Cooling System Problem

I have a 1989 Astro 4.3 automatic. There isn't a t-stat in it right now and the engine runs about 190 degrees when warmed up. It will also over heat if I put the ac on. It will also over heat if I put a t-stat in. With or with out the t-stat I can never get heat from it - it just blows cool air and all heat/ac controls do work fine. I have flushed the system, put new radiator cap on and held the heater control valve up by hand but it does no good. The heater control valve ( I think that is what it is called, it is the component with the in and out lines to heater core connected to it) only seems to work with the ac on at which time the vacume line connected to it pulls up the medal bar on bottom of componet. Should it also do this when heater is put on? Again, O did try holding that up in case it was suppose to go up but wasn't for some reason but it didn't help, I can't seemed to find the problem. No water leaks or noises from water pump and heater hoses seem strong (not collapsing). Any ideas? Could there be a retriction some where? When I was at junk yard I saw 3 Astro's, a 1988, 1989 and a 92 all with the same engine and none of them had this heat control unit. Can mine be illiminated and vacume hose plugged? Thanks

sounds like it is air locked and u need to leave the rad cap off when running with a thermostat in it, letting it run witht he rad cap off will let most of the air out to circulate in the heater core as it is the highest point in the veichle.
also if you can get your hand on a yellow rad bowl that attaches to the rad that would be a better idea
most rad shops will flush it for ya for 70 or so dollars and it will get done right
just say you had heat when it went in..hehe.
im sure that will be the fix
if not then your lines may be plugged or you may have ehaust gas getting into your cooling system from a leaky head gasket..:(
scott

Thanks for replying Scott. I just got done with a little work on it. I took off the bottom line to the heater control valve (in-let). Most of the water I put in came back out but a little when into the block it seems. I didn't have the proper connection to get a good pressure into it. I only had a piece of garden hose which fit into the heater hose. When I put it back together I ran the engine and it didn't get nearly as hot as usual and I had (for the first time) a little heat. I am going to try again tomorrow flushing the same hose with alot of pressure. Hopefully this will do the trick. If not I may have to take the head off and check for blockage. Whan I did it today there wasn't any movement in the radiator water. If this was working properly I would think that a lot of back pressure would cause the water to come out of rad with cap off. What do you think? Thanks again

if there wasnt much movment in the rad then id say your fins are gone on your water pump, so id start then and change that. also that would explain why your van overheats while the thermostat was in with ac on
scott.