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BigJacko · 12th November 2013, 14:56

I agree, Kaiser - if there is significant pressure after 24 hours cooling-down, then that implies combustion-gases are in the coolant, and they've not found their way back out via the same way they came in (ie the engine).

However, I would also expect to see some 'mayonnaise' in the radiator cap - though this isn't a given, if there's no oil getting in to the coolant - but then again, a HGF where only exhaust gas but no oil is getting past? How realistic is that? (I have no direct experience with this on my R75 and I hope never to, so it's a genuine question! My gut tells me that oil and gas together is a far more likely outcome, rather than one or the other.)

Back to first principles, though - and I call 'overfill' as the first item to rule out. The OP said that upon opening the rad cap, some pressure and some coolant came out too. To my mind, that would indicate *major* pressure if the expansion tank had been filled to the correct level prior to all this. To be lifting the water-level to beyond the neck of the tank 24 hours later, seems, well..., improbable. I would expect the OP to be reporting all manner of gurgling and bubbles at the same time too, yet he doesn't.

Perhaps the OP can elaborate - where was the coolant filled to, in terms of level? Did you cover the fins inside the expansion tank? Have you seen the MIN/MAX markers inside the expansion tank (directly below the filler opening). Have you seen the second set of MIN/MAX marks on the nearside wall of the expansion tank exterior? You would be amazed how many garages don't have a clue about how to fill an R75 coolant tank.

If you don't know any of the above, just tell us what level the coolant shows, after you've opened the cap (after cooling down). If it's still way, way over the MIN/MAX marks, I'd get that straightened out before plumping for HGF as the cause. Conversely, if there really was a HGF, and thus some pressure had been introduced, I would expect (after cooling-down) that the coolant levels would return to (true) normal, possibly even lower than normal, once the gas had vented off after you opened the cap.

If the tank has been filled to brimming, there's not going to be enough room for normal operation and expansion - remember the water-pump at high revs is enough to raise the water level by an inch or two on its own, hence the seemingly low min/max level marker. There is also no overflow pipe on this system; it's a sealed, pressurised arrangement. You'd be hard-pushed to remove a rad-cap on an overfilled system without spilling some coolant, because the end of the cap will be well and truly immersed already (so some will drip out of it as you remove it), plus any 'normal' pressurisation will doubtless cause a bit of a spill-over, simply because the level was overly high and nearer the neck to begin with.

Do let us know the score - it may be a game-changer.

12th November 2013, 14:56	#11
BigJacko Avid contributor Rover 75 Tourer 2.0 V6 Club (2002) Join Date: Dec 2007 Location: Bath Posts: 236 Thanks: 47 Thanked 16 Times in 12 Posts	I agree, Kaiser - if there is significant pressure after 24 hours cooling-down, then that implies combustion-gases are in the coolant, and they've not found their way back out via the same way they came in (ie the engine). However, I would also expect to see some 'mayonnaise' in the radiator cap - though this isn't a given, if there's no oil getting in to the coolant - but then again, a HGF where only exhaust gas but no oil is getting past? How realistic is that? (I have no direct experience with this on my R75 and I hope never to, so it's a genuine question! My gut tells me that oil and gas together is a far more likely outcome, rather than one or the other.) Back to first principles, though - and I call 'overfill' as the first item to rule out. The OP said that upon opening the rad cap, some pressure and some coolant came out too. To my mind, that would indicate *major* pressure if the expansion tank had been filled to the correct level prior to all this. To be lifting the water-level to beyond the neck of the tank 24 hours later, seems, well..., improbable. I would expect the OP to be reporting all manner of gurgling and bubbles at the same time too, yet he doesn't. Perhaps the OP can elaborate - where was the coolant filled to, in terms of level? Did you cover the fins inside the expansion tank? Have you seen the MIN/MAX markers inside the expansion tank (directly below the filler opening). Have you seen the second set of MIN/MAX marks on the nearside wall of the expansion tank exterior? You would be amazed how many garages don't have a clue about how to fill an R75 coolant tank. If you don't know any of the above, just tell us what level the coolant shows, after you've opened the cap (after cooling down). If it's still way, way over the MIN/MAX marks, I'd get that straightened out before plumping for HGF as the cause. Conversely, if there really was a HGF, and thus some pressure had been introduced, I would expect (after cooling-down) that the coolant levels would return to (true) normal, possibly even lower than normal, once the gas had vented off after you opened the cap. If the tank has been filled to brimming, there's not going to be enough room for normal operation and expansion - remember the water-pump at high revs is enough to raise the water level by an inch or two on its own, hence the seemingly low min/max level marker. There is also no overflow pipe on this system; it's a sealed, pressurised arrangement. You'd be hard-pushed to remove a rad-cap on an overfilled system without spilling some coolant, because the end of the cap will be well and truly immersed already (so some will drip out of it as you remove it), plus any 'normal' pressurisation will doubtless cause a bit of a spill-over, simply because the level was overly high and nearer the neck to begin with. Do let us know the score - it may be a game-changer. __________________ [SIGPIC][/SIGPIC]