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Water temperature of engine changed


Colin North

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I have a 1.5 BMC diesel on nbconstance. The engine cooling water has always settled at about 75C. It normally takes about 3/4 hour of gentle cruising to get to that temperature.

On my last trip (about two weeks ago) I noticed that after about an hour of running the temperature gauge had barely risen off the lower end stop. I checked the temperature of the calorifier hoses an agreed with the lower temperature.

So, I assumed the thermostat to be faulty. Have now removed old 72C one and fitted an 82C, the nearest I could get in hurry.

Run engine, takes ages to warm up as borne out by touching caloifier feed pipes. Touched skin tank, not even warm.

Every other aspect of engine running seems to be OK, just not getting hot.

Any ideas?

Thanks, Colin

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In my experience, an airlock causes a sudden dramatic temperature increase, at least in BMC 1.8 engines.

In this case, and I know it is unlikely, could it be a second defective thermostat?

Try the saucepan test, put it in a saucepan, heat it up, with a thermometer in the water, check it is working as it is supposed to.

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Yes, they are normally too hot to hold when then engine runs normally at 75C. The water temp gauge agrees with the temperatures I am getting. Currently the calorifier hoses are hot but 'holdable' at 45C. Maybe I am just overreacting but I certainly do not remember this scenario last winter.

Thanks for all of your replies.

Colin.

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Yes, they are normally too hot to hold when then engine runs normally at 75C. The water temp gauge agrees with the temperatures I am getting. Currently the calorifier hoses are hot but 'holdable' at 45C. Maybe I am just overreacting but I certainly do not remember this scenario last winter.

Thanks for all of your replies.

Colin.

There should be an obvious difference between the temperature of the 2 hoses for the calorifier heating coil until the Temp is equalised.

Sounds to me like you have a small air pocket in the coil ,so,some water is flowing but not enough to raise the temperature quickly

CT

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Thanks for your replies.

The skin tank was only a degree or so above ambient. So the heat is not going there. The calorifier is definitely working OK. The 'hot' water was cold (almost ice cold) before I set off today and I now have reasonably hot domestic hot water. There was the usual small differential between the flow and return pipes.

As I say, I might be overreacting, I will try and give her a good run tomorrow and see if the temperature will achieve the normal running levels.

Thanks, Colin

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Is it possible that there is some crack or corrosion hole that is allowing the coolant to bypass the thermostat? I know you said the skin tank was cool, but it is cooler than normal? The heat from combustion has to be going somewhere.

 

As Nick says "The heat from combustion has to be going somewhere"

Is the feed hose to the skin tank hot? My skintank always feel cool.

Water pump? are the drive belts slipping?

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I would bet money on air in the system. Systematically bleed it all round the circuit. Should cure it given that the usual suspect, the thermostat, has already been dealt with.

It's the most likely reason for bad circulation, and a bubble around the temperature sender would give a low reading.

 

Richard

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I can't see how, or why it should have changed, but nothing to do with wrong type of thermostat, is it?

 

BMC 1.8s typically need the less usual type of thermostat, known as a "bypass" thermostat, (on right, in picture below), that has an extra part not present on the more usual non-bypass type, (on left).

 

IMG_1711.jpg

 

If when you look down the thermostat housing there is a seat and hole at the bottom that would be covered by that extra "penny washer" shaped bit at the bottom of a bypass stat, that is almost certainly the type you should have.

Fitting a non-bypass stat in a system designed to have one might give unpredictable results, as it is possible for water to end up bypassing the cooling circuit.

Edited by alan_fincher
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Crack the return pipe from the calorifier to the engine, allowing it to drip when warmed up. Hold the other pipe and wait for the temperature to increase before re tightening the clip om the return pipe. I suspect it's air, I had the same problem on a boat a few weeks ago.

Edited by jenlyn
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No it isn't. The water has to go through some part of the circuit

 

My understanding is that the bypass is to allow air through when filling the system

 

Richard

 

This which has been linked to in previous threads, disagrees, and says it is possible by fitting the wrong type, to bypass the cooling circuit.

 

(My understanding is that this is the actual intention of having a bypass stat, but only in the very early stages of warming up the engine, after which the bypass should get closed off - however, without the extra plate, that doesn't happen, and water can continue to short circuit the cooling system, irrespective of any temperatures).

 

In my own experience of my own engine, (including following the water paths that exist, when the pump and thermostat were removed), what is in this article correctly states the situation for my BMC 1.8.

 

I'm not saying it is in any way pertinent to the question asked, just checking that OP did have, (and still has), the correct type fitted for their particular configuration. It's something that can muddy the waters.

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Both the old and new thermostats are as the left hand item in the picture above. There is no lower seat within the block for the penny washer. The top hose was warm,30C, but that water is heated by the exhaust manifold on my marinisation.

As I say, I will try and give her a slightly harder run tomorrow.

Cheers, Colin

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Both the old and new thermostats are as the left hand item in the picture above. There is no lower seat within the block for the penny washer.

Fair enough!

 

For the benefit of anybody who has no idea what I'm on about (!), the type of housing that needs a bypass stat, looks like this at the bottom.

 

(Once initial warm up is complete, it covers and closes that hole at the bottom).

 

IMG_1720_zps0a866364.jpg

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Both the old and new thermostats are as the left hand item in the picture above. There is no lower seat within the block for the penny washer. The top hose was warm,30C, but that water is heated by the exhaust manifold on my marinisation.

As I say, I will try and give her a slightly harder run tomorrow.

Cheers, Colin

You need to be careful here. If diesel cylinder heads get starved of coolant whilst running they can crack easily. Did coolant overflow when you removed the thermostat housing and from where the thermostat sits when you took it out ? If not the coolant level is too low in the engine and probably lower than the temperature sender which will cause the gauge to show just warm or even almost cold. Also the coolant level maybe too low to allow the water pump to send the coolant over the top ''so to speak'' and circulate around the system, ''air in it''. Though I would expect hot high pressure to have built up to boiling point and be blowing from the filler cap or overflow after running the engine for some time. Have you a remote header tank filler or just the filler on the heat exchanger manifold ? Have you tried topping up at the filler and running the engine with the filler cap off to see what happens?

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By and large bypass thermostats are used to ensure water is still circulating around the cylinder head hot spots even when the thermostats is closed. This minimises the chance of localised boiling when the engine is cold but is put under load. It might also allow air to leave the cylinder head but that is not its main purpose (unless an engine design engineers says different).

 

Certainly fitting a normal stat into a bypass stat engine would allow a degree of coolant totally bypassing the cooling circuit but how much is open to question so fit the correct thermostat.

 

Not applicable here but the/some BMC A series petrol engines had a bypass hose for a similar reason and they had a "hole" in the bottom of the thermostat housing but it was smaller and they used an ordinary thermostat. This might hold true for the rare 998cc BMC diesel.

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The bypass hole in an A series goes between the head and the pump directly below the thermostat, is about 3/8" in diameter, has a horrible corrugated hose and was eliminated on the A plus engine as it was unnecessary

 

Richard

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