No Start When Hot

My 90 Buick LaSabre will not start when hot. If I let it cool 15 minutes it starts right up. If I leave it in park running I will get a yellow service engine light, that will go out once I begin
driving. Any ideas.. 140K... runs good
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Try checking for trouble codes - see my site below or www.troublecodes.net .
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Robert Hancock Saskatoon, SK, Canada
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