97 5.7L Misfiring

[johnsonmlj]

I have a 97 5.7L Chevy that is misfiring at idle. Once the rpms have reached 700+ it stops misfiring. I have changed the plugs, wires, dist cap and button cleaned the rails and tested the fuel pressure at 57. I am at a lost as to why I'm misfiring. The local Chevy dealer has no clue either. Help please.

1997 Chevy Sliverado
2000 Chevy Silverado
and various other pieces of transportation

[chevytrucks92]

How many miles on that truck? I dont know if a cam would cause it to do that, but it definetely woudl give you a vibration if a lobe was gone or bad.

Also could be yoru TPS (throttle positioning sensor) or maybe an O2 sensor(s).

[Tumbleweedktm]

If the miss at idle seems intermitant and acts like it stumbles then catches, it might be unmetered air. The unmetered air is due to carbon buildup in the throttle body holding the butterfly open, allowing air by when it should be totally closed. This air should go through the air bypass valve and when it becomes unable to control the idle it closes more and the engine stumbles. Above idle the engine will function normally. Use a throttle body cleaner (not carb cleaner) to clean the carbon out.

[TweakedGM]

if you are going to se a cleaner don't use throttle body cleaner. get the cleaner for the vortec engines I can't remember what it is called but I know it is not a throttle body cleaner.