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8 minutes ago, Alan de Enfield said:

It sounds then like it's the PRV doing what it should do and 'blowing off'.

 

I'd still suggest that you look at the size of your skin-tanks as if the water is getting too hot you stand to damage your engine.

 

I agree.

 

The OP is being seriously misleading describing the sound as 'an alarm'. Alarms are designed to warn you about impending problems BEFORE they turn into actual problems. A PRV is designed to open at a pre-determined pressure during fault conditions. It is not an alarm it is a safety protection device.

 

If the PRV happens to make a noise letting by, this is just luck and good fortune that it did and that he happened to notice it. It could just as easily have let by silently. It is not designed with making a noise in mind. 

 

 

Edited by Mike the Boilerman
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15 minutes ago, alan_fincher said:

Without a reliable gauge, I don't think you can make that assumption.

Do you know what the thermostate opening temperature is supposed to be?
Do you know it is opening at design temperature?
At what temperature does the engine overheat alarm go off?
Has it ever?
If it hasn't gone off in recent times, can you be sure it actually works?

I think you are wrong to assume that the engine getting far hotter than the engine thermostat temperature will automatically start to produce hot engine oil smells.  In my experience you can get a pressurised cooling system well over 100 degrees, and still not produce oily smells.

If your domestic water is getting very hot, then by implication it is likely your skin tank is as well.  At the moment I don't think you have the evidence to say if "too hot".

I note you didn't actually answer questions about skin tank dimensions.

Alan, it is far too hot a day, and with the engine still cooling after this morning run I am unable to access down below in the engine room the swim tank with a suitable measure.

I will take a look later this evening while we are moored up on the Nene in a quiet backwater.

Cheers.

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2 minutes ago, Cyan said:

Alan, it is far too hot a day, and with the engine still cooling after this morning run I am unable to access down below in the engine room the swim tank with a suitable measure.

I will take a look later this evening while we are moored up on the Nene in a quiet backwater.

Cheers.

OK

An infra-red thermometer (something like this maybe), is a very good diagnostic tool for actually getting a good idea of temperatures in your skin tank, (and any other part of the cooling system), and seeing if all is as one would expect.

Well worth having in the armoury.

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34 minutes ago, Richard10002 said:

How hot would you expect a skin tank to be when the engine is running. One would have thought that the hot water in the calorifier would be close to the same temperature?

If it’s all working as it should be with an 80C stat in the engine then everything should be at around that temp. If it’s between 90C and 100C then it’s too hot. If it’s above 100C then it’s much too hot. 

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32 minutes ago, Richard10002 said:

How hot would you expect a skin tank to be when the engine is running. One would have thought that the hot water in the calorifier would be close to the same temperature?

A little above the engine thermostat opening temperate and that could be anything between about 62C and 82 to 88C.

 

If the PRV was squealing I can only assume the water in the calorifer was boiling. otherwise it would make a burp as it vented pressure and then just close again BUT a typical bloat domestic water system is pressured to 1.5 to 2.5 Bar (20 PSI to 40 Psi say) while the  coolant is unlikely to be pressurised to more than 15 PSI and usually only around 8PSI so it the calorifier was boiling the engine should also have been boiling.

 

Even with a 65 degree engine thermostat the hot water gets close to being unsafe and very hot.

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58 minutes ago, Richard10002 said:

How hot would you expect a skin tank to be when the engine is running. One would have thought that the hot water in the calorifier would be close to the same temperature?

I would expect it to be at engine temperature at the top, then getting steadily cooler towards the bottom. If it is properly baffled you may be able to feel the zig-zag flow of water as it gets cooler. If there is a rapid change in temperature from hot to cold part way down the tank, that would suggest there is insufficient coolant flow - indicative of an air lock, blockage or pump problem.

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16 minutes ago, Tony Brooks said:

A little above the engine thermostat opening temperate and that could be anything between about 62C and 82 to 88C.

 

If the PRV was squealing I can only assume the water in the calorifer was boiling. otherwise it would make a burp as it vented pressure and then just close again BUT a typical bloat domestic water system is pressured to 1.5 to 2.5 Bar (20 PSI to 40 Psi say) while the  coolant is unlikely to be pressurised to more than 15 PSI and usually only around 8PSI so it the calorifier was boiling the engine should also have been boiling.

 

Even with a 65 degree engine thermostat the hot water gets close to being unsafe and very hot.

Hi Tony, interesting prognosis!

