Even flushing won't remove crusted up sediment and rust in the water passages. I have had slant sixes that ran hot all the time until I had the head hot tanked.

It is also good advice to double check and make sure your temp sending unit is functioning properly (and is the correct one for your car).

Temp Band on what? Factory gauge or after market gauge?
Check the actual temperature either with a non contact thermometer or a known accurate temp gauge!!(non contact thermometers are not completely accurate because they tell you surface temp and not actual coolant temps but will get you close.)
I am with Dan on the radiator. but you need to verify the temperature first to see if you actually have a problem.

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This is my hooptie, there are many like it but this one is mine. With out my hooptie I am walking and without me my hooptie is broken down.
1980 dodge d150, sl6, 4sp od, air

Factory gauge. I figure if it's right at low speeds, why wouldn't it be right at high?

If you cant find the reason for your ailment why rule out anything?

Verify the temp.

Relook at everything.....especially anything you are sure of.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Fans spun the right way?

Olaf.

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Aspenized

Well, turns out Slantsix.org diagnostic master of the month for October is:

Aggressive Ted for his "vacuum leak" diagnosis.

As I thought before (ESPECIALLY during that ugly frost plugs removal), I have no cooling issue. Turns out this is a PURE vacuum leak issue. Turns out after a bit of driving that I had a couple of hose leaks. They weren't properly clamped (which they should have been), and having been in the lava hot engine bay, would expand and leak once I got to highway speeds.

The hilarious key culprit: the vacuum hose connected to my Vacuum Gauge! Yes. It is that sad.

Happens to the best of us? Not sure. But after reclamping some hoses the temp stayed stable for 200+ miles. Not a problem.

Anyway, thanks to everybody for their two cents! Love this board!

Isn't it just great to get a happy ending?
On some of his posts I could almost see him going for the poison pills to finish it all off, and then ... simply a vacuum leak.
And they all lived happily ever after!

(a bit like a Humphrey Bogart movie - just love it)

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Gary from Downunder
2 weeks ago I couldn't even spell mukanic - now I are one!