Westerbeke W50 high heat

[zdunc]

I've read everything I can on my Tartan 37 owners forum and posed these questions there and also the tech section of the owners site that shows trouble shooting our W50s for over heating. Couple of questions for advice. Before we left cruising, I noticed temp gauge creeping above 180 if over 2000 RPM (in previous years could go at 2500 for 12 plus hours and stay at 180). So, before we left, I checked the impeller, removed the water pump">raw water pump, inspected, checked flow, back flushed the raw water circuit as shown in tech section, inspected the heat exchanger (didn't appear bad and also when flushing pump and circuit didn't notice any bad sediment or blockages coming out. all was normal), replaced the engine belts (retensioned and no slipping that I can see), replaced a couple of hoses that looked worn, scraped out exhaust elbow and seemed free of obstruction to begin with, changed out zincs, put everything back together making sure to line up cap on heat exchanger, cleaned raw water strainer, removed hose and pumped salt deposit remover and fresh water through system, flushed with fresh water and left with fresh for a week. When we left cruising, it seemed a little better, but temp still seemed to rise ever so slightly above 180 if I went over 2000/2200. Read on owners forum some saying that they have to run between 1800-2000 mostly and thought maybe that was just me too.

Now, a month into cruising, I look down one time and see the gauge near the mark between 180 and 230 (guessing this is 200 since the gauge seems to count by 40 with a mark in between each 100, 140, 180. not sure why it jumps to 230). Never seen it that high before. I was running between 2000 and 2200 fighting current out of Naples, FL. Reduce RPM, temp comes back down to 180 or just over. I haven't tested to see if temp holds steady under 200 if continue at 2200 to 2400 because worried about damage.

Question:
Are these temps bad or just a little higher than normal? Is over heating anything over 180 or is over heating when alarms start going off and temp is climbing without signs of stopping?

Some other observations:
Water temp at exhaust is hot. meat thermometer read about 150 holding it in the stream best I could although couldn't hold my hand there easily due to heat.
Water flow at exhaust is about a gallon in a minute at 1200 RPM (engine RPM, not necessarily pump RPM) measured by holding a container. I have the Sherwood pump. I believe R30G-1. Can't find flow rate in any manuals on boat or on line. It doesn't appear as water is gushing out like crazy, but I can't tell any more and also can't find any reason that it would be diminished flow. Anyone know if that is the correct rate?
Water hoses going into galley hot water heater get hot making me think I have flow there and nothing to suspect an air lock.
No noticeable leaks anywhere.
Nothing visually wrong with my expansion tank cap.
I haven't checked or changed thermostat or coolant yet, but I have absolutely no leaks in that system that I can find. Correct level has stayed the same for years. I know I am due a coolant change, but from what I've read the old coolant won't affect it's cooling properties. Can I check the thermostat without draining the coolant? (I'm at anchor and would like to keep things simple at first if possible).
No idea of my prop specs, but this is a new issue with the same prop that I used to run at 2500 for many many hours and not go over 180. Only difference would be more weight in the boat since we are now loaded down for cruising.
I haven't jumped in the water yet to check for obstruction of the intake, but I just had bottom scraped and the guy said it looked good, and I do have flow. Just not sure if its the right flow.
I even added a little oil since I've got nuisance leaks but dip stick has never shown low.

I'm at anchor and don't want to start pulling everything apart...yet. Can anyone read all my info here and tell me what I might be dealing with...if anything? How do you know if a raw or fresh water pump has gone bad? Visually nothing looks wrong when they run and no strange noise or anything. Please ask away if any questions about things I may have left out.

[Suijin]

Bottom line is if it is climbing above normal and the historical temps you experienced, then something is wrong. Given everything you've done, the first thing I would do is check the thermostat. That actually would have been the third thing I'd have done, following inspecting the impeller and the coolant level.

[Shrew]

When is the last time the exhaust manifolds were replaced?? Rising engine temps and hot exhaust water, coupled with being the only component left to be inspected and replaced.

Manifolds would be the next culprit on my troubleshooting list. Unless you have stainless steel exhaust manifolds, they will need to be replaced on a regular basis. I replace mine about every 5 years. A restriction will cause this, despite you seeing water being discharged while running. It would be the reduced volume due to internal corrosion.

I also assume you've verified there are no restrictions on the intake side of things?

[zdunc]

Going to try the thermostat next. Hoping can do it without changing the coolant, then next time at dock, flush the system and add new coolant. Exhaust manifold probably over 10 years, but unsure. I've owned boat 3 years. According to PO, there was an engine "rebuild" in 2003. It's originally a 1980. It's in good shape other than this over heating mystery.

[zdunc]

Thermostat removed, here is the inside of the housing. Looks pretty good to me, I guess this is a sign of things being generally good in the fresh water circuit. Tested thermostat in water. Began to open at 180, but not fully opened until higher temps. Found a spare on the boat from previous owner (looks new) seems to open at same rate.