Gurus, I'm trying to figure out a situation and need a little input...

Last summer, while changing the 02 sensors on my '92, my mechanic had a real difficult time getting the passenger side sensor out. He ended up having to drop the section of the exhaust with the converter in it to get better access to the sensor. He told me back then that when he took the converter down, a couple pieces of the substrate fell out and he could see that the cat was cracked (not sure where), but still looked reasonably serviceable. He went ahead and put the new 02 in and put everything back and the car seemed to run okay and that's how it's has been since. My mechanic's thoughts was that something, at some point, got the passenger side cat really hot causing some damage to it, and perhaps that was also why the 02 was so difficult to get out.

The car was having onging problems with a high idle that I finally traced down to a bad ECM earlier this spring. After replacing that, the car runs very well, but now that it idles at normal RPM, I'm noticing that it has a bit of an uneven, "shaky" idle. Any roughness goes away as soon as any throttle is applied, and like I mentioned, it runs great otherwise. I was working with a tuner this past spring as well, getting my fueling fine tuned for the Bosch IIIs that I'm running, and he mentioned to me that he was seeing that my BLMs were consistantly somewhat higher on the passenger side than on the driver's. Plugs and wires are a year old. EGR valve and EGR solenoid were replaced last year, and the Opti is original as far as I can tell. The car has 67k miles.

So, my own personal diagnosis is this: Since the cat has deteriorated a bit internally, I now have either slightly less backpressure OR a slight blockage (not really sure which) on the passenger side, and this is causing the leaner BLM readings on one side along with the uneven idle. This condition is only really noticeable at idle, as under acceleration, the engine is producing enough additional exhaust flow to overcome the difference between the two sides.

Think I'm barking up the right tree?

Since the O2's are prior to the cats, I would guess that if the issue is exhaust related it is more likely some kind of exhaust leak, i.e. extra air is getting to the sensor and giving a false-lean situation. Alternatively these cars are prone to split BLMs at idle.

Since you have a shaky idle, there could be other issues that are hard to diagnose - maybe your idle speed is too low in the tune (happened to me once..), could be a number of other things, maybe a sticky injector, loose plug wire, sticky IAC.. who knows...

Good luck!

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Yea, quite possible.

Have you checked for codes on the dash?

You can do a check on the cats if you have access to an infrared temp gun.
With the car fully warmed up, run it for 5 min @ 2,000rpm.
Then hit the exhaust pipe about an inch in front of the cat and an inch behind (on both cats).
The temp at the rear should be hotter. If they are close to even, the cat isn't working right.
I got this method from Rick Anthony, it works pretty well.

Diagnosed my Silverado this way and swapped cats to fix a check eng. light.

someone, and I don't know who, is making a oem cat install for the LT1 using a random tech cat.

I thought it was TPiS, but it's not showing... it was in one of the corvette mags last summer as a "new product".

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I saw it by just Googling Randon Tech. Damn pricy $$$$

@djxib: You may be on to something with the idle speed. In the tune, the idle is set to 550 RPM, in gear, once everything is up to temp. That's actually the factory setting. The car has a new Professional Products 52mm throttle body that you can actually tweak the idle on. I upped it a few weeks back and it did help to smooth things out a bit. I know that when I turn my A/C on, the ECM raises the idle to 600 RPMs in gear, and it's smooth as silk. Maybe with its age, the different injectors, different TB and different tune, 600 RPMs is right where the engine wants to idle now. The IAC is brand spanking new, and the injectors are from FIC and were just installed at the end of last summer as well. The idle isn't really a BAD situation, just a bit annoying.

@BillH and @Bogus: No codes. I use an Auto X-Ray scan tool to see live data and also to scan for codes, and everything is clean. Like I mentioned, the car runs absolutely great with the exception of this minor little shake at idle, and the unbalanced BLMs that I'm seeing on the scan tool. Knowing that the '92 LT1 was a batch fired system, I guess I could see how idle quality and fuel trim balance could be affected. I was just wondering if perhaps this situation with the catalytic converter might somehow be contributing to it. Also, I did see those Random Tech cats and they are a bit pricey. Magnaflow offers a direct fit/bolt-on high flow cat for C4 Corvettes, and I've seen them on eBay for $160.00 per side. If I replace the cats, I was leaning toward a pair of those. I was just trying to determine if my theory of a differential of exhaust flow between the two sides due to a damaged cat could potentially cause the symptoms that I'm seeing. Ya' know, more or less reassure myself that replacing the cats will have a reasonable likelihood of fixing the problems before I take the plunge and spend the time and money.

I think maybe I'll look into picking up one of those temp guns and trying the cat test (should we call it a "cat scan"?). I believe I've seen them at Harbor Freight for about $40.00.