2000 pontiac sunfire starts only when gas pedal is held to floor, idles rough and white smoke comming to exhaust changed IAC valve..no change

Try cleaning the throttle body and IAC idle air control valve.

It can be issue with IAC( idle air control valve) causing the problem.

There are many other possibilities causing these idle issues. Engine idles rough.

There are few help links with solutions to troubleshoot such idle issues.

I suggest you refer the link below and follow its troubleshooting details:------

Click the link below:-----

Ford Expedition IAC valve replacing

http://technoanswers.blogspot.com/2012/12/ford-expedition-iac-valve-replacing.html

Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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Stumbling and stalling at idle after engine rebuild?

http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html

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1999 Ford F-150 Idles rough and quits when gas pedal is pressed less?

http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

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Cold start problem and vehicle shudder around 2500 to 3500 rpms

http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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GMC Sonoma Stalls and idles low

http://technoanswers.blogspot.com/2012/10/gmc-sonoma-stalls-and-idles-low.html

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GMC Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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1996 Camry intermittent engine failure while idle?

http://repairhelpcenter.blogspot.com/2011/10/1996-camry-intermittent-engine-failure.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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High Idle Problem

http://repairhelpcenter.blogspot.com/2012/10/high-idle-problem.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed?

http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html

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High rpm in neutral? Inconsistant high RPM in neutral?

http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html

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Vehicle stumbling at idle and shows code 54?

http://schematicsdiagram.blogspot.in/2012/03/vehicle-stumbling-at-idle-and-shows.html

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Rough idle, staling and Erratic shifting?

http://technoanswers.blogspot.in/2012/03/rough-idlestaling-and-erratic-shifting.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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These details will help.

Thanks.

Check for a vacuum leak.hoses,carb,base gasket.

There are few possibilities that can cause such problem.
I recommend you few help links related to your problem.Click the link below and follow the troubleshooting;-----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------
Brake pedal stuck? http://howtobyme.blogspot.in/2012/03/brake-pedal-stuck.html

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Car only starts when gas pedal is pressed? http://technoanswers.blogspot.com/2012/01/car-only-starts-when-gas-pedal-is.html

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Car won't start without hitting the gas pedal? http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-without-hitting-gas.html

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emits whitish bluish smoke and bad fuel smell from exhaust

http://schematicsdiagram.blogspot.com/2011/12/van-emits-whitish-bluish-smoke-and-bad.html

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White smoke and Acrid Smell? http://whoisbyme.blogspot.in/2012/07/white-smoke-and-acrid-smell.html
---------- Car stalls and throws white smoke from exhaust? http://repairhelpcenter.blogspot.com/2012/01/car-stalls-and-throws-white-smoke-from.html

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This will help.
Thanks.

Check the help links related to this problem.Click the link below to go through its troubleshooting procedure :-----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html---------
Car won't start without hitting the gas pedal? http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-without-hitting-gas.html
-------Complete loss of brake pedal then all gets normal? http://repairhelpcenter.blogspot.com/2012/01/complete-loss-of-brake-pedal-then-all.html
------Car dies when gas pedal is pressed? http://schematicsdiagram.blogspot.com/2011/12/car-dies-when-gas-pedal-is-pressed.html
--------Pushing on the gas pedal, but the car slows down? Gas pedal to the floor and the car slows down?

http://technoanswers.blogspot.in/2012/02/pushing-on-gas-pedal-but-car-slows-down.html

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After replacing brake rotors and pads, brake pedal remains on floor? http://technoanswers.blogspot.in/2012/02/after-replacing-brake-rotors-and-pads.html

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accelerator pedal sensor? http://technoanswers.blogspot.in/2012/02/2009-chevy-impala-accelerator-pedal.html

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For rough idle issues and problem troubleshooting,go through the help links provided below :----- van idles rough?

http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html

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will not idle when cold?

http://technoanswers.blogspot.in/2012/02/ford-explorer-40-will-not-idle-when.html

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runs lean and idles rough?

http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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This will help.

Thanks.

Dirty throttle body . Does it run good when you step on the gas pedal ? driving down the road . Don't guess , do fuel pressure testing . Videos on youtube showing how , fuel pressure tester under $30 most part stores .

So IAC has been replaced. Sounds like a dirty throttle body, EGR valve, or vacuum leak. DId you do the maintenance yourself. If so then you should be able to inspect Throttle Body to determine if black and dirty.. this can cause intake problems leading to your issue adn rough idle. EGR valve sends exhaust gas back into combustion chamber to reburn it and make car more environmentally sound. If stuck open it can allow too much exhaust into combustion chamber causign a lean mixture, too much air can cause this.. Vacuum leak can also allow too much air in. Start with easiest and check Throttle body, then EGR valve, finally for vacuum leaks. Depending on your skill the EGR can be cleaned as can the throttle body.. a good link for TB cleaning is http://www.popularmechanics.com/cars/how-to/repair/1272341. A vacuum leak may be harder to diagnose. You are compensating by depressing gas pedal telling ECM to send more fuel to motor to overcome this issue thats why it will run and start with gas pedal pressed.

if its white smoke look for coolant getting into the cyls, black smoke could be leaking injectors

Check your idle air control valve.

Check for vacuum leaks. Check PCV hose and other vacuum hoses. Spray carb cleaner around intake, throttle body and vacuum hoses while idling in park. If spraying anywhere causes a change in idle. that area has a vacuum leak.
Possible IAC motor dirty or defective. Remove it. Clean the IAC plunger tip and the throttle body passages with air intake/throttle body cleaner spray can. Reassemble and start vehicle.