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My trailblazer has been running fine until the other day I started it up in the morning and it ran rough for a second then went to high idle (3K) then rough idle, then high idle, then cycled between 2k -3k for about 30 seconds and then ran normally. The next day I started it and it ran at high idle for a few seconds slowed down and when I pulled out onto the road it went to a 4k-5k rpm. I shut off the car and looked under the hood and found some paper towels for mice on top of the last few injectors (outside of the engine). Went back home and removed the air intake so that I could vacuum out all of the debris, checked the hoses (one on the bottom and one small on the right side and they looked OK. I made sure that the butterfly was not sticking and reassembled. The car ran fine after I put it back together but I question if this could be a sensor thing (as soon as I get the right temp/humidity it will come back) or if the paper towels on the top of the engine could cause this type of behavior?
 

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2003 gmc envoy_slt
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Have you removed and cleaned the Throttle Body lately? Mine was begining to have a rough idle and I did the Throttle Body clean on it (do a search and you will find plenty of threads on this) and It improved.
 

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2004 gmc
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Even if the coolant temp sensor failed, the desired idle RPM table in the PCM has no entries above 1200 RPM:



So I would also conclude it was a sticking butterfly valve. With no traditional PCV valve, the I6 throttle bodies gunk up horribly on the backside with blowby gasses. Some owners have to clean them every 30K miles.

http://forums.trailvoy.com/articles.php?do=viewarticle&artid=91
 
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