idle drops when put in prk.

[Hvymetlclo]

I have this problem that i can't figure out. I have replaced the IAC valve twice within a one yr. period and my truck has about 105k on it. My problem happens when i put my tranny into park and the rpms drop down to 450-500. My TB is pretty clean, the MAF has not dust or oil build up. what could b causing this?

 

[swynx]

i have no clue what could be causing it might just need cables tightened, i no in the 85 nissan hardbody we own, it idles at about 1k with the heater on and if u turn it off it goes down to about 500, i doubt this helped but maybe...