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post #21 of 30 (permalink) Old 17-03-2017, 09:53 Thread Starter
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Thanks Icony.
Car lost next to nothing all week. Drove her home yesterday (9 miles). Level OK at start, level OK when I got home, down this morning. Unfortunately it has been very wet, so any tell-tale wet patch under car hidden by rain.
I'll get the cover off at the weekend (hoping it stays dry) so the hunt goes on.
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post #22 of 30 (permalink) Old 17-03-2017, 20:15
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my guess is you have an air lock... air expands with heat.. so trapped air not in the header tank... but at some other point, expands and contracts as it gets hot and cold... that trapped air is there, because if fluid is leaking... its is replaced with something... either air in the header tank. or air getting in where the fluid leaks out.. or both..

its probably both because you top up the header tank, but still you see rise and fall of coolant level..

if you have a leak to atmosphere, and you bleed the system... initially you might see a slight improvement... since if you are lucky , you remove the trapped air..

but still until you have the leak.. the fluid getting out and the air getting in will continue.. this is because the system is semi sealed..

although the vent cap prevents air getting out.. until 1 bar... it doesnt like a vacuum.. so will let air in... and it does that as the trapped air contracts...

so will the leak point... if the outside pressure is greater than the pressure inside the system.. air will bleed in through the leak point or the vent cap

The second problem is the pressure in the system... as the coolant heats up.. it expands.. and as it cools down it contracts.. its not a pure hydraulic system.. devoid of expansion and contraction due to temperature or inside and outside pressure influence..

it reacts to them all.

so.. if you have a leak.. you wont get the correct running pressure dependent on how much fluid is leaking and when.. and what the pump is doing and what the temperature is..

its a very complicated thing.

and the sum of that.. is the fluid level rises and falls...

i know for certain now all that happens..., the gauge , tells me the influence of the pump on the system pressure, and what the idle pressure is with a warm engine and the pump running...

around 0..5 bar

thats 0.5 bar higher than ambient pressure at sea level.. and i live within about 150 feet of sea level.. at Holme Next The Sea

so if i get a leak.. at idle... its going to be pushed out at 0.5 bar , at whatever temperature it is..

and so on.. if i get a leak after hard drive on a hot summers day... and the coolant temp is high suddenly the leak lowers the pressure... and what leaks boils on exit and makes steam..

now.. maybe the leak is a pin*****... that doesnt change what happens except finding it is like looking for a needle in a haystack,,

the coolant evaporates as soon as it leaks into the atmosphere - if not it wanders along driven by air currents and engine shape et al.. until it evaporates or finds the lowest point and forms drops that drip.. somewhere else... it becomes gravity dependent as to where it goes.. and external engine temperature dependent on whether it evaporates or not..

Last edited by Incony; 17-03-2017 at 20:49.
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post #23 of 30 (permalink) Old 19-03-2017, 19:43 Thread Starter
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Took her for just enough of a run today to get hot. Parked up on top of some clean dry cardboard. Nothing while idling, or giving it a few revs. Engine turned off, nothing. Came back 15 minutes later . . .


Doesn't solve anything, but thank God for a bit of confirmation water is getting out. I thought I was losing my marbles faster than she was losing coolant.
It actually seems like something happens as she begins to cool and depressurise. Each time I saw a dribble of coolant it was after she'd been standing 5 to 10 minutes. Any Guru have an insight into that one?
Seems to be where the underfloor coolant pipes start, but right above is the thermostat and a whole tangle of hoses, so still plenty of choice.
Hey, at least I know it's not my imagination!
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post #24 of 30 (permalink) Old 19-03-2017, 20:37
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what you must also understand, is that a warm normal operating engine.. temp... includes the heat in the oil..

while the pump runs that heat is dissipated and absorbed by the coolant... because the pump is circulating it.

what happens when the engine stops? does the oil suddenly stop being hot:? no..

so for some time after the engine stops heat percolates upwards from the hot oil..

it continues to make the head, where the coolant sits stationary, heat up... the oil temp is very high..

