1993 Jeep Grand Cherokee 4.0l engine in CA.
Fail first round of smog test the HC at idle failed by 8 pts (Max is 100) Measured 108.
Did a full tune-up. New plugs, cap and rotor...tested all wires as good resistance. Changed the oil and filter. Ran fuel injector cleaner. Installed a new Air Sensor in the exhaust pipe.
Did a re-test and it failed again but by only 1 pt (101).
I had also checked all vacuum lines and found no leaks.
Any ideas on what to do next?
It has 198,000 miles. Maybe a carbon cylinder cleaner??
It runs great but this is making me crazy.
I have heard of moving the idle a little higher will get it to pass...that true?
Here is a picture of the results.

 

Was the jeep nice and warmed up when you tried to smog it? The cat needs to be nice and warmed up to work fully. If it was cold that would explain why the numbers look so good at the 25mph. I always leave the jeep running while waiting for them to be ready to pull it on the machine.

Max