I recently had a pretest smog check done on my car 84 camaro Lg4 V8 (carbed computer controlled ) the car failed due to high CO & NOX all visuals passed except gas cap was bad.The #'s or results were [CO (%)@15MPH max 0.82\AVE\0.13\meas\0.79 = pass NO (ppm) max 1125\ave\399\meas\1890 = fail 25 mph CO (%) MAX\0.62\AVE\0.11\MEAS\0.74 = FAIL\ NO (PPM) MAX\955\AVE\332\MEAS\1828 = FAIL Timing set at 6* btdc pass Since the test purchased a new CAT the guy that did the work(installed CAT) showed me old CAT and pointed out that it had some big chunks missing inside it ,basically fully deteriorated inside.Can anyone tell me if a new CAT would or should bring my numbers into the permissable passing range or should I also replace the EGR too just to be safe?Also what does the difference betwween a positive back pressure or a negative back pressure EGR have as far as effect regarding emissions (is it a propriatary thing or can they both work or be functionaly used without changing anything?

thanks

 

A fresh cat should make a significant difference in your results all by itself.

I wouldn't touch the EGR unless you have strong reason to beleive it's bad (you don't have that just yet).

Knock that initial timing back down again, at least for the emissions test. Advanced timing raises cylinder pressures and temps, which causes increased NOx and CO, to some extent. Exactly what's failing you right now. Set her back down to 0* BTDC initial timing (stock setting) for the emissions test and bump it back up when you get home.

If you don't know how old your O2 sensor is, I'd replace it at the same time you get the fresh cat put on (very easy to do while they're under there). I always replace the O2 any time I put on a fresh cat.

 

Thanks for the info.The reason I asked about whether or not the pos back pressure EGR would perform the same as the negative one is this,I went to the junkyard found a Trans Am that had the identical emissions tag or sticker under the hood(#'s matched completely,motor same etc...)I purchased the carb,distributor,maf sensor,EGR,EGR solenoid at least that's what I think it was called and put all these parts on my car"except for the EGR valve because I noticed it was stamped "N" instead of "P" and "P" is what came with my car from factory.So I am still in a holding pattern on whether I should or could use the "N"back pressure type since it along with all the parts I mentioned above came off the TA with matching emissions label,I mean my motor looked exactly the same in every aspect vacumn routing,componet locations,etc...except mine of course is a Chevy and the other car being pontiac still GM motor correct?Just a bit curious as too why the TA was outfitted with a "N" EGR and mine was outfitted with "P" EGR?????And btw the car runs about 100% better since I swapped the carb and distributor,MAF etc...just need to get it smogged,I live in California and we know what a bastard it can be to pass a 3rd gen camaro w/comp controlled Qjet....

 

Damon, he lives in the land of the smog *****, he can't change his timing by more than 2 degrees either way from factory or he automatically fails.

stevog66, most aftermarket cats don't do much for NOx.
with your NOx that high, i can see you having a problem with your EGR system.

the type of EGR valve does make a difference.
a positive back pressure valve requires some back pressure in the exhaust to work, a negative back pressure valve requires a slight vacuum in the exhaust at the valve to work.
a non-back pressure type EGR valve would be best, it doesn't matter if there is back pressure or not for a non back pressure valve to work properly.

like Damon said, if your O2 sensor is old, replace it with a new Delco sensor.

 

I checked Napa and Autozone for non-backpressure EGR came up with nothing can you tell me where I can find such a part?Tognotti's maybe?And you are right about the "smog *****"...lol

 

That's part of the reason why I told him to knock the timing back to 0*- it's the stock timing setting for an LG-4. The L69 HO motor used 6* initial stock, but not the LG4.

 

Well I took the car down to the test only smog shop and it passed with timing set at 6* btdc actually that is what the manual calls for.Did not touch a thing since the pretest failed except changed the CAT.The new CAT did the trick,definitely a dramatic change in the results!Thanks for all the input guys.I am attaching the results from both tests to show what a difference just replacing the CAT made.

 

Also some premium gas for the test might help, slower more even burn, find it with ethanol as most premiums will have it. A fresh oil change can make a bit of difference if you're close, and run it out the highway at a steady cruise for several miles before testing. (Just for future tests if needed.)