'92 GMC failing idle HC smog test in California

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 retest.
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
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Don't get me started on Calif's new pissassed smog BS.. Those dirty rotten bureaucratic mofo's need to keep adding more tests and tighter limits so they can justify their pencil pushing jobs....Got a "Test Only" notice this time around and I have a new engine in the 91 S-10
Please Arnold...FIRE THEM ALL
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ken wrote:

Don't you really want to say "Terminate" them?
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Just as long as they don't have a chance to say: "We'll be back"
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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 retest. 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" ##########@@@@@@@###########
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)
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Brian Orion wrote:

If you can see arcing on the wires you need to replace them anyway.
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Brian:
Cap & rotor were replaced about 6 months ago when I R&R'ed the intake manifold gasket (what a fun job!).
Idle is a tad bit rough. I'll try the plug wires and see how that improves things. It's a tough call because I didn't really notice the truck running poorly in the first place. I do know this, it's going to cost $30 each time I retry the smog test, so I'd like to get it in the best shape possible (without spending a fortune) before I try again.
Thanks for your reply.
Dennis
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Anything that causes a missfire will raise HC -- ignition, valves, leaky intake gaskets. Sounds like you need attention to the ignition. I'd be surprised if it's your oxy sensor. When mine failed, it failed across the rpm range. It would go in and out of closed loop. A scanner would tell. Dan
On 15 Aug 2004 08:19:34 -0700, snipped-for-privacy@gcid.net (Dennis) wrote:

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Hi,     Your HC's are unburned hydrocarbons. These fail because of poor ignition of the gases. You can check your wires. A good quality wire is Motorcraft. If you buy them at Kragen's auto they are warrantied for life assuming you keep your reciept and don't rip the boot off at the plug.     A culprit may be your air injection system if you have one. What the air injection system does is feed a fresh air charge into your exhaust manifold at the heads. This fresh charge ignites the gases which in turn burns the unburned hydrocarbons. If your truck is running good this is probably your problem. Have you heard poping out of the exhaust when letting off the throttle on older cars? This is due to the leaking exhaust which sucks fresh air into your exhaust system. The poping is gases being burned in your exhaust. The same thing as your air injection. Good luck to you. Jessie
Dennis wrote:

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