Kind of a long story, but I bought my '03 Monte about a month ago. Shortly after I bought it I got the dreaded P0420 code. I took the car to a shop and had the cat checked out. They said the cat was fine. I planned on replacing the cat anyway to be safe, but I have been in no hurry to get it done since they said it was ok. Today, when I left work for lunch, the car started running really rough and it sounded like there was a rod knocking or something in the engine while I was at a stop light. The check engine light was flashing so I threw my scanner on it when I got home. The code was a P0304. I cleared the code and started the car. I let it run a few minutes and the light started flashing again. This time I got a P0300 code. Tonight after work, I pulled all 6 plugs and looked them over. They were all about .020 out of gap. They didn't look like they were in too bad of shape. The biggest thing that I found was that plug #1 wasn't tight in the head. I re-gapped all the plugs, but the car still runs like crap.

The noise which sounded like a rod knocking or something was really worrying me. I jacked the car up and slid underneath it to see if I could get a better idea of where the sound was coming from. It was twice as loud under the car, and seemed like it was coming from the cat. If the cat is bad, could it be making that noise? I've never had a cat go bad before. I'm hoping it's the cat and not something in the engine. The oil is full and is clean.In fact it looks like it was just poured out of the jug. I'm planning on pulling the cat off tomorrow evening and seeing if the noise stops. Until then, I figured I'd ask you guys if anyone has ever known a bad cat to cause the misfiring and if it could be making that noise.

 

My 02 SS was doing the same thing last week ! It ended up having a cracked Rear Exhaust Manifold.

 

Good info. I hadn't thought of checking that. Thanks for the help.

 

If you have a mechanic's stephescope, it might help you isolate where the noise is originating (or get you close to it).

I had a 3100 in my wife's Grand Am that I believe had a sticking valve issue. Under recommendation of a knowledgible friend, I threw in an additive like Motor Honey, noise went away. I had to do that for the following 2-3 oil changes and finally no need for additive and the noise never came back. BUT, the car had no check engine lights OR performance problems.
Looking at P0300, appears related to mis-firing.
http://sm.gpona.com:9001/si/showDoc....m&cellId=25812

And looking at P0304, there is a TSB about possible crossed injector wires:
http://sm.gpona.com:9001/si/showDoc....460291&from=sm

I am suspecting you have a 3400. If so, to get to the injectors the upper plenum has to come off, I'm reluctant to jump to that step since you noted a loose spark plug. I'd first inspect the plug wires for signs of damage, check for vaccuum leaks and if you can get access to a scan tool, see if the PCM is reporting any cylinders misfiring (if you have more then one, see if they go back to the same coil pack).

Also, if you feel inclined to check the injector wires, the only reason those should have to be crossed is if someone recently serviced something in that region (such as replaced an injector or two, changed LIM gaskets). It's just not a typical area to be in. IF you have any knowledge of someone servicing anything around the injectors, I wonder if perhaps one is not properly seated (which could make a vaccuum leak too).

 

Unfortunately a whole bunch of things can be wrong with the P0300 code. And yes, the Cat can be one of the problems.

You need to now check all your ignition parts, plugs, wires, coils. Your fuel could be an issue too, if the car is starving it can throw this code. Fuel filter, FPR, injectors.

You could also have a bad vacuum leak, or a clogged up EGR (stuck open) making the car run super lean. A bad oxygen sensor (bank 1) could also make you run lean or rich.

 

Really good info guys. Thanks a lot.

 

Let us know what you find please. Im curiose on wat its gonna be.
Brian

 

I was looking at the car again last night and noticed that it is making an air leaking type of sound around the #4 plug at the head. Not sure what that means, but I noticed it, so I thought I'd put it out there.

 

I'm not a master at the firing order, but if I am correct that's the center plug on the back. I did not catch if you have a 3800 or 3400 but either way, there should be a large vaccuum line in that region going to the brake booster.

I good check for a vaccum leak is get a can of starting fluid and mist it around an area you suspect might have a leak. If it DOES have a leak, the fluid will get sucked in and you'll hear/notice a change in engine performance for a second.

If the noise is truly close to the head, it might be something else (I'm guessing, but maybe a bad gasket, hopefully not a cracked head). You can try using a mirror and with the enigne off see if you can get a look around there.

 

Well, I have it down to just a code P0304. I pulled the number 4 plug again and it was soaked in gas. I checked the spark at the wire, and it had plenty. I slid under the car and used a stethescope to listen to the cat, and all the usual places in the engine. The noise seems to be loudest in the heads. I did the starter fluid trick with no results. It made no difference. I checked the resistance on the number 1 and 4 coil. It checked out fine. So, now I guess the next step is a compression test. I'm wondering if it doesn't have a valve issue. Maybe a burned valve or a stuck valve.