Mystery overheat and hot fresh bilge water - MD2020

[Ric]

The overheat alarm sounded on my MD2020 after it had been running for about three hours at 2500rpm (ie not a lot).

On investigation, I found that about a litre of coolant had boiled out of the overflow tube into the sump below the engine, and that the bilge was full of a good few bucketfuls of hot, fresh water.

I thoroughly checked the engine, and there were no broken or displaced hoses. I changed the impeller, even though the old one seemed fine. I topped up the coolant, restarted the engine and it has been fine ever since. Likewise, no more water in the bilge.

Can anybody suggest what might have happened? I can only think that the hot fresh water in the bilges must have come from the calorifier.

 

[tillergirl]

One possibility is that you had sucked up a bit of weed over the water inlet stopping water flow. When you stopped the engine, the suction stopped and the weed fell away as though nothing had happened. I've had weed do this and a jelly fish although the latter got stuck in the strainer. Impeller surviving is not unknown (mine did). Only difference with me was my coolant decants into a litre bottle so I was able to put it back!

I am sure there may be other answers but this suction thing has happened three times to me with a 2030. Not that the make/type of engine has much to do with it.

 

[Keith-i]

If you did have an overheat situation your calorifier would have reached maximum expansion capacity and the pressure relief valve would have dumped excess hot water into the bilge. Sounds like it was doing the right thing.

 

[pvb]

If you did have an overheat situation your calorifier would have reached maximum expansion capacity and the pressure relief valve would have dumped excess hot water into the bilge. Sounds like it was doing the right thing.

Several bucketfuls???

 

[Leighb]

Was it fresh as in drinkable, or not salt but containing antifreeze solution? I would have thought the two should be readily distinguishable. This would pin down the source at least.

 

[Falling Star]

This has happened on my Merry Fisher. Cured by removing the pressure relief valve and cleaning the crud off the seating. If the problem re-occurs it may be necessary to replace the valve as I think the spring weakens over time.

 

[David2452]

Several bucketfuls???

Absolutely, if over a period and a decent sized calorifier.

 

[Keith-i]

Several bucketfuls???

I would think possible. Valve lets off, fresh cold water gets pumped in and cycle starts again.

 

[pvb]

Absolutely, if over a period and a decent sized calorifier.

I would think possible. Valve lets off, fresh cold water gets pumped in and cycle starts again.

I have to disagree. The OP said the engine had been running for 3 hours before the alarm sounded and he found several bucketfuls of hot fresh water in the bilge. Keith suggested that the calorifier's PRV had released water to relieve pressure caused by the expansion of the water. The amount by which water expands at these temperatures is fairly small, so any release of water by the PRV would be minimal.

Now, on the other hand, if the PRV is faulty, it's possible that lots of water was released - but that's an entirely different fault scenario.

 

[David2452]

Depends how hot the thing was running before the alarm went off and I maintain that it is possible and indeed I have seen it happen with the PRV simply doing its job on a hot running engine, turning the restrictor valve down a little to reduce the hot coolant flow cured it, trouble was it just made the engine run hotter. On both occasions water was being released through the skin fitting sensibly provided for the purpouse and was whilst narrowboat owners were performing the usual battery charge and make hot water run at fast tickover with noload. Regulating coolant flow to prevent calorifiers getting too hot is covered in the Indel training courses and like radiators it is important to balance the in and out temperature.

 

[rivonia]

Hmm we had the very same with our md 2020. The way we discovered it was steam coming out of the exhaust pipe. Switched off engine, changed impeller (it was perfect).Released the rad cap the there was no water. Filled up + antifreeze. It then worked perfect. So conclusion. Blockage on the intake as it has worked perfectly ever since.

Peter

 

[pvb]

Depends how hot the thing was running before the alarm went off and I maintain that it is possible and indeed I have seen it happen with the PRV simply doing its job on a hot running engine,

The PRV does exactly what it says - relieves pressure. The pressure caused by coolant getting to say 95 degrees rather than say 80 degrees will be minimal, there simply isn't that much expansion. If the PRV does trigger, it should only release a tiny amount of water before closing. If it's faulty, as I commented before, it might not work that way.

