# 05 Engine Misfire me Stumped (long)



## MostlyStock (Aug 31, 2005)

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?


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## jpalamar (Jul 22, 2008)

Shot in the dark. Clean the MAF? Maybe do a compression test? Atleast its cheap and can elliminate things. Clogged fuel filter? Fuel pressure?


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## MostlyStock (Aug 31, 2005)

*thanks*

Yeah, I was going to test compression next, could fuel pressure cause this, I mean when it runs with open manifolds it runs fine, drives fine, so i was discounting fuel pressure since I couldnt see any way for the fuel pressure to matter in the whole cat on versus cat off running and not scenario...


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## GuatoLT1 (Jul 22, 2009)

My buddy had an 96 Z28 LT1 with clogged cats and the car wouldnt even run. Took us a while and did almost everything you did, then took the cats off and it ran perfectly. His had about 80k milles. I was going to suggest your fuel pressure regulator. But since it runs fine with open headers, sounds like its your cats.


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## MostlyStock (Aug 31, 2005)

*Solved*

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.


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## jpalamar (Jul 22, 2008)

I would replace them all. I belive these would work for you. GM LS9 Valve Springs Matched Sets - Lingenfelter Performance


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## Rukee (Feb 8, 2007)

:agree if 1 broke, then how can you trust the rest of them?


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