Volvo: smog test failure, smog test, smog report

Question
I am in California, and had to have a smog test recently. My car is a 1985 Volvo 240.  It was classed as a "gross polluter" at idle speed, but passed the test at higher rpm.  I checked the following already:
-Engine compression test, all ok.
-spark plugs, wires, rotor, distributor cap, all ok.
-Timing ok.
The engine does seem to idle a little rough; I dont know what else to check, which could be causing it to fail the smog test at idle only.
Any advice you could give me would be greatly appreciated.
Thank you,
Paul Simcoe

Answer
Emission problems can be a pain to track down, as you've already discovered.  The fact that the car only fails at idle is definitely unique in my experience.  I would check your oxygen sensor, the car's ECU, the catalytic converter, and the frequency valve of the injection manifold.  Beyond that, I would review your smog report card, make note of the specific gas you failed on, and post the question on www.brickboard.com in the 240 section.  Depending on what gas you are failing on, it can mean different problems, but thats a bit beyond what I know.  Members of brickboard, however, will be able to take you a little further if my suggestions don't reveal any problems.  Good luck!