Overheating question

[Analgesic]

I have a 1988, M 25 XP Universal.  Memorable afternoon on Boston Harbor today. The "chance of showers" turned into an all out squall with a 50 kt gust and low 40s sustained for a few minutes. Seeing it coming, we reduced sail to minimally unfurled jib and started the engine a few minutes before it hit.  Guests smelled something and engine temp was at max. (I think my high temp alarm is broken).  After a nerve wracking 30' running and hove to, the wind died and we ghosted to our mooring.  There was no squealing as I have read in other overheating posts.  I found the  impeller looked great and had free flowing water at the through hull.  I restarted engine, saw perfect water flow in exhaust, saw nothing abnormal in the engine but after a few minutes the temp hit max again. The only other data is that the galley tap produced no hot water despite the engine temp.   I assume there is a problem with the coolant pump.  Any thoughts?  Of course I'm supposed to take a big group out to see fireworks in two days which doesn't seem too likely right now but if there is a quick diagnosis and fix, I'd be thrilled.  Thanks in advance for your help. 

[Jon W]

Hi Brian, not the way to end the day. You have good raw water flow out the exhaust and your coolant level is fine correct?

Have you done any work on or around the engine recently? Has the engine been running at normal temperature until this?

[Stu Jackson]

Unsaid is if coolant level was checked.

Check hoses to water heater.  See Critical Upgrades topic.

[KWKloeber]

@Brian

If you have good seawater flow out the exhaust obviously the issue has to be the Hx or the closed coolant system.  It is a VERY slim chance that the coolant pump is bad (impeller can't move water.) Bearings could be bad (a squeal) but the pump will still turn.  If the impeller is bound up you will burn up the engine belt.

If the HX is blocked, you won't see good flow out of the transom.  But it might be worth removing/checking the end cap on the inlet (stbd) end of the Hx.

If the exhaust manifold is full of coolant then the situation I see is a stuck thermostat.  However, that doesn't make sense because the WH should still make hot water (unless something else is going on with it) because with the TStat closed coolant will still flow to the WH. 

If coolant is moving, the short hose from the TStat cap to the Exhaust Manifold inlet would be hot.  And the long return hose between the Hx and the coolant pump will be cool.  See for coolant flow directions:
https://groups.io/g/Catalina30/wiki/9626

My gut says "air lock" based on everything you describe.

[Analgesic]

Thanks for the quick replies.  Coolant level is normal.  Winter work included changing out coolant, new engine mounts, cleaning out heat exchanger with new gaskets, new impeller and new fan belt.  Six weeks into my sailing season, the engine temperature has been about 140 consistently until now (our cold ocean water) and perfect hot water from the tap.  I haven't touched anything since May launch. 

[Matthieu G]

Maybe it helps or not, but last week we had an air lock in the coolant system (I changed the hoses and had the HX cleaned). After a few minutes on the engine, the temperature gauge read over 200, but the engine was not particularly hot. Was that really the engine? The gauge? Finally we found that there was no coolant in the thermostat housing so the probe was taking the temperature of the air (which heats up faster than the liquid I imagine). Conclusion: I bought a laser thermometer to get a second opinion if my instruments tell me that the engine is too hot.

[scgunner]

Brian,

I too suspect an air lock, not unusual when you change your coolant. Run the motor if the heat exchanger remains cold to the touch it's air locked also known as a big giant air bubble. It's easy to eliminate, run the motor with the radiator cap off, once it starts to warm blip the throttle a few times up to around 1500rpm that will drive any trapped air out the radiator opening. You may want to put some rags around the opening because when the air bubble hits that opening it's going to bubble up and take some coolant with it. The H/X should start to warm immediately indicating the air bubble has been cleared. You'll have to top up the coolant afterwards.

[Jon W]

When you changed the coolant, did you follow the steps to "burp" the system? If coolant level in the reservoir is normal, raw water flow is good, no hot water at the tap, but temp gauge reading very high I suspect an air lock as well.

[Stu Jackson]

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

[Ron Hill]

Brian : It also sounds to me like an air lock in the internal coolant line.  Burp the engine!!

A thought

[Analgesic]

So, important details to update and nothing is ever simple...I told you that I checked the coolant level.  Well, the overflow tank showed coolant in the normal range but in the aftermath of the storm and the 240 degree temp reading, I didn't open the radiator cap. I naively thought the level was fine.  Well, today I did and coolant was empty!  I then found evidence of a leak where the hose to the water heater attaches and coolant in the bilge.  I fixed this, BUT, nothing is ever simple.  I filled up with fresh coolant, burped the system once, twice, three times following the different options recommended but in the end nothing worked.  Temp still rapidly rose, coolant hoses to the heat exchanger remained cold as did hoses to the water heater, all the while with perfect raw water flow.  So, does this mean a faulty thermostat?  I wonder if having no coolant could damage/destroy the thermostat.  Does anyone know if this can be opened and serviced/fixed or do I replace the thermostat?  Is there anything else left to check?  I'm saddened that my 20 year tradition of watching the best local fireworks from the boat will end but I don't see any other simple answers. 

[KWKloeber]

5% the TStat is stuck. 
95% chance you still have an airlock.  Probably in the coolant pump so the impeller is trying to move air, which it cannot.  Fill Fill Fill thru the top of the coolant pump.

You could try to put a bypass hose between the Pump/TStat housing in place of the WH hoses and burp again.  That might get you on the road for fireworks. 

[Analgesic]

Well Hallelujah!  We're back in the game, leaving for fireworks in 30'!  Thank you Ken and everyone else.  For my last attempt,  I filled the water pump from above with coolant as Ken suggested and then repeated the coolant into the Tstat lower hose until it flowed out of the Tstat and everything is running perfectly!  Of course, just to provide a little anxiety, as the engine heated up there was some light smoke coming off the engine blown by the alternator.  I'm pretty sure it was spilled antifreeze evaporating because I couldn't locate a specific source and it ended after about 10'.  So, thanks again and happy 4th to everyone in this great community!

[Ron Hill]

Brian : Glad to hear that you finally got it!!

The way I "burp" the engine is - I disconnect the heater hose to the thermostat and take the cap off the reservoir. I have a small PAR hand pump the I put into the disconnected hose and pump the coolant into the reservoir until I get a steady stream of coolant. Then I reconnect the hose back to the thermostat and run the engine. You might have to open the butterfly valve on top of the thermostat till fluid comes out, but you should be good to go!!

A few thoughts

[KWKloeber]

GREAT!!!
Dontcha love when a plan comes together?  And without blood, cut fingers, and iodine.

Enjoy the light show and be safe.