Pressure In Coolant System

[matt.leese3]

Hi all,

I have recently brought a Ford Focus 1.8TDCi Duratorq 2008. When I came to check the coolant level in the system (cold engine) there was no coolant visible in the expansion tank. When removing the expansion tank cap, the system lets out pressure and the coolant floods back into the expansion bottle through the large hose located at the bottom of the tank. Surely when the engine is cold there should be no pressure in the coolant system?

Thanks in advance!

[artscot79]

Its a closed system so theres always pressure in the system even when cold just less than when its hot simple car basics they really should teach the cooling system is a closed loop and is pressurised the pressure raises the boiling point of the coolant without it the coolant would boil away this is why in the manual it will tell you to use a rag and slowly release the pressure http://auto.howstuffworks.com/cooling-system7.h

[Guest]

I have a 1.6 petrol and mine does the same, well not so much but it will drop to below the minimum until the cap is removed. If you Google it, as I did many seem to do it.

There is a really mixed input, but the most common reply seems to be, start engine and if it goes to the max don't worry!

a little hard to check. If you put more in it will just push it out when it gets up to temp.

[mjt]

Its a closed system so theres always pressure in the system even when cold

I wouldn't totally agree with that statement Arthur. Let's start with a cold system and you've just fitted the cap. The engine heats up and the pressure rises as you described. Then the engine cools down and the pressure drops. If it cools to the initial temperature the pressure should drop right back to the initial value, i.e. ambient, that existed when the cap was fitted. If there's still residual pressure that would imply a leak of combustion gas into the cooling system.

Another point is that if the coolant is disappearing from the tank when under pressure that suggests there must be an air space somewhere else in the system otherwise where does it go? Water isn't compressible. This implies that not all the air has been purged from the system during filling. The normal scenario is that the level rises in the tank when the engine is hot, as the coolant expands, then drops back to the MAX line when the engine cools down.

Incidentally the modern systems don't work quite as described in the link you posted as the pressure cap is now fitted to the overflow tank. The idea of a separate tank was, and is, that the coolant in it stays cool to prevent loss by evaporation.

[artscot79]

The cooling system is always pressurised after initial use excess pressure is vented but it will always have higher than atmospheric pressure even when cold theres always residual pressure as long as the cap isn't removed sorry but the purpose of the expansion tank its initial purpose is to allow expansion of the coolant under temperature and expansion its not cold its always just below boiling evaporation doesn't occur as that's the point of the pressure cap which increases the pressure up to 15psi preventing coolant from boiling

[matt.leese3]

Update: I have given the cooling system a dose of radweld as a precaution, the car appears to be loosing coolant and gaining pressure in the system. I have 'burped' the cooling system by jacking the car up at the front and running it with the cap off. There appears to be no avail.

[mjt]

The cooling system is always pressurised after initial use excess pressure is vented but it will always have higher than atmospheric pressure even when cold theres always residual pressure as long as the cap isn't removed sorry but the purpose of the expansion tank its initial purpose is to allow expansion of the coolant under temperature and expansion its not cold its always just below boiling evaporation doesn't occur as that's the point of the pressure cap which increases the pressure up to 15psi preventing coolant from boiling

Sorry, can't agree. Evaporation can and does occur without the water boiling, how do you think puddles dry up? The water evaporates to form water vapour in the air. Hot air can contain more vapour than cold. When the pressure is vented some of this vapour will be lost. As I understood it the idea of a remote tank was that it allowed for expansion but, because the coolant did not circulate through it, it remained much colder than that in the radiator, so the air in it contained less vapour, so less was lost when pressure was vented. Maybe modern systems don't work this way but I'm sure when expansion tanks first appeared that was the rationale.

[matt.leese3]

Update: Jacked up the front end and 'burped' the system again, as the engine got up to temp the coolant bottle started oozing white foam (see pic). It continues to do this for the 30 mins it was running for. The white foam appeared to be coming from the top right hose entering the expansion tank.

After burping, I put the cap back on and within a minute or two the expansion tank emptied again. No sign of leaks. Still no mayo on oil filler cap.

[mjt]

That doesn't sound good. White foam usually means oil mixed with the coolant. It's beginning to look suspiciously like a blown head gasket.

[jeebowhite]

that is possible, although the foam for the head gasket is ordinarily in the oil cap, so I would check that to be on the safe side.

It sounds more like the water in the coolant has seperated from the antifreeze and is boiling in the system, and is bubbling out. However, when the system is under pressure, you will find the bottle "drains", as the level will reduce and after cooling will rise again. This is because the expansion tank heats up and puts pressure on the fluid, forcing it further into the pipes, and into the radiator.then, as the system cools, the pressure releases, the liquid flows with equilibrium and settles back.

[matt.leese3]

That doesn't sound good. White foam usually means oil mixed with the coolant. It's beginning to look suspiciously like a blown head gasket.

This was my initial fear, I know the mayo substance doesn't appear quickly too. This would explain why the expansion bottle is a bit dirty looking on the inside I guess?

It would also explain why, although appearing to loose coolant, it is not pouring it all over the street.

I'm not sure what the next step should be, either:

• Try an oil additive to fix a gasket leak
• Take it to a garage and let them diagnose further?

Please forgive my poor knowledge on this Is there anything else I could try in order to get a better idea?

Many thanks.

[matt.leese3]

that is possible, although the foam for the head gasket is ordinarily in the oil cap, so I would check that to be on the safe side.

