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Discussion Starter #1
Seeing strange (to me) pattern of coolant loss, that could be head gasket but not 100% convinced so would appreciate feedback on these symptoms:
1) Car is generally fine around town and at low speeds, it can be left at fast idle for an hour and loses no fluid. There are no fluid leaks around radiator, hoses, heater or back of the engine. All hoses/pipes between radiator and header tank are clear of blockages. Have replaced the thermostat, and tried K-Seal, neither fixed problem but k-seal was useful diagnostic indicator as i could tell that only visible place coolant being lost externally was via header tank cap.
2) After motorway runs, low coolant light comes only after engine drops down to idle
3) With exhaust gas leak detection fluid kit on header tank, and engine constant revving at 2000 rpm, fluid in swan neck trap stays same colour, and no bubbles are passing through fluid so no pressure building. However, when the engine is then allowed to idle there is a big release of gas/pressure through the fluid which then changes colour to indicate combustion products present. This is consistent with coolant level dropping after motorway run.

If this were a classic combustion chamber to coolant gallery head gasket failure I would expect the pressure to build up and blow coolant out when the engine was at fast idle and/or at motorway speeds, but it doesn’t and there is no evidence of excess pressure building up inside the large radiator hoses. So i’m wondering if this issue could be caused by a leak elsewhere (such as the EGR cooling jacket) as it only ever seems to manifest when revs are dropped.

On one occasion i had driven 40 miles avoiding motorways, when i stopped coolant level was normal. Then as the engine cooled lots of glugging as gas from within engine escaped into header tank with corresponding drop in fluid level.

So it looks like the exhaust gas is accumulating somewhere in the block/head when at high speed, and only being allowed to escape when the water pump is idling and/or stopped.

Before I incur the expense of a strip down to skim head and replace gasket, I thought it worthwhile checking if others have seen identical symptoms, and if there is anything else that could cause them?

Thanks in advance for your help!
 

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2 litres generally do not suffer head gasket issues and as you rightly say if it was a gasket issue it would be far worse.

We had a 2 litre citroen C5 we bought it with a known gasket issue as guy had overheated it towing a caravan the size of a lorry i just removed the thermostat and my wife used it like that for 6 months she just topped it up daily :)

I think it may well be worth bypassing the EGR cooler with a bit of pipe just to eliminate that as the cause.
 

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Seems like air being trapped in the system. I would reverse flow the rad and the block first and remove the hoses to check to see if, internally, they are not collapsing. Also disconnect the heater matrix hoses and reverse flow that too.
 

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Discussion Starter #4
Thanks for the comments guys👍, a couple of questions
1) the thermostat housing gasket is part of the thermostat, so whats the best way to seal this if the thermostat is omitted?

2) regarding reverse flushing everything,
i have done some random flushing already when i was changing the thermostat (tried garden hose in top rad hose, hose in bottom rad hose, hose in header tank, hose in thermostat housing). The heater gets nice and hot, and once the thermostat is open the main rad gets hot all over with no cool spots.

is there a ‘flow diagram” (or such like) somewhere that shows the direction of coolant flow through these engines?
 

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Reason for removing thermostat is to stop engine getting hot when it IS faulty as the thermostat would not work correctly if engine is full of compression gasses
 
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