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Rough idling

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by randyc1959, Feb 5, 2012.

  1. randyc1959

    randyc1959 New Member

    I own a 2003 Ext. cab Z71 with 130,000 miles. Only when the climate control system is on it idles rough and the lights, inside and out, slightly dim and go back and forth to normal. Like I said this is only when I'm running the a/c, heat, defrost etc. I recently had the plugs and wires changed as well as the fuel filter. Another problem I have, and this may or may not be related, whenever the heat or A/C is running a noise occurs about once every 5 minutes or so that sounds like a clunking coming from behind the dash. It sounds like the fan is catching on something or hitting something. It does not affect the performance of the a/c or heat at all. Whenever the defrost only is running it doesn't do this. Thanks for your help..
  2. donyms

    donyms Active Member ROTM Winner 1000 Posts

    Just a thought, I know several people with 1999 - 2006 Silverados that have put all kinds of plugs in their trucks and they would not run right until they put the AC Delco plugs in. I have not had that problem because I only put AC Delco plugs in my truck and I check the gap to make sure it is right, this includes the so called pre gaped plugs. They are not always gaped properly. Also I have had allot of problems with new plug wires mostly ones from Auto Zone. I now buy quality OEM type wires at Advance or O'Reilly's and my tune ups are quick and easy now with out having to go back and mess with it. Now with all that said. When your truck does not idle good the AC coming on will put more of a load on the motor and effect the idle and that can effect the output of the alternator. Now all that said, a vacuum leak can cause a rough idle also, check carefully for brittle, rotted vacuum lines, PCV, hose and brake booster diaphragm leaking will also cause this rough idle. I don't know what's wrong with your truck but I have addressed these problems before with success so I just want to share my experience. Good luck. :sign0011:
  3. randyc1959

    randyc1959 New Member

    Thanks a lot. I'll check it all out..
  4. michiganman517

    michiganman517 New Member

    Altenator is a good bet, but also might be your IAC valve (idle air control). I have an 02, and a few years back, during cold weather, I noticed the truck idling rough, stalling out sometimes before it had warmed up. Low enough idle to dim lights, etc. Knock on wood, I've not had to replace that altenator yet. Remove your IAC valve and shake it. If you hear a rattling, its a good thing. If you don't hear it rattle, you could soak it in some carb cleaner, etc. Eventually, I replaced mine shortly once I found out how inexpensive they are.

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