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My car keeps failing emissions but only when it is idle. What can be the issue? on 1991 Mercury Grand Marquis

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I have failed every year. My car is too old for a diagnostic so how am I to know if the repair person is really fixing the issue or just adjusting it to pass.
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1991 is very easily disgnosed!! Maybe try another shop. However a lot of things can happen in a year's time!
IDLE RPM TEST HC ppm reading 282 allowed 220 FAIL CO% reading 0.11 allowed 1.20 PASS RPM reading 786 allowed 1250 max CO+CO2% reading 15.7 allowed 6.0 min
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It's possibly a tiny bit too lean when I see .11CO and HC's too high. That usually means a small vacuum leak affecting only one or two cylinders, one or two injectors that are a little lean so that they don't provide as much fuel as the rest of the injectors. Now that being said I still have to consider that without the O2 readings I can't comment on how efficient the Catalyst is at cleaning up the exhaust. If there is O2 making it out the tailpipe then forget the idea that this is too lean, the catalyst isn't doing its job of cleaning up the exhaust. Some of the other things that can elevate the HC reading would be spark timing that is over advanced, and even carbon deposits in the combustion chamber. Your not failing by much, that can be a little tougher than one that is failing by a ton.
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