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1999 4.3 Vortec: Bad misfire/sputter under light to moderate acceleration. Starts great & idles great. Runs good under heavy to full throttle. Just bucks and sputters like crazy when accelerating normal when cold. After warmed up... I mean after running for 30 plus minutes it gets better. The longer it runs the better it gets. I can let it idle and warm up before driving for 20 mins and the issue may not be quite as bad but is still bad. When it's sputtering while driving, and I hit the gas it smooths out until I level off again. It gives the P0171 & P0174 codes, with a P0300 as well.

I already cleaned the MAF with special cleaner.

Any help would be appreciated.
 

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B4U Task Force Admin
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25,663 Posts
Might want to clean the egr also.
 

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B4U Task Force Admin
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25,663 Posts
Get the gasket with the screen in it to keep out the big chunks.
 

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Been there Done it
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9,207 Posts
Here's a complete rundown on the 171&174 combination.

I'd start by looking for a vacuum leak or a leak into the air intake between the maf and throttle body. Leaking breather tube or PCV?
 

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Look at you vaccum lines running to you driver side fender they rot and fall apart. There is a 3 way line connection one of the lines going into your right fender if you have a 5 speed the t connection is almost on top of the clutch fluid filler cap.
 
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