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2004 chevy trailblazer_ls
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I chased the same hi idle, P0171 code for years. Tightend the intake manifold, cleaned the throttle body, checked for vacuum leaks, etc., etc. All the sensors were reading within range, I simply couldn't find it. FWIW, the fuel tims were in the range of -3 +3, LT and ST, on a flat road steady speed, so correct.
Eventually, a P0410 popped up (SAIS valve), tried cleaning it, didn't work, so I replaced it. The hi idle went away for about 6 months. Go figure.
Over time, it happend less and less, and rarely happens now. When it does, I clear the code, the idle comes down, and it's good for many months.
I've read on one of the TB forums on person noticed on a customers car the air cleaner cover wasn't quite on correctly, he reset it, and a P0171 he wasn't even lookin for went away.
 
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