so.. its easily able to make an unsealed coolant system reach near boiling point.. if its at atmospheric pressure,, which a coolant system with a leak... might be.. the coolant therefore heats up and expands still in the engine head...

and so it leaks more when the engine isnt running after one stops it... until the heat from the oil dissipates...

in a truly sealed system, with no leaks... that heat is absorbed by the stationary coolant in the head just the same.. but it cant get out via a leak.. so the heat and pressure is absorbed by the whole stationary coolant... and all one will see.. as i have, - is it takes a long time for the pressure in the system to drop .. mine took 50 minutes to go from 0.5 bar to 0.2 bar..

why.. ? because the coolant was still absorbing heat from the oil, that kept the pressure due to expansion.. at a very slowly decreasing level -- and as the heat went.. the expansion went with it.. and the pressure fell...

but.. as ive already said skellum... if that pressure is 0.5 bar... and the leak you have needs .5 bar to push any fluid out.. then that is what you see.. after you stop the engine...


once the engine cools .. the leak stops... its a pressure dependent hole you have somewhere..

it is as easily likely its leaking when the engine is running.. but its evaporating as fast as it leaks..

so you dont see it... or it only leaks at certain pressure... a pin pric hole ( miss spelt on purpose)

Last edited by Incony; 19-03-2017 at 20:46.
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post #25 of 30 (permalink) Old 20-03-2017, 09:28 Thread Starter
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Thanks for your insight Icony.
Whilst the engine is running the coolant continuously recirculates between hot engine and cool radiator. When the engine stops the circulation stops. With no further combustion there is no new heat being generated, but the engine and its oil will be holding a reservoir of heat, which can still raise the temperature of the water within the engine.
Hadn't thought of that at all- I'd been wondering how a cooling engine could force water out . . .
Next- Inspection cover off, run her hot, then get under and just wait.
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post #26 of 30 (permalink) Old 20-03-2017, 23:19
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i hope you find it... and its a hose failure.. its not that the oil gets any hotter.. its that it is a heat reservoir and can go much higher than stationary coolant can dissipate quickly... certainly above boiling point of water at 100 degrees C - thats only of real consequence if the pressure in the coolant cant reach 1 bar, and stop the coolant boiling... because there is a leak..

its the only reason i mention it.

how long does a chip pan of oil take to cool down... try that yourself with a thermometer.. get the pan of oil to say 110 degrees.. and see how long it takes to reach 98 degrees... and thats just an open pan of oil, not a sump and engine all that temperature when the engine stops.

that heat must go somewhere... and the closest thing above it... is the coolant in the head, surrounded by a hot engine bay.
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post #27 of 30 (permalink) Old 21-03-2017, 03:31
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looks like its either the beginnings of the water pump failing or gasket in the water pump area , brought on by a dodgy thermostate.
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post #28 of 30 (permalink) Old 21-03-2017, 06:30
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Originally Posted by Andy400 View Post
looks like its either the beginnings of the water pump failing or gasket in the water pump area , brought on by a dodgy thermostate.
You might turn out to be right, but I'm not sure what you have read in this thread to draw that very precise conclusion.

The F/TF water pump is on the offside, the OP states possibly seeing coolant on the floor on the nearside...


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post #29 of 30 (permalink) Old 21-03-2017, 18:59
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the floor doesnt look level and seems to be making the water drain over to one side , so allowing for water to run along the
bottom edge of the engine until it finds the first place to drip or run off? as the car would also be slightly off level.
Leaky TF_zpszwwdcav3.jpg Photo by maretta_03 | Photobucket

its all just a guess as like the rest of us we are not at the car and cant see exactly whats going on...
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post #30 of 30 (permalink) Old 27-03-2017, 08:34 Thread Starter
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Hi All.
Seems to be leaking at the PRT.
Ran her up to working temp yesterday, turned the engine off and waited.
A couple of minutes later I got a steady drip running off the PRT. I got the inspection cover off to check if it was coming from a hose higher up and running down, but didn't appear so.
Couldn't get far enough under the car to see exactly where it's coming from, but it's progress; either the hose connection top of the PRT, or some damage to the PRT body itself.
Thanks for all the suggestions.
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