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I have an LS1 with ET Heads, HPE S-Cam, 1 7/8 Kooks, a three inch custom exhaust systems including a magnaflow x-pipe and magnaflow turbo mufflers, and a ported stock TB. I had Thunder Mountain cats on until yesterday. A few weeks or even a month ago in the most severe heat we've ever had here since I owned the car I had it tuned as I replaced the TSP springs with Patriot Golds. The car laid down 420/367 and the AFR was 13.1 accross the range (trust me, it was perfectly flat).

Things have been sort of screwy for us lately and I laid off driving a while and took her out a few times in the last few weeks. Immediately I noticed the car crackling and popping and even backfiring. It was inconsistent though, spurious, and unpredictable. I put her back on the dyno and the AFRs were at around 13.4. Horsepower was up, given I had added 3.91s since the last pull I was putting down 415/362. I'm told 3.91s can cost as much as 15/15. Since it was much cooler I figured I had picked up some horsepower from the cooling off. Even on the dyno though it backfired when he shifted into third.

The tuner surmised I had a blockage in my cats which had been dented. I cut them off yesterday and replaced them with a straight pipe. Looking at the units I took out there was some compression and closing of the elements but nothing I would think would be severe. The crackling and popping has reduced but I still get a random backfire (not nearly as pronounced or as frequent as before).

We concluded the car was making good horsepower, the curves were smooth with no gaps from spikes or anything of the nature. Leak down test showed 4% across all eight cylinders. There seems to be nothing measurably wrong with the internals. Nor was the reason for the backfire apparent in the tune or the OBDII though I did briefly throw a lean code a couple of days ago.

Questions:

What could cause a tune to deteriorate (plugs are less than a month/1000 miles old)? By that I mean what could cause the AFR to go from 13.1 to 13.4? Weather? Temperature? Given same dyno each run.

What could cause a backfire going into third under power (a little aggressive lift and shift)?
 

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wrp said:
I have an LS1 with ET Heads, HPE S-Cam, 1 7/8 Kooks, a three inch custom exhaust systems including a magnaflow x-pipe and magnaflow turbo mufflers, and a ported stock TB. I had Thunder Mountain cats on until yesterday. A few weeks or even a month ago in the most severe heat we've ever had here since I owned the car I had it tuned as I replaced the TSP springs with Patriot Golds. The car laid down 420/367 and the AFR was 13.1 accross the range (trust me, it was perfectly flat).

Things have been sort of screwy for us lately and I laid off driving a while and took her out a few times in the last few weeks. Immediately I noticed the car crackling and popping and even backfiring. It was inconsistent though, spurious, and unpredictable. I put her back on the dyno and the AFRs were at around 13.4. Horsepower was up, given I had added 3.91s since the last pull I was putting down 415/362. I'm told 3.91s can cost as much as 15/15. Since it was much cooler I figured I had picked up some horsepower from the cooling off. Even on the dyno though it backfired when he shifted into third.

The tuner surmised I had a blockage in my cats which had been dented. I cut them off yesterday and replaced them with a straight pipe. Looking at the units I took out there was some compression and closing of the elements but nothing I would think would be severe. The crackling and popping has reduced but I still get a random backfire (not nearly as pronounced or as frequent as before).

We concluded the car was making good horsepower, the curves were smooth with no gaps from spikes or anything of the nature. Leak down test showed 4% across all eight cylinders. There seems to be nothing measurably wrong with the internals. Nor was the reason for the backfire apparent in the tune or the OBDII though I did briefly throw a lean code a couple of days ago.

Questions:

What could cause a tune to deteriorate (plugs are less than a month/1000 miles old)? By that I mean what could cause the AFR to go from 13.1 to 13.4? Weather? Temperature? Given same dyno each run.

What could cause a backfire going into third under power (a little aggressive lift and shift)?
Did you check to see if the header gasket had a leak in it, that could cause the backfire issue with no other effects.
 

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Discussion Starter #3
Wretched 6.0 said:
Did you check to see if the header gasket had a leak in it, that could cause the backfire issue with no other effects.
Good suggestion, I'll check that out. I assume you mean the gasket between the head and the header. I replaced the gaskets between the collectors and the flanges as I usually suspect them when there are issues.
 

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If you are running in Speed Density, Open Loop or with fuel trims turned off, your going to lean out with the colder air.
 
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