rough idle & difficult to stay running in the morning

2003 GMC SIERRA
97,000 MILES • V8 • 4WD • AUTOMATIC
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CREED427
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I have a 2003 GMC Sierra 1500, 4wd, 97,000 miles, 5.3L. A couple weeks ago, it began to idle rough and is difficult to stay running 1st thing in the morning. It will often die on me unless I feather the throttle. After about 15-30 seconds of feathering the throttle and revving it to between 2-3K, it will stay running, but still idles rough and drops to about 600 rpm at idle. It only seems to have difficulty idling after sitting for long periods (i.e. over night)

The Service Engine Soon light cam on, so I hooked it up to a diagnostic scanner, which showed code P0300 "random/multiple cylinder misfire detected". I have replaced the fuel filter and spark plugs (the center electrodes were pretty worn, but all cylinders looked like they were burning good), but the problem still persists. I don't see any vacuum or other hoses unhooked or cracked. The symptoms seem to be similar to a choke malfunction on a carburated engine. Any thoughts?

Thanks,
Chad Reed
Nov 24, 2010 at 4:10 PM
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DENNYP
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Check the intake manifold gaskets for vacuum leaks when you first start it. Spray some carb cleaner around the gaskets with the engine running. If they are leaking, you will notice a change in idle.
Dec 8, 2010 at 8:11 PM
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CREED427
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If it was a manifold gasket or other vacuum leak, wouldn't it run rough and have difficulty idling all the time? I only have the symptoms when it's cold outside and the truck has sat overnight. Once it's been running for a few minutes I don't have any symptoms.
Dec 8, 2010 at 8:36 PM
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DENNYP
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There is a problem with early model 5.3l intake gaskets. They leak when the engine is cold, then seal up as the engine warms up. This is why you must perform the test on a cold engine. The updated gaskets can be purchased at your local GM dealer.
Dec 9, 2010 at 1:39 AM
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CREED427
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I have sprayed carb cleaner around the intake gaskets several times now in cold weather (below 20 degrees) and I haven't noticed any change in idling. It still dies on my until the engine has ran consistently for 20-30 seconds (which requires my assistance) and it still idles rough. Are there any other suggestions on what may be causing this?
Dec 20, 2010 at 6:03 PM
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DENNYP
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You can try cleaning the throttle body. It's possible there is some carbon buildup inside it, but your description sounds like leaking intake gaskets.
Do you have a scan tool that will show engine data?
Dec 22, 2010 at 5:12 AM
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CREED427
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I have already cleaned the throttle body. There was a little buildup, but it wasn't real dirty. I don't have a scan tool that shows engine data, I guess I'm going to have to break down and take it to a mechanic who has one.

thanks for all your help.
Dec 22, 2010 at 3:07 PM
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DHINTON4
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I have a 2003 Chev Suburban w/ a 5.3L. This winter when it started to get below 30deg.F, I noticed a rough idle. Then we got a check engine light. I checked codes for free at a local autoparts store. Got P0174 and P0171, which were System Too Lean on Bank 1 and Bank 2. I sprayed intake as noted above when symptom was present and sure enough - idle was affected. I changed the intake gaskets ($75) and cleared the codes and so far it has been running well. The intake seemed easier to remove/replace than the older ones, since there are no water jackets in the heads. It took about 4hrs.
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Jan 25, 2011 at 2:22 AM