Posting as an FYI because it took me awhile to find solution to this issue. Temps perfect at all speeds except idle on the port. After re checking impeller that I changed earlier, new thermostat even though I didn't think that was it and replacing impeller housing (read about a minute crack being the issue on one forum) still couldn't figure out. Took temps at various spots on both engines and then started pulling hoses and comparing water flows. Turns out water flow restriction between strainer and engine was being caused by a bad seal on the strainer cap and it was sucking in air and preventing full water flow! Ugh, so simple. As an aside- my intake strainers actually sit at and slightly above the water line which I haven't seen before on other boats. Is this common with Luhrs? Good news is with the seacock open I won't flood on a leak issue.