Hi..I have a 95 GMC Yukon..failed the low idle portion of the test (NJ) Reading was 726...Standard is 220..Co% reading was .73..standard is 1.20...The truck passed the high idle portion of this test..Just wondering what I can do to bring it down..egr valve was replaced exactly 4 years and about 30,000 miles ago..converter was replaced 2 years ago..however..the repair shop put on smaller exhaust pipe and a generic (small) converter. they did the last emissions test for state inspection, so I dont know for sure if it passed, for all I know they put the wand into another vehicles tail pipe..anyway, just need to know besides a basic tune up what I should do?
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1995 GMC Yukon Question: failed HC idle test
Answer #1ZeeTech September 15, 2010, 15:35Master
Incorrect cat. converter and restricted exhaust flow could cause this. I assume the air filter is clean.
Dirty or bad MAF sensor could also cause high HC.
Answer #2mastertech6371 September 15, 2010, 15:51Master
when was the last time you did a tune up with plugs?? common for the plugs to cause high hc.
Replygoodforbrains, September 15, 2010, 17:45Rookie
It is def due for plugs and cap etc..I'm going to do that tomorrow and take it back through..I was just reading that plugged egr holes in the intake will also cause high hc..the test for that is while running push up on the egr valve, if engine stalls there is no clog..if engine stays running there is a clog in the intake. I'm also going to remove the throttle body and clean all around the MAF sensor like zee suggests..there's 185,000 miles on the engine and still runs amazing..lots of oil changes..I will repost tomorrow..thanks