The heat exchanger is the large cast piece that hold the coolant. If you look for the temp probe at the front of the engine that's where the thermostat should be.Glad you found the elbow was plugged It can be a B-tch to clean out. Most i've seen on this site just buy a new one. But it can be done. Some take it to a rad shop or try sandblasting.
Good luck
Thanks Don!
I brought the elbow, (with the other two elbows attached), home today. With a vise and large pipe wrench I removed the 3/4" thread x barb street ell and the 1 1/2" street ell. I found deposits clogging the 3/4" water jacket inside the mixing ell.
Before I broke them down, I blew through the 3/4" water ell and my breathe went through. Why water didn't go through I do not know. Did my breathe go through the water jacket and exit back through the exhaust? I don't know. I hope this was the problem.
I took the pieces and soaked them in straight muriatic acid for about two hours. They came out like new!!!!! No chiseling, scraping, brushing, sand blasting. It was too easy. I'll neutralize them in some baking soda after they are all clean.
I hope to hell this is the problem. Water pumped with good force up to the elbow last night. What else could it be?
I'll find out in two weeks after I reassemble the exhaust system.
Thanks again.
Mike