MisFire/Popping Sound
 

All,

I just had a used motor put in with 75K miles after my original blew with 144k. After installing the engine the shop tells me its misfiring and recommends new VVT's and new plugs. I have replaced 2 of the 4 VVT's and I still get the P0300, P0302, P0304 and P0306 Codes. My CEL is on and car seemed to be running fine until a week ago when it now has vaery high idle intitially then comes back down to around 1k but starts shuddering. I shut it off and when I restart it seems to be better. It also makes a popping noise when I start it that sounds like backfiring but its coming from the engine compartment. do any of you know if the VVT has anything to do with the misfires? I haven't read anything to suggest that. I am going to clean my MAF and possibly seafoam based on the fact that I bough this motor used from a salvage yard and have no clue if its all carboned up. Also wondering if it could be solved by a reflash since this isn't the original motor and possibly causing ECU issues? Any recommendations would be greatly appreciated.

So you've replaced 2 of the cams or are you talking about timing chains (VVT's)?

Is the cam timing correct or still off due to chain stretch?

Chevy has definitely done several updates to the software since the 2010 LLT was first produced (one was a timing chain stretch compensation)- so to be sure yours has the most recent, I'd take it to a dealer for a flash update.

Your CEL's are misfire codes on the passenger side cylinders- you could pull those plugs and see how they look, but it seems like it could all come down to cam timing. :confused: