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Hi everyone,

I have a 99 Suburban with a 5.7 Vortec with 306,000 + miles (gotta love GM motors). It runs great, with one exception. In the last few weeks, I have been getting a really rough idle at cold startup, but after I let it run for a minute or two, it is fine. The issue occurs only after the truck has been sitting a while (like overnight). If I turn it off and let it sit for 2-3 hours before restart, it still runs fine...

I have searched and searched and cannot find any "known" issues with that truck/engine to cause this. All idleing issues I can find are after it warms up. I recently had a fuel pump put in (AC Delco) due to poor performance. The problem did not start until a couple of weeks after that, so I doubt the issue is related to the fuel pump replacement UNLESS the pump was bad.

No codes are being thrown. I am going to run some Sea Foam through it in a couple of days when I can get to the autoparts store, but was hoping to find some suggestions before then so I can pick up parts if necessary.

Thanks!

David
 

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I know this is a very old post, just curious if you were able to figure out what the problem was with your truck ? I have an 88 suburban that has been doing the EXACT same thing for the last couple weeks. . Thanks in advance if you can help !
 

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Supersubdriver:

Here are some things that I've found to be exacerbated by cold weather, in ascending order of difficulty:
  • Dirty throttle body
  • Dirty MAF sensor
  • Leaky intake manifold gasket
First two are super easy to fix and you probably ought to every now and then on general principles.
Last one - if you spray starter fluid around the manifold and the idle changes, you might have a leak.
 
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