Just fired up the boat for the first time. The first thing I always do is to check for water flow out of the tail pipes. Port side ran good,temps and pressures good.Nice even water flowing from both tails. Started starboard motor and noticed port side had very little water coming out.Checked the port header and it got quite a bit hotter than he starboard. Is this just a worn impeller or do I have a bigger issue? Drained all 4 headers and rechecked,same result.

Start with impeller might be that easy. my luck is never that good though

Were you on a garden hose or sitting in the water while running

I was running off the garden hose,hooked to the quick connect by the heat exchanger.

Check the impeller first.
525= closed cooling, check the front heat exchanger for possible "stuff"/ restriction. remove the right, (facing motor) end cap and clean it out, if no change, remove both end caps from Rear oil cooler, clean ends, push some "soft" wire or something through the holes, some might be pluged.
If right (facing motor) side of headers heating too much you got low water flow. Check you hoses, and the restrictors in the headers and tails.

The port side always runs slightly warmer, particularly when running on the hose. I veryifyed this by running a scan and the port exhaust temp is always 10-20* warmer than star. This has been the case with every merc motor I've owned.

When my boat is in the water the port header is noticably cooler. When I run on the hose it gets pretty warm, even though a decent amount of water is coming out the tips.

What year is your 525? Do you have the bypass hoses on your headers? There is a merc service bullitain that eliminated the bypass hoses. i have the parts, but haven't done it yet.

When I had that problem it was a restricted oil cooler. I was getting good flo but just not what it needed to stay cool. It is a PIA to get to on the 525's. I think the big laptop scanners can show water preasure at different parts of the system.

I think I will back flush first,then pull some hoses off and try to isolate. I don't think its the impeller,the other header is pumping tons of water.