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1993 Chevy Suburban 5.7L Is ignition timing 0 or 8 degrees BTDC?

The first smog test I took my chevy passed emissions except the technician told me I need to set ignition timing to "0" to pass it and get the smog certificate. So I took it to the shop to adjust ignition timing it to "0". I take it back to the station to test only. He told me the emission is too high and the timing is correct. But why my emission is different than before? Can you tell me what is wrong with my truck.
Answer:

Sounds like a classic "It passed when the timing was off... Shouldn't emissions get better with the timing correct?" situation. Your Chevy's ignition timing should in fact be set to 0 degrees also known as Top Dead Center (TDC); with the in-line grey & yellow spout connecter disconnected. During the first smog check, ignition timing on your Chevy Suburban was set to 8 degrees Before Top Dead Center (BTDC) actually making the timing retarded. When a vehicle's timing is retarded it lowers cylinder compression, lowers combustion chamber temperatures, lowers engine performance and power, AND also lowers emissions; under light load and no load conditions... And so your Chevy passed the emissions portion of the smog check but failed the ignition timing part. This is why the smog inspection process checks both emissions output and ignition timing. For those who know, it would be too simple to retard their vehicle's ignition timing and pass the smog check. I'm afraid at this point you will need to have a certified smog check repair station conduct a hands-on diagnosis with the ignition timing set properly to find out why your Chevy failed the smog check. The reasons it failed can be many and there is no way to tell which emissions part is defective nor how much it will cost to get fixed. I recommend you use our online VIR diagnosis system to have one of our smog technicians give you a better idea of what your Chevy's problem might be. If you used a SmogTips.com certified smog station for either of your first smog tests, our online diagnosis is free. Go to www.smogtips.com/smog_evaluations.cfm. posted by SmogTips Support

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