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[KWKloeber]

@KG

How old is your Hx? 
If it has a pinhole between the closed and seawater sides (a common occurrence over enough time) you will gradually lose coolant and eventually overheat.

My Hx was totally in freshwater and IIRC it lasted about 12-15 years before it developed the pinhole. 

Less likely, but if you have a pinhole in the water heater between the potable water and engine coolant sides, you can lose coolant (depending on whether you keep the potable system pressurized) 

The proper way to troubleshoot this, rather than chasing shadows around, is to (FIRST) pressure test at the "radiator cap" using a tester (loaner from an auto parts store.)   That will reveal whether you have a tight system or a leak (a myriad of possibilities - hose clamp, hose chafe, thermostat gasket, Hx pinhole, water heater pinhole, head gasket, exhaust flange gasket, other cause.) 
ONLY AFTER TESTING, isolate the leak source (another post depending on the test results.)

[BJeansson]

Keelson,
Thank you for your post. I have the exact same issue and concern as you describe.
My throttle has a stop so that I cannot rev the engine above 2100rpm, as soon as I go over 2000 and push towards 2100 I notice that the temp increases towards 195.
My overflow tank has been bone dry for the last two season and coolant level in the engine never needs topping off.  I had the HX completely cleaned out before the last season but no change in temperature behavior. No leaks to my knowledge but I do have some white smoke in the exhaust.
I keep wondering why the throttle is limited and not able to reach crushing rpm of 2600 but on the other hand then it will overheat. By the way, with 2000rpm I reach cruising speed of 6 knots.

[Noah]

You definitely have some problems. I AM NOT A MECHANIC, but have no problem offering some observations.
1. The engine should be able to reach WOT, 3000+(?) and not have an RPM stop/limiter at 2100.
2. The coolant overflow bottle should never be “dry”.
3. I suggest you bite the bullet and hire a competent mechanic to inspect.

[waughoo]

You might be over prop-ed.  I have a prop on my boat that only allows max rpm of about 2250 - 2300.  At 2k, I go about 6-6.25kts through the water.  At idle, she goes about 2.5-3kts 8-|  I have heard this was a strategy to lower the fuel consumption while achieving the max cruise.  Though I haven't gotten there yet, I plan to re-prop to get the engine to reach max hp/revs.

[Noah]

Interest thought Alex. What size prop are you currently using?

[waughoo]

Noah,

I didn't know to look when the boat was out of the water upon delivery.  It looked to be quite "power boat" looking though.  Very large ears and very little blank space between them when viewed from directly astern.

[Noah]

How about you take a swim and get back to us on that?

[waughoo]

Oy!!! The last temp I saw for the slip I sit in was 54 degrees!!  The divers bring heated water to pour in their wet suits!

[BJeansson]

Thank you for the feedback Noah and Alex,
Keelson - I hope this is valuable to you as well.
I think this can be valuable information, for what it is worth my prop on the M35 is a Michigan Sailer 8/89, 15RH12.
Based on boat being produced 1990 with hull #1024 could it be possible that the Prop has been an original if produced 8/89?
My cruising speed does not seem to suffer with around 6 knots at 2000rpm, seem to be running at about 185 temp but the cruising temp should be 165 temp. When I cruise at full speed the stern sit pretty deep in the water being pulled down by the prop.
I will fill the overflow tank and try to burp the system before I bite any bullet with an experienced mechanic. Engine has been working like a champ with 2000-2100rpm during long motor cruises.
Appreciate all the feedback.

[Breakin Away]

I think this can be valuable information, for what it is worth my prop on the M35 is a Michigan Sailer 8/89, 15RH12.

Is that 3-blade or 2-blade?

If that is a 3-blade prop, then IMO you are probably WAY over-propped. Most 3-blades for this boat are 15x9 (mine is, and my M35B has 17% higher horsepower than your M35). Some people get away with 15x10 on the M35B. I've never heard of a 3-blade 15x12 on a boat/motor this size. You're probably lugging your motor, generating a bunch of soot that could plug your exhaust elbow, and pushing the limits of your HX capacity. Remember, if your motor runs slower, your raw water impeller is pumping less water, which may be why you are seeing steam.

Many people fool themselves into thinking that lower RPM always equates to better fuel economy. It doesn't work that way. Would you drive your car in 5th gear all the time?

I second the suggestion to hire a good mechanic. You've got multiple warning signs in several areas, and they're probably interconnected.

FYI, your M35 is spec'ed for a max speed of 3200 RPM. You are so far below that that you really need to do something about it. https://www.westerbeke.com/universalengines.htm

[KeelsonGraham]

This has been a very useful discussion. Thanks folks. Apart from the new overheating issue, my engine starts in and instant and runs as sweet as a nut. So i’d always assumed it was fine. Back in the water in the first week of April so I’m going to take a closer look at it then.

[Noah]

Another thing to check when diagnosing RPMs is to make sure your tach is reading correctly—using a laser.

[Breakin Away]

Regarding strobe tachometer, I stumbled across this yesterday $16. I already have one, but it cost a lot more than this:

https://www.amazon.com/Non-Contact-Tachometer-Precision-Backlight-Functions/dp/B07T25BCB1

If you do test, you should do it on the main motor pully. If for some reason you decide to do it on your prop shaft or coupler, be sure to account for the gear reduction ratio to calculate the engine RPMs.

[BJeansson]

Thank you Breakin Away, greatly appreciate your feedback.
The prop is a 3 blade.
I'm well aware that the max RPM is 3200 and I think cruising should be 2600, but as I mentioned the throttle is blocked not to exceed 2100 and then it runs hot.
You are right about the soot, plenty of that.
My prop “drag” when going reverse and parking is brutally strong.
So I suppose these are all signs of an over sized Prop. I think this prop has been mounted for the last 20 years.
The engine runs great, no issues during the two seasons I have owned this C34.
Thank you for your input.

[lazybone]

A dirty, fouled injector(s) will cause all the symptoms described.
The engine will be low on power but will run hotter than normal.  It will probably also reach max rpm in neutral.  The reason for the elevated temps is because the fuel is still igniting in the cylinder, just not efficiently.

Buy one good (denso) injector and try it out. Install it in different cylinders.  See if it helps.  If it helps replace all four.

They’re consumables.