Coolant pressure cap is rated at 8 psi. No signs of coolant being passed beyond the cap, ie; No deposits of coolant at the end of the overflow hose. Must assume that engine temp was not excessive on this occasion. In fact there have never been a sign of engine overheat ( cam cover paint is not blistered!)

I will now remove the panels to expose the H.W. tank and check the PRV is clear and for any signs of discharge through the valve.

 

Any thoughts on a thermo control for the water supply  from the engine through the calorific heat exchange coil?

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6 minutes ago, David Mack said:

I would expect it to be at engine temperature at the top, then getting steadily cooler towards the bottom. If it is properly baffled you may be able to feel the zig-zag flow of water as it gets cooler. If there is a rapid change in temperature from hot to cold part way down the tank, that would suggest there is insufficient coolant flow - indicative of an air lock, blockage or pump problem.

David, we will check the temperature difference from top hose into the skin tank and the return, lower hose into the engine. Previous examinations revealed a significant difference but not checked in 2018 :( Need to borrow a 'Widget' to measure the temperatures, as described by Alan Fincher, with an IR Thermometer.

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10 minutes ago, Cyan said:

Hi Tony, interesting prognosis!

Coolant pressure cap is rated at 8 psi. No signs of coolant being passed beyond the cap, ie; No deposits of coolant at the end of the overflow hose. Must assume that engine temp was not excessive on this occasion. In fact there have never been a sign of engine overheat ( cam cover paint is not blistered!)

I will now remove the panels to expose the H.W. tank and check the PRV is clear and for any signs of discharge through the valve.

 

Any thoughts on a thermo control for the water supply  from the engine through the calorific heat exchange coil?

 

Unless you ave a thermostatic mixing valve on the calorifier hot outlet I doubt the is any thermo control of domestic water supply. I also doubt you have any temperature monitoring on it either. However with a  second-hand boat nothing can be taken for granted.

 

I would be looking in the area where you think the noise was coming from for something like an inverter that may have  been complaining about something.

 

If the pressure switch on the domestic water pump stuck closed then the PRV might make a noise as it constantly vented water.

 

I am very interested as to what is going on.

 

 

 

 

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8 minutes ago, Cyan said:

Must assume that engine temp was not excessive on this occasion. In fact there have never been a sign of engine overheat ( cam cover paint is not blistered!)

I still think you are on shaky ground in concluding the engine is not getting too hot.

If it is not being adequately cooled a water cooled boat engine with a pressurised system is easily capable of ending up with the coolant at a temperature much above the natural 100C boiling point of water.  The fact that it is pressurised to (in your case) 8 psi means it will not boil until a very significantly higher temperature, and any anti-freeze present will raise the temperature it boils at even higher.   However if you were to whip the pressure cap off it could suddenly boil at atmospheric pressure. (Please don't unless you are really confident it is below natural boiling point!).

However even at these elevated temperatures I'd not expect to see damaged paint on engines, or necessarily smells of hot oil.

If you have no reliable working gauge, I still don't understand why you are discounting that the coolant in the engine may be at a much higher temperature than the intended figure determined by the thermostat.

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15 minutes ago, Cyan said:

David, we will check the temperature difference from top hose into the skin tank and the return, lower hose into the engine. Previous examinations revealed a significant difference but not checked in 2018 :(

Assuming a skin tank is not air-locked, the thermostat is open, and the circulating pump working well, the usual indication that a skin tank is not sufficiently large to keep the engine at thermostat temperature, is that there will be very little temperature gradient from top to bottom - it will be hot over its entire surface.  That is to say that the water being returned to the engine has not cooled enough to be able to keep the engine temperature where it should be, which is what then causes the coolant temperature to continue to rise above what it should be.  Many boats will survive this situation, because as the skin tank gets hotter than intended there is a bigger temperature difference to the canal water cooling it, and hence rather more heat can be dissipated.  The real issue arises if the temperatures rise to the point it may boil, (at well over 100C, because of the pressurised system), and the skin tank still can't dump heat fast enough.

However I'm running ahead of things here.  If you actually have a good temperature gradient across the tank, even when running flat out, such that water returned to the engine is sufficiently cooled, then I think it is unlikely your skin tank is too small.

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2 minutes ago, alan_fincher said:

I still think you are on shaky ground in concluding the engine is not getting too hot.

If it is not being adequately cooled a water cooled boat engine with a pressurised system is easily capable of ending up with the coolant at a temperature much above the natural 100C boiling point of water.  The fact that it is pressurised to (in your case) 8 psi means it will not boil until a very significantly higher temperature, and any anti-freeze present will raise the temperature it boils at even higher.   However if you were to whip the pressure cap off it could suddenly boil at atmospheric pressure. (Please don't unless you are really confident it is below natural boiling point!).

