Fan always when engine stops

[SD1too]

Hello Tim,

Let's take this slowly and simply. Please do the demist test as follows:

1. Start the engine from cold.
2. Press the windscreen demist button on your air conditioning/heater control panel.
3. Get out of the car and walk to the radiator grille.
4. Can you hear the fan running continuously (not pulsing on and off)?

Please report back with the answer.

Do not take your car to the garage.

Simon

[COLVERT]

Originally Posted by Lee T

With the header tank cap off the system cannot fully pressurise, so what would cause an overflow? When I bled mine using the hoover pipe method (credit to Big Russ for that tip) I only lost a little when the air burped out and when I removed the pipe from the header tank.

The water pump will force the water around the system faster than gravity will take it down through the radiator. The end result will be the radiator will fill right up and over flow. ( atmospheric pressure at 14 psi and gravity not able to keep up with the demand from the pump. )

With the cap on this can't happen because the water has to flow around the whole system at approximately the same pressure. ( Low at the pump inlet and higher at the pump outlet.) The differential determining the the speed and direction of the flow.

The tube you used with a higher water level made use of gravity and the increased weight to aid the flow of water down through the rad.---Cunning stuff physics isn't it.--Also as the water cools on its journey through the the rad its density increases and its weight.- ( ie. gravity then gives it a little help on its way. )

Think about it.--Lol.

[Lee T]

My original statement and question to you was based on header tank cap off and hoover pipe inserted, so under those circumstances we agree on no overflow

[Ducati750cc]

Originally Posted by COLVERT

The water pump will force the water around the system faster than gravity will take it down through the radiator. The end result will be the radiator will fill right up and over flow.

With the cap on this can't happen because the water has to flow around the whole system at approximately the same pressure.

The tube you used with a higher water level made use of gravity to aid the flow of water down through the rad.---Cunning stuff physics isn't it.

Think about it.--Lol.

The engine water pump via its design, is a centrifugal pump which by utilising an impeller with vanes and a volute (or diffuser vanes, two systems) cast into the pump housing, creates an area of low pressure at the eye of the impeller.


This in turn is connected to the radiator to pump connection, the bottom hose, this pressure reduction draws water, or allows water to flow from the radiator and then pushes it through the engine.


So the water is sucked out of, not pushed out of the radiator, the correct term is that the pressure differential causes water to move from the radiator, but most folk understand the word suction easier.


Gravity has nothing to do with it at all thankfully !


With the cap on this can't happen because the water has to flow around the whole system at approximately the same pressure.


Understanding the subtleties of flow, pressure, (not the same things, though can be interdependent) and paths of least resistance, explains why some, but not all by any means, cold cooling systems exhibit some water loss with the pump running and the cap off, yet some vehicles don't exhibit this at all, why is that?


There are lots of variables, too many to go into here, as to what the influences are that determine what cars will exhibit a loss and those that, under the same circumstances don't, one is path of least resistant, in the scenario you describe, for whatever reason, the return of water through the pipework and radiator to the bottom hose and pump offers a higher resistance path than that of the open filler cap, so the water goes that way.


Yes, physics and fluid dynamics are clever things.................when understood.

[timbo6063]

Thanks to everyone.

I open the cap of the expansion tank and and fluid pours out so I can not put more in. I have not carried out the tests as I have lost faith in my ability to predict whether I am going to do further damage by any action.

The garage has not got back to me. I am in Wimborne Dorset - any ideas about a good garage that others have used and had satisfactory service?

I am loath to give up on what is a very comfortable and spacious car yet (leather and wood etc.) - clearly not worth a new engine economically.

I then phoned the RAC and found that our membership required us to pay £40 (!) for them to have a look at it (each and every time) when we nominally had home start.

So it sits outside my house now.

Tim

[timbo6063]

Hello

Lots more advice - thanks - will take time to digest and action.
Will come back when it stops raining - probably Saturday A M.

Tim

[T-Cut]

Originally Posted by timbo6063

I open the cap of the expansion tank and and fluid pours out so I can not put more in.

Hello Tim, not wishing to bombard you with ever more suggestions on what you should do next, but your repeated mention of the header tank erupting coolant has me concerned. Does this happen when the engine's cold/not running? When you open the pressure cap when the engine's stone cold, there should be no significant hiss of pressure released and definitely no eruption of coolant. If there is, you have much more going on that an air lock.


TC

[COLVERT]

Originally Posted by Lee T

My original statement and question to you was based on header tank cap off and hoover pipe inserted, so under those circumstances we agree on no overflow

Yes, in the method you were using.---

[SD1too]

Originally Posted by timbo6063

I have not carried out the tests as I have lost faith in my ability to predict whether I am going to do further damage by any action.

Tim,

You have come onto the forum to ask us for advice. I have asked you to carry out the "demist test" as a basic starting point. This involves pressing a button on the centre console and using your eyes and ears. How can you possibly believe that is going to "do further damage" to your engine?

If you really want advice we are happy to give it, but not carrying out our suggestions and asking for garage recommendations instead is sending us a message that you don't trust our advice.

You need to make a decision Tim. Do you want us to guide you through what's necessary to reach an accurate fault diagnosis? If so, then please give us the result of the "demist test" and answer T-Cut's question.
On the other hand, if you want to let a garage decide that's fine, but the two approaches don't mix.

Simon

[stevestrat]

Originally Posted by SD1too

On the other hand, if you want to let a garage decide that's fine, but the two approaches don't mix.

Simon

If you go down the garage route be sure to use the right garage, one who knows their way around 75s. If you use the map below you can see there are two garages near you that have been recommended by members ( https://www.facebook.com/BridportMG/ & http://www.mikerolls.co.uk/index.html) and a forum trader (cb750chris) a bit further away.