I agree, my BBC did the same thing but actually a little worse. I would have to shut it off and refire it to get the coolant moving again. Mine started doing it out of the blue too. I was driving down the freeway and it overheated so I pulled the thermostat. It was fine until I put another thermostat in, then it did it again. I asked the question on here and someone recommended putting the bi-pass on. Once I did I haven't had a problem since. PS: I had a few holes drilled in my thermostat and it wasn't enough flow.
This! But I see you already solved the issue.... I have a Chevy van that mystified me for weeks.... It was the collapsing lowerhose finally.
Nice!! I know the 5/16 was a typo To the bypass ,absolutely that fixes it too. Also keeps water flowing through the block and by the stat all of the time to prevent hot spots during stat closed conditions including warm up...
Whats really fun is when the lower hose starts to delaminate and the inner part of the hose collapses and the outer stays full diameter.Will drive a guy crazy. Good luck.Have fun.Be safe. Leo
The lower hose isn't really a suspect in a situation like this. It's the first start, cool engine, and the fact that the temperature comes back to normal after the initial spike would eliminate the lower hose as a suspect, at least in my mind. A lower hose issue usually shows up one the engine gets up to operating temperature, and the heated coolant makes the hose soft enough to collapse. The fact that the hole the O/P drilled in the thermostat fixed the problem indicates that it was a "no flow" situation where the thermostat wasn't seeing as much heat as the sending unit was
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