pishta said

I poked a lb of crap out of my 67 when I had the freeze plugs out. Open the rad and start the cold motor and watch the coolant flow. if its a stat issue, youll either see no flow (until it opens) or youll see flow off the bat which is not good either unless the bypass is showing you a little ancillary flow. Id check the pump again, rebuilt stuff is hit or miss sometimes. Does it run well outside the temperature issues? Valves adjusted .010/.020? Here is an online reference for an Aussie Slant but all the specs are legit, 'cept the Bosch distributor!
Chrysler 225 Slant 6 Engine Rebuild Manual : Free Download, Borrow, and Streaming : Internet Archive

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AJ/FormS said

What does that tell you? What it tells me is that whatever air is coming thru the rad at lower rpm is either not at ambient, or there is not enough of it coming. Maybe the hot air is just going round and round and round........ Or if the stat was out, the water was not circulating properly.

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