Dodge Repair: 1991 Dodge Voyager, throttle position sensor, vacuum problem

Question
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I replaced all the vac lines and pcv valve.  I stills idles low, but runs.  when you are at a stop light or sign it barely keeps the rpm's above 500, and it sometimes drops below for a second.  Is there a way to change the idle to have it idle up a little higher?
Followup To
Question -
When I put my van into drive the RPM's drop to below 500, and it stalls.  But when it is in park or neutral it stays above 1000 RPM's.  What could make this happen?  Any suggestions on how to get this fixed?
Answer -
classic vacuum problem... check all vac hoses and pcv valve..

Answer
ok doug sounds like a tps sensor problem  "throttle position sensor" have a mechanic check this part for you as it needs to be calibrated to a specific voltage to correctly work.. sounds as if it might have came out of adjustment. dont worry what you have is a minor problem nothing a little time and patience wont fix!  joe