The cycle of heat up in idle that I see:
Intake manifold starts to heat up from around 150F (after driving), and goes toward 190F. As long is it is under 190F, engine stays under 210F.
Then the manifold continues to heat up but the engine coolant starts to climb as well. BTW, transmission temperature starts to climb as well, even if it is in park and no effort on it whatsover. If I would not have Derale ecooler, I think that my transmission would go into 220F area just in idle.
Then manifold and coolant catch up. At around 231F - same temperature for coolant and manifold.
Then manifold continues to climb and reaches to 237F, while coolant stayed at 231F.
The only way how Intake manifold can be hotter than the engine coolant is if it soaks the heat from manifold or the cats.
When manifold was 237F the idle start to become little rough and I decided to test restart of the engine - I shut down and restarted after less that 40 seconds - the engine barely started, typical to fuel rail heat soak.
So, my conclusion that heat soak from teh exhaust is what is major contributing
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