Generation IV External Engine - Please help with difficult troubleshooting problem

I need help troubleshooting a wierd problem. I have a 2005 GTO with an LS2. Bone stock with 88k miles. I will describe the problem and then what I have done so far to troubleshoot it, not trying to be a jerk, but please read what I have already tried before responding, as I have tried alot... Thanks in advance for any suggestions. I am completely stumped.

This car has been running perfectly. One morning i Drove 15 miles round trip with no problems whatsoever. About an hour later started back into town and all of the sudden it started running bad, check engine light flashing, no power, would not idle. Limped it home and put a code reader on it, p0300 random misfire. Alway 300 never isolated to one cylinder.

Below is what I have tried, with the results.


* New plugs and wires. Same result p300 will not idle
* Cleared codes left battery disconnected overnight. Same p0300 no idle.
* Drained fuel tank replaced with 10 gallons fresh fuel and a bottle of heet fuel dry, also drained fuel lines, removed fuel rail and drained, blew out rail, then reassembled everything....same p0300 and will not idle
* Disconnected all o2 sensors. Same p300 will not idle.
* Feel exhaust coming out of tail pipes and it "seems" like less pressure out of right side.
* Disconnected exhaust at Muffler from cats back leaving cats attached Same p300 will not idle
* Removed cats, open manifolds,no p300 code, just codes relating to missing o2 sensors, now it will idle correctly. So I think I have found it. But the cats don't look clogged and don't exhibit back pressure when hitting with compressed air. Since I don't believe that both clogged simultaneously i continue below.
* Remembering right side having less pressure i put them on one at a time. Left one first. With left cat on and right one off, still idles fine no p300 code, just codes relating to missing o2 sensors. So seems to me like it has to be right side cat. Order new hi flow replacement direct bolt in cat from magnaflow. Bolt it in, start it, same p300 will not idle. WTF? If I remove this cat it idles and runs, put it on, and it will not run or idle and sets p300 random misfire codes. O2 sensors in or out no difference.

Seems like any back pressure at all on right side causes the problem, but wife won't drive it open manifolds :)

Any ideas on what else to look at here, without just throwing parts at it?

Found the problem, it is a Broken Valve Spring on number 4. I see that this is a known problem on the older LSx engines but was supposed to be fixed before the 2005 Model Year engines. It does have the yellow springs, is completely stock and not run hard at all. Seems like i got lucky and it didn't drop or cause any other damage, but I haven't pulled the head, am going to replace the spring and pressure test it to see before pulling the head if necessary.

I have to question whether this is because of a defective spring or are they all at risk and need to be replaced. 88k miles, rarely run very hard. I am trying to decide whether to replace them all or just the one. And if I replace them all, whether to replace with stock or with some aftermarket, and if aftermarket which ones?

Thanks for any help.

I am glad you found your problem! It seems like you would of heard that valve spring?

Call up TSP and get a set of dual springs.

How bad did it damage the head and seat?

Not meaning to resurrect an old thread or anything but I started having a similar issue the other day. Its been parked since other than to drive it across the street to the parts store to read the P0300 code. I quickly checked my wires and they weren't coming into contact with anything (stock wires, too short to touch manifolds but i realize they could be giving way being 10 years old). The car doesnt seem like its falling on its face or anything while the check engine light is blinking.

I came across this thread in a search and at first it seemed like my Cats would be the culprit as your issue was looking to be. I recently had a true dual system put on my APS twin turbo'd 02 TA (rear O2's programmed out) and had the exhaust fabbed to accept my modified stock Cats (cats had their ends chopped off and V-Band Clamps welded in so that i could easily swap them with race pipes). I'm going to try and remove the cats and run it open dumped to see if the code stops (hopefully later tonight after work) but I'm confused as to why your broken valve spring was causing the issue seeing as how the code disappeared/reappeared with the removal and application of the cats respectively. I'm hoping in my case its the cats and not a broken valve spring. I have emissions coming up next month and I need this fixed fast. Go figure this happens right before i take it in to get tested.

Anyone have any insight into what would cause the code to disappear and resurface like that with the cats going on/off?

If the removal of my cats fixes the problem what should my next step be? I obviously need the cats for emissions but I wont pass with a blinking check engine light.

If removing the cats doesnt fix the issue from what i've gathered it looks like i should check the plugs and then wires next. If it comes to it and i pull the plugs, what exactly should i be looking for? Same with the wires.

Sorry for all of the questions, just kind of in a bind as emissions are creeping up on me fast and i don't want to prolong this ordeal.