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Idles hard, stalls when idling and starts hard.

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I have been having this problem for a while now, it's annoying. When I try to start my truck it cranks and cranks for a while before it starts, sometimes pressing gas peddle with help it start faster. But even after it starts it will rev up to around 7 to 800 RPM then die down to around 400 then will do that a couple times, sometimes will stall out and sometimes it will finally run at around 5 to 600 rpm. When it is just sitting there idling sometimes it will get idling hard and rough even vibrates the truck. Sometimes when I let it sit there and idle for a while it will just stall itself out, this happens when I'm driving to, for example if I am sitting at a red light idling then I start to drive it will stall out. I just put new spark plugs and wires on it and a new fuel filter. I have a 99 gmc Sierra 1500 4.8L if anyone could help that would be great.

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Keep in mind I wasn't trying to jump to conclusions or sound presumptive - I'm just pretty sure that year Sierra was included in the TSB for the manifold gasket. Definitely do some testing first. Fuel Pressure, vacuum pressure, and trouble codes if present would be helpful for diagnostics.


txab can correct me if I'm wrong because I'm away from home and don't have my Haynes, but that fuel pressure spec I would assume to be 55-62.


This sounds an awful lot like the problems I was having last week: http://www.gm-trucks...howtopic=143012

Get some more information, and good luck.


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On a 99 you have an idle air control valve (IAC). It sits on the side on the throttle body, sounds like it may be clogged or sticking. It's simple to remove, then clean with throttle body cleaner.

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