4 minutes ago, Reptoid Overlords said:

When it is hot, what happens when you squeeze the upper and lower radiator hoses(cap on)?

Both hoses are hot and hard to squeeze, unlike a cold hose that squeezes easily.

3 minutes ago, Zed Head said:

Here's another easy test of the radiator...

Yep, this is a good idea. I'll pour another beer and try this one.

Have you investigated the hissing noise at the thermostat housing you mentioned in the 1st post?

When I replaced my intake/exhaust manifold gasket I removed all the stuff in that area. When I put the thermostat housing back it hissed and leaked a tiny bit.

A bit of cooling system sealant(permatex grey) stopped it.

Problem has been solved.

First, I thank you all for your help. You group of guys are what keep me wrenching on a car that's 2 years younger than myself.

Secondly, I'm an idiot. I'm not generally seen as an idiot, I hope, but here's what happened:

When I had the radiator out and was flushing the inside with Muratic acid/water, I pushed large corks into the inlet and outlet so I could really shake/rattle, and roll that stuff through the tubes inside. (you already know where this is going) After that, I decided to paint it before sticking it back in. Apparently, I didn't see the cork in the bottom outlet since it had a fresh coat of paint on it. How's that for an oversight? No harm done, thankfully.

Oh, and my Temp gauge settled under the 1/2 mark afterward.

 

 

 

 

Holey moley, that's one corker of a story.

At least you're a new cooling expert now.

Good find!

5 hours ago, jeremy93ls said:

Problem has been solved.

First, I thank you all for your help. You group of guys are what keep me wrenching on a car that's 2 years younger than myself.

Secondly, I'm an idiot. I'm not generally seen as an idiot, I hope, but here's what happened:

When I had the radiator out and was flushing the inside with Muratic acid/water, I pushed large corks into the inlet and outlet so I could really shake/rattle, and roll that stuff through the tubes inside. (you already know where this is going) After that, I decided to paint it before sticking it back in. Apparently, I didn't see the cork in the bottom outlet since it had a fresh coat of paint on it. How's that for an oversight? No harm done, thankfully.

Oh, and my Temp gauge settled under the 1/2 mark afterward.

Thanks for being Honest and a good laugh.

Been there done that! I left a rag in my engine one time. Luckily I recalled before starting. 

Q - Overheating after modifications, what am I overlooking?
A - The cork you left stuck inside the radiator outlet tube.

Glad you found your smoking gun and fixed the problem, and I think we've all been there, done something like that!    

Yes, thank you for a good laugh. 

I won't even start on all of my oversights, that should be my username instead of siteunseen.  

William of Ockham strikes again. 

Yep, that cork would cause some cooling problems. 

I was going to suggest a new radiator.

On 10/28/2017 at 12:19 PM, Zed Head said:

My 76 has done the same thing in the past.  It's either the fluid heating up and expanding or the water pump collapsing the lower hose, reducing overall system volume.

So, what I  implied was normal is probably actually a sign of a clogged/low flow radiator.  I saw this on my old radiator, which didn't cause overheating but had problems keeping temperature down on the hotter days.  It was marginal.  Interesting too that the suddenness is most likely the T-stat opening, like you thought.  

More clues for future cooling problems.