It sounds more like the water in the coolant has seperated from the antifreeze and is boiling in the system, and is bubbling out. However, when the system is under pressure, you will find the bottle "drains", as the level will reduce and after cooling will rise again. This is because the expansion tank heats up and puts pressure on the fluid, forcing it further into the pipes, and into the radiator.then, as the system cools, the pressure releases, the liquid flows with equilibrium and settles back.

I cannot see anything suspicious when looking at/around the oil filler cap

When the engine is cool (left for 24hrs+) no coolant appears to be in the expansion tank. But on removal of the expansion tank cap the system lets out pressure and coolant floods back in through the big bottom hose.

[jeebowhite]

that does sound about right, depending on where you filled it too before you put the cap on...

[matt.leese3]

that does sound about right, depending on where you filled it too before you put the cap on...

It never rises to the same level as it was after topping up the system initially which is worrying. After a moderate journey (70mi) then left to cool, it will probably be around 2cm lower in the expansion tank.

Thank you for your help on this James

[artscot79]

Try replacing the cap if its not venting excess pressure it will over pressurize and hold it over pressurized instead of slowly venting it out the cap should be changed every few years

[matt.leese3]

Try replacing the cap if its not venting excess pressure it will over pressurize and hold it over pressurized instead of slowly venting it out the cap should be changed every few years

Thanks Arthur, for the sake of £10 on eBay its worth doing!

[jeebowhite]

agreed with the cap replacement. I am not surprised though, as the guys said, a closed system will still hold some pressure, even if it has dropped down by 2cm thats fine.

I always top up to the max line, and leave it there, I have had a leak recently so done it a lot, and recently did it after the thermostat housing was replaced. Now its fine, and maintaining pressure nicely.

[matt.leese3]

Cap ordered, I will give that a try and continue to check the coolant. Watch this space!

Thank you for your help guys, it is much appreciated!

[artscot79]

]When

the engine is cold the pressure in the cooling system is equivalent to

atmospheric pressure. As the engine warms up the coolant circulates the

engine via a two-stage thermostat. A two-stage thermostat allows coolant

to actually circulate rather than remain still. This is to prevent

hotspots forming and uneven cooling. The coolant warms up with the

engine and in doing so begins to expand in warm up stage the hot coolant flows directly through the exdpansion tank and through the bottom expansion hose into the pump and round the engine aiding in faster warm up times. The pressure therefore begins

to rise. Eventually the engine gets too warm and the coolant can no

longer have a cooling effect, and so the thermostat opens allowing

coolant to pass through the radiator dissipating its heat into the air

which passes through it excess or expanded coolant flows minto the expansion tank. The engine now begins to cool again and the

pressure falls. If the engine gets too cool the thermostat closes and

the process begins again. The pressure in the cooling system is directly

proportional to the temperature. The higher the temperature the higher

the pressure and vice-versa. If the engine gets too hot the cooling fan

cuts in to generate airflow through the radiator. The pressure cap acts

as an overall safety valve in the event of the pressure getting too

high. Most caps release at about 15psi. The cap has a second valve, the

purpose of which is to allow air into the cooling system as the engine

cools down and the coolant contracts. If this wasn't the case the

cooling system would be a partial vacuum after the car had been stood

overnight which sounds like youre issue

[iantt]

I would get a test done on the cooling system for combustion gasses. Decent garage will have the kit to test, only takes 5 mins to test

[james1089]

I would replace thermostat housing and gasket also check the narrow pipe from thermostat housing to header tank is not blocked. On cooling down air is drawn in from the leak in the housing which is then trapped in the middle of the cooling system eventually building to the point where it's forced out the cap.The cap is where the air should be drawn in on cooling down not the leaking housing that's why it's over pressured even when cold. (Hopefully) Please not radweld If you do fix it get flushing, HG is rare on this engine and everytime it's changed it doesn't seem to fix it along with water pump so I would explore all other options first.

[matt.leese3]

Update: I have changed the cap and thermostat, the three way hose connected by a plastic T-piece that connects between the expansion bottle, radiator and engine had a bulge and the beading was exposed. The hose instead of being brittle as I would have expected was like a putty on the inside which I suspect was turning the coolant black. I cut off around 2 inches of the hose and refitted (a temporary fix whilst the replacement is being delivered). The hose now appears watertight and undamaged, and after a couple of hundred miles the coolant is still pink/red as it came out of the bottle new. However, after a long run I am still having to top up the coolant.

[jeebowhite]

Might be better once you replace the hose fully.

Sent from my GT-I9195 using Tapatalk

[matt.leese3]

Update: A photo of the expansion tank cap and tank itself, the small top hose on the bottom right of the pic is blowing bubbles through with the returned water which I am assuming shouldn't be happening and should just be a steady stream of coolant. I cannot see any more hose issues than the one that I intend to replace, so I will run the car today with the heater turned fully to cold as I assume this would isolate the heater matrix branch of the cooling system and see if that changes anything. If this combined with a changed hose doesn't appear to fix the issue, I will assume the worst and take it to a garage for a cooling system pressure check/combustion gas check. The color of the expansion bottle surely seems to suggest this may be an issue.

[artscot79]

youve either got a blown head gasket or the oil cooler is leaking into the coolant replacing hoses wont sort anything youll need the problem fixed then replace all the coolant hoses or remove and clean them all that scum is oil and coolant mixing its a diesel so its common many garages misdiagnose it as a cylinder head the oil cooler gasket leaks mixing with coolant