right now so it was kind of a surprise to see the code. Ever since then it had been stalling and misfiring till were I am now. 
 

The P0420 code will normally come back a day after clearing the code. 

I haven’t just been sitting around not trying to fix it so here is some things I’ve replaced I’m not made of money either so even if there is still I should of tried replacing I can’t afford too.

Ignition Control Module

Crankshaft Position Sensor

Serpentine Belt

MAP Sensor

PCV Valve

Spark Plug Wires

 

if anyone has a clue on what I need to check let me know and I’ll gladly do it, or if you know what the problem child is. Thank you for reading I probably said more than I needed too.

[Last Indian]

Well, first off the mechanic you went to needs to go back to class. Cats go bad! And when they do, depending on what has gone awry, they can do many different things. Code P0420 is a catalytic system code! There are many things that this can be, but one of the first is rule out the cat & the two oxygen sensors.  To do this on your own may present some issues depending on your capabilities & tools. At nearly 300,000 miles the bolts at the back of the cat that connects to the back exhaust system will more than likely need cut off & replaced. You also need a replacement or a way to check the cat for both efficiency & flow. A replacement could be a non catalytic pipe, but you still need a way to link the o2 sensors & their feedback. 
You could also be looking at some fuel delivery issues with regard to the missfires. So check your fuel line pressure at the fuel rail. You could & more than likely do have some fowling of your injectors unless you have maintained the system well. So if you don’t use a good fuel treatment like STA-BIL & Gumout’s Regane complete fuel system cleaner I would suggest doing so.

Edited March 19 by Last Indian

[Evans7]

I’m talking on 2 different forums but this is probably relevant.
 

Compression is fine and the doesn’t seem low on power. Had the chance to hook up a more expensive scan tool to the car and did a relearn on the crankshaft position sensor. The car won’t misfire during idle until under load like I thought.

This may be concerning, according to that scan tool underload cylinders 1 - 4 are misfiring, although this was tested before doing the spark plugs.

Finally replaced the spark plugs instead of doing just wires and the problem still is there, although I can’t get the code to actually pop up again the check engine light flashes, but it’s misfiring like it did before. Tested Coil packs once again, the top coil pack seemed weaker but I was told that was not close to enough to cause the misfire.

The car struggles to go faster when you’re going like 70mph onto 80mph, I don’t know if that just has to do with the misfire or if that could be something else.

I got told once again that the timing chain is probably stretching.

What else could I test or is there any parts of concern?

I did read your post but right now after clearing the codes my P0420 didn’t come back and I feel like there was more important and easier stuff to check before digging into the exhaust system. Today I will test fuel delivery and stuff. My fuel trim is good when it was checked. 

[Last Indian]

Well if for now you ignore the possible cat issue & focus on the misfire issue I would suggest pulling the injectors & test them. To me this is the most obvious scenario if the fuel pressure is good & coils, etc check out. Lack of fuel will cause misfiring. With injectors that are fouled or plugged this will get worst as rpm increases. Remember this is a port injected system! Which means there is always fuel laying in the intake manifold. So a weak or plugged injector may not feed its cylinder the proper fuel, when the intake valve on that cylinder opens it can rob fuel from an adjacent cylinder intake area, causing a multi cylinder misfire. You can make a simple setup by getting a spare injector pigtail wire, a little giant pump, connecting hose, 12 volt supply, some gas & a container for the injector to spray into.

[Evans7]

I’m getting free Coil packs, so the ones in the car are getting replaced and then I’m getting the car diagnosed for free from a reputable shop near me, and I’ll just do the repairs, but I’ll definitely let you know what the problem ended up being because it has me stumped (clearly). 

[Evans7]

I’m getting free Coil packs, so the ones in the car are getting replaced and then I’m getting the car diagnosed for free from a reputable shop near me, and I’ll just do the repairs, but I’ll definitely let you know what the problem ended up being because it has me stumped (clearly). 
 

It was indeed the catalytic converter, I stopped getting misfire codes after replacing everything for spark.

Edited March 26 by Evans7