Paul, good report.  That's a pretty old way to do it, and can acvtually scare new skippers since the engine has to overheat before the T stat opens to allow air out of the petcock.

Here's what we recommend: (Maine Sail suggests installing an automatic air vent in lieu of the petcock; we don't have room above our engines for that, but it's a great idea)

Engine Overheating 101 - How to Burp Your Engine (Reply #6)  http://c34.org/bbs/index.php/topic,4518.msg26462.html#msg26462

I'm not sure if this a perfect way of bleeding the system.
I use a Stant automotive radiator pressure checker. I fill the system, pressurize with the pump. Start engine and bleed at the thermostat housing.

This does two things helps get air out and I can get for leaks. 
my 2 cents

Paul.  Great post.  Thank you!
my C36 #2164 (2003 year) began to run a bit hot late last year, about 180°. 
I also did a complete cooling overhaul:
-removed and boiled out/cleaned out the HE heat exchanger-some of the tubes had pencil zinc debris.  I thought that would solve the issue.  It didn't. 
-changed the impeller in my Sherwood raw water pump. 
-new thermostat
-nearly all new hoses (except to water heater). 
-new high quality hose doubled hose clamps, checked to see if I clamped a hose off the nipple restricting flow.
-checked and cleaned the raw water strainer.
-checked the raw water thru hull for blockage, new hose. 
Still no change. 180-185°
-changed the temperature sending device, no change. 

I'm about to change the temp gage itself in a last-ditch effort to remedy this running hot issue. 
But, after reading your post, I'll try the high reving after thermostat opens. 
I will let you know. 
Steven Jones

I'm following this thread and currently in the process of doing the same thing to my M-35 engine. That is, replacing the water pump, (Oberdorfer type M-202N -908 from Pumpsandpartsonline $309.) and new water heater (Atwood/Whale from Defender $300.) and new heat exchanger (type 302631 from Lenco $375) thermostat and hoses.
Question; What do you consider to be the ideal operating temperature for your engine ?
Down here in the Gulf of Mexico, the average water temperature is usually in the 60-80 f. and our engine has always operated at about 180 f. at 2300* RPM.(Checked with a IR temp guage)
Thanks,
Bill

I know this is an old post but wanted to share my experience with my M25 "overheating".  Had a diver clean the bottom on a Friday, Saturday went for a sail, no issues, temp in normal ~160F range. Sunday took her out again and the engine temp was up around 170+.  In my mind I'm going through all the issues that are possible: debris clogging the raw intake due to diver? Low oil? Low coolant? HX needs cleaning? Intake strainer clogged? So I started with the easy: oil-check, coolant-check, strainer-removed and completely cleaned out. There was a bit of mud settled in the bottom, about 1/2", maybe that was it? Put it all back together, fired it up - no still running hot.  But, as I was putting the engine cover back in place, the admiral, watching the temp guage from the cockpit, says the temp just dipped down to 150 and back up again. Huh? What the heck?  Wait let me pull off the cover and tell me what happens. We went thru this a few times, moving the cover back and forth and seeing the temp guage bounce around... then it dawned on me: Electrical. I tapped on the wiring harness next to the engine a few times, sure enough the temp guage was jumping. So the cover was rubbing against the wiring harness for the wires running to the cockpit. I traced the wiring from the temp sensor on the engine to the harness, pulled the wires, cleaned, applied corr-x, put it all back together and voila temps reading normal. Took her out again this past weekend and no temp issues.  SO moral of the story: Check all your electrical, wiring, harnesses, etc. before tearing the engine apart looking for mechanical problems.  You might just save yourself many hours of frustration. 

SLEEPLESS IN SEATTLE.... OVERHEATED IN TORONTO

Well this is certainly an old post at 7-9 years since the last addition, but let's see if my return to it might get some fresh thoughts....

I've asked about overheating several times over the past 11 years since buying our '91 Mk 1.5 with its M35A and have applied all of the valuable suggestions, but the problem continues.

Originally I could run her up to 2000+ RPM and she held at ~180 degrees consistently over several seasons.  Then she began to run hotter and hotter every year, burning off more and more coolant with every lake crossing under power. Last long run in 2024 I had to hold the RPM under 1500 at about 5 MPH to prevent her from overheating and shutting down.

One time years ago I let the impeller age-out and the rubber debris blocked some heat exchanger tubes (at the time I also soaked the HE in muriatic acid overnight), but other than that issue being solved, the gradual increase in temp problem continued until last season I saw pink coolant around the lower gasket and reckoned it was time to pull the engine.  Turned out it was a growing hole due to corrosion at the underside of the aluminum housing behind the coolant pump (didn't need to have pulled out the engine!), but the guy who rebuilt the 4 injectors said two of them were blocked and this could have contributed to overheating, so I hoped that I'd solved the contributing factors.

By this 2025 season every single thing mentioned in all the posts above, other than checking the wiring from the engine back up to the back of the gauge, has been gone over 2-3 times (new hoses, new impeller, no clogging, exhaust riser nipple checked for build-up, raw water flow through the intake/filter/HE/exhaust pipe, etc., etc.).  IR gun showed that the engine was running 20 degrees cooler than the gauge, so I bought a new temp sensor and thermostat (over $300 CDN for the pair from Westerbeke) -- she STILL runs at 230 degrees F at 1600 to get her up to a steady 6 MPH (our standard motoring speed for 11 yrs).  At 1500 and ~5 MPH she runs at 220 degrees (verified via TWO IR guns).  My frustrated mechanic has bowed out at this point saying that there's nothing wrong with a diesel running at over 200 degrees...

[NOTE from my diesel mechanic that the coolant SHOULD NOT be 'burped' at the thermostat by the screw-valve and certainly not by removing the housing/thermostat.  Doing so runs the risk of air being introduced in the housing versus running the engine with the coolant tank pressure cap off and watching for the BURP of coolant to bubble out and over the reservoir (yeah, makes a mess).  This is a WAY more accurate and visible manner of ensuring it has burped, and this may have to be done 2-3 times while the gauge rises to overheating range at above 240 degrees momentarily over a period of a few minutes after getting it warmed up.]

So it's SUPER frustrating to read so many fellow owners repeat that, as was the case during my early years of owning this old girl, they can run up to 2300 RPM at ~180 degrees (buries the transom above 1800 RPM and gets to max hull speed at about 7-8 MPH, but you could probably pull a wake-boarder...).  My last remaining effort (I'm pretty sure it's not a wiring or gauge issue since the IR gun confirms the temps, but I'm open to suggestions) was to consider a larger heat exchanger, but reading more posts here it sounds like the HEX we have is already the larger 3" diameter by 17 5/8" length vs. the earlier M25 size.  I'm not seeing any references to larger ones being used on the M35 to address the problem from anyone else, so I'm back to square one, not understanding how my system has been 'corrupted' over time without a workable solution...

I have seen some lower cost HEX options I could try from China, but it seems like there must be a solution if other C36 owners with the same engine are operating in tropical salt water at NORMAL coolant temps. 
 

I am not clear what temp sensors you are referring to? There is a butterfly valve found on the top of the thermostat that can be twisted open to release Air from the system. Side that there is a electrical connection to a temperature sensor that feeds into the gauge in the cockpit. I have run the engine with the thermostat in and out and replace the thermostat with a brand new one and still have the same problem.