 

[Falling Star]

The PRV does exactly what it says - relieves pressure. ................... If it's faulty, as I commented before, it might not work that way.

I think that's what is happening. As the PRV gets older, the closing spring gets weaker. When the valve triggers then the weakened spring doesn't shut it very quickly. Also, any build up of calcium etc. under the seating will allow excess water out.

 

[David2452]

The PRV does exactly what it says - relieves pressure. The pressure caused by coolant getting to say 95 degrees rather than say 80 degrees will be minimal, there simply isn't that much expansion. If the PRV does trigger, it should only release a tiny amount of water before closing. If it's faulty, as I commented before, it might not work that way.

Its not only the water that expands at a ratio of 1.28% with a rise from 20c to only 80c which is conservative for a hot (not overheated) running engine, but the steam it produces as the pressure drops on release, there is also an amount of air in the system, it should not happen on a properly balanced system with an accumulator in the right place but with no restrictors and no accumulator or one in the wrong place it does. I used to replace PRVs with no effect until I learned this. That said, in this, or any case the PRV should not be discounted as apossible culprit.

 

[pvb]

Its not only the water that expands at a ratio of 1.28% with a rise from 20c to only 80c which is conservative for a hot (not overheated) running engine,

So a 40 litre calorifier might need to leak out around 0.5 litre of water, not allowing for the expansion of the container itself due to heat. Hardly several bucketfuls!

It's nothing to do with expansion of the water due to temperature. It's most probably a faulty PRV.

 

[David2452]

So a 40 litre calorifier might need to leak out around 0.5 litre of water,

Yes,and then a couple of minutes later another 0.5l and so on, as I said the PRV should not be discounted but neither should other posibilities.

 

[pvb]

Yes,and then a couple of minutes later another 0.5l and so on, as I said the PRV should not be discounted but neither should other posibilities.

Errr... no. There'd only be another 0.5 litre if the calorifier mysteriously refilled itself with cold water which mysteriously increased in temperature to 80+ degrees in "a couple of minutes". Sounds far-fetched.

 

[David2452]

Errr... no. There'd only be another 0.5 litre if the calorifier mysteriously refilled itself with cold water which mysteriously increased in temperature to 80+ degrees in "a couple of minutes". Sounds far-fetched.

Firstly the calorifier would not mysteriously refil as it would only be 0.5l down, but the 0.5l would be replaced by cold water added to the 39.5l of already hot water as it is designed to do, it would then only take a couple of minutes for the temperature of the only slightly cooled water to increase to the previous level.

 

[pvb]

Firstly the calorifier would not mysteriously refil as it would only be 0.5l down, but the 0.5l would be replaced by cold water added to the 39.5l of already hot water as it is designed to do, it would then only take a couple of minutes for the temperature of the only slightly cooled water to increase to the previous level.

Oh dear, this is tedious. Yes, it wouldn't take long for the "only slightly cooled water to increase to the previous level", but then it wouldn't expand to the same degree, would it? So you wouldn't get another 0.5 litre out of the PRV, indeed the PRV most probably wouldn't operate, as the pressure wouldn't be high enough. Basic physics.

 

[David2452]

Indeed it is getting tedious Pete, regardeless of theory it happens, I see it happen from time to time in my daily grind ergo I know it happens from time to time and sometimes replacing the PRV does not cure it but balancing or an accumulator does, sometimes practical obsevation and memory of a decent sized sample is more useful in diagnosis than theory. The first cal is, I agree and have previously said the PRV, but it is not always the case and simply assuming it is when other factors are present e.g. the unusually high temperature expeienced in this case when all was previously OK is not the way to go. For instance I now easily exclude a PRV with a five minute pressure test so know if its that or not, so can save the time fitting one when it is not needed, and it isnt always the PRV at fault.