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03 Sierra, 4.8, erratic idle

gapost

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gapost Enthusiast

gapost

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I've got an 03 Sierra 4.8l that I recently replace the heads on. It ran perfect for a couple of months, but recently, I've been getting an erratic idle. It seems to start when the engine is warm and more obvious when in idle and in gear, but you can feel it when in park.

 

It's weird because it runs very smooth, then drops in rpm for a second or two, then back to running good. The time between these drops is variable; sometimes one second and sometimes 40 seconds. I replaced the throttle position sensor and put in some Seafoam but no change. I guess it could be one of the ignition coils, but with no check engine light, it's pretty hard to narrow that down. Any suggestions appreciated.

gapost Enthusiast

gapost

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No codes, but I only have a basic code reader, not a tech 2 or similar.

 

I cleaned the TB when I replaced the heads. Changing the TPS didn't affect the issue.

 

I will try to check the fuel pressure, but the fuel pump was replaced less than 20,000 miles ago.

 

Thanks for the suggestions so far.

gapost Enthusiast

gapost

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I put a fuel pressure tester on and it's a steady 52, even when it misses. What about O2 sensors?

XYZ Enthusiast

XYZ

Posted
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There was a TSB on the wiring harness plug that connects to the throttle body, wiring breaking within the insulation, is it possible this may have happened when you were reconnecting everything after the cylinder head work?

  • 3 weeks later...
gapost Enthusiast

gapost

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Just an update. I replaced the plugs and wires with AC Iridium and MSD wires. Runs perfect now. I don't know what I was thinking when I re-did the heads and used the old plugs. That was stupid as I purchased the truck a couple of years ago and haven't replaced them. Btw, they were Autolite.

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