Pontiac Repair: high idle, 2003 gmc savana, intake manifold gasket

Question
i know its not a pontiac, but i have tried all over the place for an answer to no avail.  it is a 2003 gmc savana 3500 with a 6liter 8.  it will idle from 550 to 1500 or higher. tried replacing map sensor on top of intake manifold near back. still doing it.  the only way it idles properly for any time at all is if i depress the gas pedal two or three times agressivly before turning on the power, then starting. it might run properly for anywhere from 10 seconds to at most one minute, then back to high idle. even while in gear it continues. hard on the front brakes, am unemployed and trying to save money, but its going out the tailpipe at about 7 to 9 mpg instead of the 14 to 15 i was getting. thanks for you help in advance

Answer
Hey rick:

a few things I see your from the great white north. I have seen a lot of older vans like yours with Engine control module issues. it's upunder the hhod and exposed to the elemnts and the case corrodes and splits open and the water and road salt get into it and cause all kinds of issues. high idle can be cause by a Vacuum line leaking or a lower intake manifold gasket leaking more so cold and once they warm up the gasket seals up again and the idle issue is back to normal. You could have a TP sensor issur where it's reading wrog and telling the engine computer the throttle is open and it's not causing a high idle. Any service engine soon lamp on? if so get it scanned at one of the big box auto parts stores and see what comes up it can give a direction to look. Good luck :)