Looking for advice with cooling system , Any suggestions? Help !!

I have a 25 HP mariner 2 stroke 1998. 2 weeks ago while out fishing , lost my cooling water...
Got boat back home with no damage by using trolling motor. Removed thermostat as had no flow, checked in water , lots of flow with out thermostat... Put thermo in hot water and it did not pop open. I put a new thermostat in and water is flowing ... However, when running at full speed I lose my cooling water ! Once I slow back down the cooling water pumps again ???? Could the impeller be damaged even though it pumps cooling water through at low to medium speed ?

Re: Looking for advice with cooling system , Any suggestions? Help !!

Check pee hole for blockages.Also check plumbing from tstat.When this happens,are you overheating?

Re: Looking for advice with cooling system , Any suggestions? Help !!

I keep
A close eye on cooling water , Im pretty sure I would def overheat if I was to let it go but I don't think I've done any damage , other then probably going to require a new seal around head.

Re: Looking for advice with cooling system , Any suggestions? Help !!

You may have a "non problem" here.

I don't know your particular engine, but perhaps, as in most bigger mercs I've contended with, much of the cooling water is bypassed through fixed ports, spring loaded valves (poppets) etc. The t-stat only opens when water gets hot and lets off a little more. The Tell tale shows only water that has gone through the t-stat. If you're running in cold water, it's common for the stream to stutter or stop. Water pressure and flow are just fine, the t-stats just close because it's cool.

I find that the best use of the tell tale is to determine that the engine is warm enough to blast off from the landing without damage. Once running, I rely on a water pressure gauge and for catastrophic failure wake up, the overheat alarm.

If it's been a year or so since you've changed the water pump impeller, do it. They take a set and lose efficiency as the rubber gets old and tired. It may look good, but might be weak. Usually, however, the tell tale will remain fairly strong till the pump pitches blades. (hotter water opens the stats, feeding the tell tale)