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paulrenecyr's picture
Joined: 6/25/14
Posts: 10

I know this has been covered before, but I just wanted to add my two cents.  I had a problem with the engine overheating when I ran it above about 1500 RPM.  I decided it was time for a complete cooling system overhaul anyway so that is what I did.  New fresh and seawater pumps, hoses, clamps, heat exchanger, thermostat, full flush and new antifreeze.  This was on a 1985 M25 engine on the C36.

Although I was extremely careful in trying to avoid any trapped air in the system, I still ended up with some.  I would run the engine without a thermostat and after a few minutes it would begin to run a little too hot.  I would try the air bleeding valve on top of the thermostat housing, got no air out.  I would shut down the engine and check the coolant level and there would be no change.

Finally I remembered reading somewhere about running the engine revs up to wide open momentarily as the engine temp begins to climb into the too hot range.  It said you may need to do this several times.  Sure enough after several tries I finally got any air pumped through and was able to add coolant.  I could then run the engine and it remained down at about 130 to 140 degrees.  I put the new thermostat back in and the engine runs at a consistent 160 through all RPM ranges.

So if the air bleeder does not work for you try revving the hell out of the engine.

Paul Cyr
C36  "At Last"
1985 hull# 369
Oriental, NC

stu jackson c34's picture
stu jackson c34
Joined: 12/3/08
Posts: 1270

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


clennox's picture
Joined: 3/31/14
Posts: 212

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

paulrenecyr's picture
Joined: 6/25/14
Posts: 10

I tried the bleeder valve on top of the thermostat housing, with no luck.  I was also running the engine without a thermostat in place in case there was some junk flowing through the system.

Yes I was a little scared of revving the engine hard as it was overheating, but you really don't do it for very long.  Rev it a few times, stop let it cool, repeat, until temp starts to drop.

Was glad I had a good working engine during Hermine's visit last weekend.  Didn't need it, but glad it was ready.

Stevenjones's picture
Joined: 6/29/07
Posts: 74

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


paulrenecyr's picture
Joined: 6/25/14
Posts: 10

Our problems sound similar but have a few differences.  My engine would run hot when I pushed the RPM's past 1500.  I would start to run 190 to 200 degree temps.  That is what prompted me to go through the entire cooling system.  After the rebuild my engine would overheat within minutes even at lower RPM's.
In other words, I made things worse.

180 to 185 degrees is not too bad if you can run up to about 2500 RPM's. Not ideal maybe, but not bad.
If you decide to do the high RPM "burping" method, I suggest you remove your thermostat while doing 
the high rev procedure to avoid any potential clogging issues.

Before I replaced the temp gauge, I would buy a cheapie IR temp gun.  They are about $15 for the cheap ones.  If you aim it at the thermostat housing you can compare readings to your cockpit gauge.
You can also trace temps through out your cooling system as well as come up with some baseline temps for future troubleshooting once you get the system right.

In its self the IR temp gun is worth having on board.  It makes reading temps of shafts, stuffing box, etc. very easy.

Good luck.

Catboat Willy's picture
Catboat Willy
Joined: 12/22/14
Posts: 305

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)

IslandTime's picture
Joined: 12/18/17
Posts: 31

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. 

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