However even at these elevated temperatures I'd not expect to see damaged paint on engines, or necessarily smells of hot oil.

If you have no reliable working gauge, I still don't understand why you are discounting that the coolant in the engine may be at a much higher temperature than the intended figure determined by the thermostat.

Alan you are correct,

 

Without a reliable and accurate temperature gauge/sensor there is no way of knowing the engine is overheating or not! I will investigate installing a temperature gauge within the control panel. An overheat warning buzzer, combined with a gauge is the ideal way to monitor the engine coolant condition. Until then we can only speculate.

 

Thanks again for your interest :) 

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4 minutes ago, Cyan said:

Alan you are correct,

 

Without a reliable and accurate temperature gauge/sensor there is no way of knowing the engine is overheating or not! I will investigate installing a temperature gauge within the control panel. An overheat warning buzzer, combined with a gauge is the ideal way to monitor the engine coolant condition. Until then we can only speculate.

 

Thanks again for your interest :) 

For now one of Dr Bob's infra red thermometers will do, even a cheap one.

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4 hours ago, alan_fincher said:

However I'm running ahead of things here.  If you actually have a good temperature gradient across the tank, even when running flat out, such that water returned to the engine is sufficiently cooled, then I think it is unlikely your skin tank is too small.

 

 

Not necessarily. 

 

If there is a very slow flow rate through the skin tank there will be very little cooling power yet a big temperature difference between in and out.

 

Could caused by be a collapsed hose (internally) or some vanes snapped off the water pump impeller. 

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9 hours ago, Mike the Boilerman said:

 

 

Not necessarily. 

 

If there is a very slow flow rate through the skin tank there will be very little cooling power yet a big temperature difference between in and out.

 

Could caused by be a collapsed hose (internally) or some vanes snapped off the water pump impeller. 

 

An often overlooked cause on boats that use a vane type Johnson or Jabsco cooling pump but an Isuzu and most other modern engines do not. Their water pumps have a solid impeller. If any raw water is involved then a very valid comment but I doubt it is here.

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We've a plan to put into a marina in Braunston / Napton area in about 10 days to a fortnight, we'll be able to order thermometers etc., on line from there. John (Cyan) will be back to share what he discovers, including (maybe) a fix. 

 

Thank you everyone for every post ?

 

 

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1 hour ago, Tony Brooks said:

 

An often overlooked cause on boats that use a vane type Johnson or Jabsco cooling pump but an Isuzu and most other modern engines do not. Their water pumps have a solid impeller. If any raw water is involved then a very valid comment but I doubt it is here.

 

I'm reasonably sure I've read on here about the rigid vanes on a centrifugal, solid impeller water pump snapping off too when the impeller is made from plastic. I can't remember any of the details though! This is why I posited the possibility. 

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Along the same lines as MtB. 

I have seen in a car engine, the water pump impeller made of pressed steel.  Fine whilst the antifreeze had anti-rust properties. 

This impeller had rusted away completely!

The engine was over heating but had a sound head gasket. 

 

Bod

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1 hour ago, Mike the Boilerman said:

 

I'm reasonably sure I've read on here about the rigid vanes on a centrifugal, solid impeller water pump snapping off too when the impeller is made from plastic. I can't remember any of the details though! This is why I posited the possibility. 

The whole rigid plastic impellers were known to  fall off some "French" cars but I have never known t on a boat engine. The pressed steel impellors I have seen but its not many always looked like stainless steel to me.

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3 hours ago, Jennifer McM said:

We've a plan to put into a marina in Braunston / Napton area in about 10 days to a fortnight, we'll be able to order thermometers etc., on line from there. John (Cyan) will be back to share what he discovers, including (maybe) a fix. 

 

Thank you everyone for every post ?

 

 

Jennifer, you can borrow my infrequent red thermometer for a few weeks if you get back to Napton.

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32 minutes ago, TheBiscuits said:

Don't you use it very often? :D

Infrequently. 

 

It’s outrageous!

 

Edited by WotEver
Autowrong got the incorrect it’s
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2 hours ago, Dr Bob said:

Jennifer, you can borrow my infrequent red thermometer for a few weeks if you get back to Napton.

Dr Bob you are a STAR! Does the 'infrequent red thermometer come with instructions though ?

 

{In hushed tones} think John should be encouraged to buy one of these for his 'arsenal' tool kit though.

 

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