1. Home
  2. Chevrolet 4x4 Trucks
  3. '92 GMC failing idle HC smog test in California

'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

Reply to
Dennis
Loading thread data ...

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

Reply to
ken

Don't you really want to say "Terminate" them?

Reply to
Stephen Young

Just as long as they don't have a chance to say: "We'll be back"

Reply to
ken

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.
Reply to
Brian Orion

If you can see arcing on the wires you need to replace them anyway.

Reply to
Mike Powers

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

Reply to
Dennis

Reply to
Dan Dunphy

Reply to
Jessie Herrera

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.