Dennis wrote: " My '92 GMC 1/2 Ton 4X4 (350) with 140K miles is failing
the California idle smog test. The test allows 120 Maximum HC (PPM) and
my truck tested 175. The oil was clean at the time of the test. The 2500
rpm test for HC shows only 24 PPM out of an allowable limit of 180 PPM,
so it is fine at higher rpm's. I have since installed a new air filter
(the old one wasn't in bad shape) and changed the spark plugs. After
changing the plugs, I waited until dark and started the truck to see if
I could see any arcing from the plug wires. None was apparent at idle,
but when I revved the motor to approx 2500 rpm, I did see 4 or 5 blue
flashes at various points. I plan to change out the plug wires and then
Here are my questions for the group:
1. Is it likely that changing out the plug wires will help? BTW, they
are OEM and have never been changed.
2. How do you test the oxygen sensor, and if it was bad, would it cause
the idle test failure?
3. Any other tricks for passing smog would be greatly appreciated.
Thanks in advance for your help. These California smog tests are a
nightmare if you don't pass!
Dennis I think you answered your own question.Replace those old wires
and make sure they are routed properly.How about the cap and rotor? They
12 years old too?replace them if they are worn.Check for vacuum
leaks.<---- (real important)There is a process to check for vac.leaks
with a volt-meter on some f.i. vehicles.
How is your idle,is it nice and smooth or are there misses?
I'm sure I'll be corrected if wrong,but high HC is caused by misses or
too rich a mixture and if you have arching wires,well,it's kinda
obvious,but get a nice smooth idle and you should pass.
Best........Brian (long time veteran of Cali smog wars)