'93 Silverado high idle

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Old 04-15-07, 05:42 AM
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'93 Silverado high idle

My '93 silverado 4.3 v6, manual transmission (240,000 miles) runs about like it always has, but is idling higher and using more gas than it has in the past. It has always gotten around 17.5 mpg until the last few tankfuls, when it dropped to around 14.5. The weather and my driving habits/locations have not changed from years past.
I took it to Autozone, but their electronic code readers do not test vehicles that old. They gave me a little key to stick in the A and B ports of the truck's computer hookup so I could count the flashes on the dashboard. It came up with a code of 12, which basically means nothing is wrong. That's the only code it showed.
Does anyone have a suggestion as to what would cause the higher idling before I take it to a mechanic for diagnosis? The plugs, wires, distributor cap, thermostat, and alternator have all been changed within the past 6 months. The alternator was most recent, about a month ago. The high idle has been a little more recent than that I think.
The truck is in good shape and despite the high mileage, I'd like to keep it as long as possible. I hate the thought of a monthly payment. Plus, I've gotten a bit attached to it in the past 14 years.

Pecos
 
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  #2  
Old 04-15-07, 06:57 AM
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Join Date: Dec 2005
Location: Arkansas
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High idle can be caused by a vaccuum leak. Check all your main vaccuum hoses, particularly the big ones from the brake booster to the intake and from the intake to the PCV valve. That would also cause the decrease in gas mileage. Also be sure to check the vaccuum hose going to your MAP sensor.

Other possibilities, just off the top of my head, might include:
Bad or dirty idle air control motor
Faulty MAP (manifold absolute pressure) sensor
Faulty MAF (Mass Air Flow) sensor
Unmetered air entering system...IOW an air leak AFTER the MAF such as in
the air intake tube going from the air filter to the throttle body...check the
clamps to make sure they are tight.

Hope this helps.

Joe
 
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