1986 LG4 Barely Fails California NO Emissions Test

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11-05-2013, 10:00 AM

eca1987

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Join Date: Oct 2011

Location: Southern, Cali

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Car: 1986 Trans AM

Engine: LG4-305-Edelbrock Intake/Q-Jet Carb

Transmission: Automatic

1986 LG4 Barely Fails California NO Emissions Test

Greetings fellow Thirdgen’ers

Update folks. I found a small vacuum leak due to a cracked cap where two lines converged. I re-tested and now passed.

Thanks for your assistance.

Big Ed

It’s time for my biennial smog check here and California and I failed, just barely, the NO emissions test. Here were the test results:

HC (PPM)
@ 15 mph - Measured = 21, Max = 101 - Pass
@ 25 mph - Measured = 15, Max = 69 - Pass

CO (%)
@ 15 mph - Measured = 0, Max = .54 - Pass
@ 25 mph - Measured = 0, Max = .40 - Pass

NO (PPM)
@ 15 mph - Measured = 757, Max = 762 - Pass
@ 25 mph - Measured = 664, Max = 643 - Fail

Bet these numbers would pass in all other 49 states without issue, Argh!

Here is the status of the car:

New Magnaflow CAT Back installed March 2013.
Quadrajet Carb rebuilt and adjusted to a perfect 50% duty cycle using the duty cycle feature of the multimeter using rebuild/adjustment instructions on the Thridgen.org website.
Idles perfectly at 650 rpms.
Intake manifold replaced with Edlebrock Performer intake manifold in 2013.
Brand new EGR valve replaced in 2013
New distributor cap/rotor, spark plug wires replaced in 2013.

It is noteworthy that the car had not been driven for about a year prior to the test. I did, however, put a half tank of 91 octane and drive it around for about 30 minutes including a trip on the freeway before the test and the car drove great.

Test results document indicates the engine timing is set to 1 degree BTDC. Smog Test Technician indicated that test can be conducted at +- 3 degrees of OEM spec which I believe is 0 degrees BTDC for this engine. Can anyone confirm?

Technician suggested retarding the engine timing several degrees to make the fuel mixture richer and to reduce combustion temperature which should reduce NO emissions. I only need a reduction of 19 ppm or ~ 3%..

Any suggestions greatly appreciated.

Thanks,

Big “Ed”, eca1987

Last edited by eca1987; 11-07-2013 at 02:47 AM.