^%#van

bought a 93 astro cl "z" engine 160km. ran good but not much heat..no trouble codes for 3 day's, I decided to fix the thermostat yesterday and it was stuck(as I suspected)open, fired it up watched as the heat guage came to life, the service engine came on and the idle started to surge, now i have heat but van does not want to idle, was expecting a code for the coolent sensor but it is 45(rich idle I think),thought i might have damaged sensor changing the thermastat as the nut holding the damn ground wires was siezed but it looks ok, plugs, wires, egr, air filter have all been changed, should i try a new coolent temp sensor or maybe the O2? as these are about the only 2 things i can think of other than the spider, but the van did run great untill i changed the thermostat.???
could it be the computer was compensating for the thermostat and now that i fixed it it is still compensating? would clearing the ecm help?
update: i think i got it fixed, i've changed the coolant sensor and the temp would rise 1/2 way then fluctuate between 1/2-1/4 but no surging, i also found a bad (new wire)on #2 plug i've driven it for a week and it has'nt gone into a surging fit yet but the temp guage still seems to bounce around alot, mabe an airlock or blockage, it has rear heat so i'll have to research it a bit before flushing and filling it.

I have a 98 Astro LS and need to change the thermostat. Where is the drain for the radiator?

i, use a prestone kit that you cut the upper hose and put in a funnel and a drain, run the engine and it collects all the old stuff in a supplied container and u dump the new stuff in the funnel with the engine running...flushes entire system from the top, no mess, pull the funnel out reattach the hose with supplied fitting and clamps and away ya go