I've noticed that my coolant level was routinely just below the cold fill level on the expansion tank but each time I would remove the cap to top it up built up pressure within the tank would release and the coolant would come back to where it was supposed to be.

I know that modern coolant systems run at a pressure above atmospheric to increase the boiling point of water but for it to keep that pressure when cold seems odd.

Anyway today it was a bit more odd as after a 200 mile run in the car over the weekend I started it up this morning to a coolant low warning so off with the engine, popped the bonnet, removed the cap and sure enough a surge of pressure released and the coolant came up. Not as far though this time and I had to put in about 500ml at a guess of some 50/50 mix I make and keep for these times.

I have done some research and typically there are three main culprits to this. Firstly the coolant cap not releasing pressure correctly so I will order a replacement to rule that out.

Secondly a pocket of air trapped somewhere in the system that compresses under system pressure, draining coolant from the tank and then when the pressure is released the air decompresses and expands pushing the coolant back to the tank.

Lastly, and what I really don't want to consider... head gaskets. It is known for combustion gases to leak past the head gasket and pressurise the cooling system without evidence of oil in the coolant, or coolant in the oil (neither of which present). A CO2 sniffer test of the coolant expansion tank would help confirm this but it could only be happening under certain variables (temperature, load...)

Am I worrying too much? Any advice? please help 2010 Range Rover Autobiography 5.0 SC
2005 Lotus Elise 111R

I'm left feeling somewhat sombre this evening unfortunately regarding my ongoing coolant issue.

After replacing the expansion tank cap, using a coolant system pressure tester and finally a CO2 sniffer test it has sadly pointed at one large and costly issue to fix... the head gasket(s).

The replacement cap didn't make a difference. The pressure tester showed no loss of pressure when pressurised to 15psi and left for upwards of half an hour on a cold engine. So finally the last test of a CO2 gas sniffer showed quite quickly the presence of CO2 leaking out of the expansion tank with the engine warm...

This all points to a failed head gasket which could either have been because of a manufacturing defect that has developed or more likely a overheat that occurred at some point in the cars history.

Am I correct in assuming that there isn't another system on the car that would introduce combustion gases into the coolant? I know that on some diesel engines the exhaust can be used to heat up the coolant to more quickly bring the engine up to temperature but I don't think this is employed on petrol engines. 2010 Range Rover Autobiography 5.0 SC
2005 Lotus Elise 111R

Evening Martin,

That's some really, really bad luck. Your car sounded sweet as a nut when you were here with it a few weeks ago!

On modern diesels I believe EGR coolers can put exhaust gases into the cooling system but there's nothing like that on the 5.0

David. 2002 4.6 Vogue SE - Alveston Red with Lightstone Leather
2012 Range Rover 5.0 SC Autobiography in Indus Silver with Jet/Ivory Interior
2012 Range Rover 4.4 TDV8 Autobiography in Baltic Blue with Sand Interior
2013 Range Rover Sport SDV6 HSE Black Edition in Santorini Black with Ebony Interior

Hey David, it certainly is... really unfortunate...

I got around to fitting the deployable side steps last weekend after refurbishing them so there was that at least to be happy about, really delighted with them by the way  2010 Range Rover Autobiography 5.0 SC
2005 Lotus Elise 111R