Mystery Problem. At wit's end!!

This issue is with my 1994 Astro van that has a (non vortec) 4.3L-Z. I am the second owner so I don't know if this is the original engine. I believe it is. It has app. 350,000 miles and has been an awesome vehicle as all 6 of the previous Astro vans have been. My last van got 750,000 miles on it!!
I bought this van in 2002 and at that time, had no problems with it. For the last 2 years, this engine won't heat over 150 degrees. I have flushed the radiator out, changed out the 185 thermostat and had 8 mechanics look at it. I even put a 195 thermostat in it and then it over heats in 2 minutes. I hate to spend the money for a diagnostic test if it is not going to tell me what is wrong. I just got back from a shop that put cardboard in front of the radiator, which didn't help at all. They did unclog the EGR valve for me though, thank heavens. It idled so rough.
Does anyone have any ideas as to what the problem is?
Thank you so much!

Pandae4

Are you sure the heater shut off valve is open? It closes when a/c is set to MAX- look for the valve on top of HVAC case near blower motor- also are both heater hoses hot? could be a temp door problem if they are- if not- then flush out the heater core- as it may be clogged...*

8 mechanics WOW are you sure they are older than the van?

Are you sure the heater shut off valve is open? It closes when a/c is set to MAX- look for the valve on top of HVAC case near blower motor- also are both heater hoses hot? could be a temp door problem if they are- if not- then flush out the heater core- as it may be clogged...*

8 mechanics WOW are you sure they are older than the van?

I know. Makes ya wonder huh? I have tried the top of line with all of the newest technology mechanics to the back yard kind. The EGR valve was fixed by a shop I pass on the way to work. Three local and well known auto parts stores don't even have the OB1 meter to read the codes on the older vans anymore. The diagnostic test to read the codes now cost $65 to $100. I am going to have to learn how to take a paper clip and read the codes myself.

The hoses are getting slightly warm. Not even enough to warm your hands.
When the engine does get a little warmer, usually on warm days, the heater blows out "warmer" air. This shop I used today said that I may have a bad thermostat. I have replaced it several times, not solving the problem. I am thinking about buying 5 of them to see if one works, since they said that they have gotten numerous defective thermostats from the local stores.(Don't want to mention any names.) I believe if I could get the engine to warm up, if I could get heater to work.
What is the temp door? They did check the heater core, I think. Isn't that on the passenger side on the floor?

Thanks for your imput, brcidd !!
Pandae4

Is it blowing hot air when the heater is on? I had a 92 that wouldn't get warm on the temp dial or blow hot air, it ended up being the hot-cold switch on the dash. It was broke inside and was stuck in the down (cold) position. I replaced the switch and my temp came back to normal .

Is it blowing hot air when the heater is on? I had a 92 that wouldn't get warm on the temp dial or blow hot air, it ended up being the hot-cold switch on the dash. It was broke inside and was stuck in the down (cold) position. I replaced the switch and my temp came back to normal .

Not at all. The water is not heating up in the engine, therefore, I don't have any heat. The hot/cold lever is working fine. The hoses aren't even heating up.
On my next day off, I am going to buy those thermostats and just try all of them. I don't think it is going to work, but I will try just about anything. This is also causing havoc with my emissions. Anymore ideas other than what brcidd suggested?

Appreciate your ideas. Thanks!!!

Pandae4

I would think that its either the thermostat or a clogged heater core. Your sure you installed the thermostat correct? im not trying to be a smart ass but Ive seen it done before. You got me on this one, Im stumped!

You can Test the thermostat in a saucepan with boiling water. Be sure the gasket/seal is properly installed. I've seen a couple of these:

http://mysite.verizon.net/strey/files/StuckThermostat.jpg

Has anyone checked the fan clutch for proper operation? If it's locked up it will continously draw cold air across the radiator and depending on ambient temperature, not allow the engine to heat up.

I wonder if maybe you have a water pump running backwards , ie: if you had a serpentine belt but got a pump for a V belt it could run in the opposite direction . This happened on a friends jeep but it led to overheating rather than not warming up. If the thermostat is properly installed then maybe it is the fan clutch or maybe the timing is retarded a whole bunch.