Ok so this afternoon I bled the system again - twice.
First time I did cold, working my way from the lowest bleed point to the highest, the lower three dribbled immediately with no air, but by the time I got to the highest point (heater matrix elbow) nothing came out, and on checking the expansion vessel I realised the level was right down near the bottom. Whether that was due to losses through the previous 3 bleed valves (I did let a reasonable amount out, especially through the radiator bleed valve, that flowed quickly before I could get it closed again) or whether the level was low after boiling some coolant out yesterday (and then contracting when cooling over night) I'm not sure. Or maybe releasing some trapped air yesterday caused the level to drop.
So I topped it up again and it started running out the matrix elbow bleed point, and naturally I dropped the cap (after about 20 failed attempts to start the cap on its thread) and had to go searching under the car for it in the damp and wet.
Fortunately it was found and finally went on.
I then went for a drive to get it nice and hot with no problems noticed, and then came home to bleed while hot, this time the level was fine. (so maybe the level was a bit low to begin with the first time)
I gave the fan low speed a good check over to make sure that it was working, which it was, and I was able to let it idle until the fans came on to slow speed with the cap off without any bubbling whatsover - the coolant was calm and flat with just a little bit of vapour rising off it, as it should be. I then went for another drive to try to provoke it into playing up again - which it wouldn't.
white exec wrote:That all sounds like an air-lock, probably in the head, that would cause localised boiling, just a tiny bit of which can produce enough steam to create pressure and eject coolant when the cap is released.
Simon, I did say when you were asking about adding Forte, that you should bleed the system (and properly). It could always be something else, of course, but part draining a Citroen cooling system and not bleeding it thoroughly is not the best.
With only one litre removed I thought I would have got away with it, but apparently not! If I had to guess I would say an air-lock started in the heater matrix pipe and stayed there for a few days then for no apparent reason got lodged in the head.
Also, a genuine 90-95C is really nothing to be worried about. One of the 2.5 thermostats doesn't open until 88C, and fully open until 100C ! 70-80C sounds much more of a problem - positively luke-warm.
Not really - the V6 dashboard thermostat is known to read 10-20 degrees lower than "actual" temperature due to its sensor location, which effectively measures the return flow from the radiator. So if the thermostat is open and you are moving at speed or the fans are on the reading will routinely drop down to 70-80 even when the real temperature measured by a Lexia is about 93 degrees, because it is measuring the cooled water coming back from the radiator. This is because the sensor that the ECU uses is located near the thermostat housing and gets a true reading.
I've checked via Lexia recently and the thermostat starts to open at about 88 degrees, low fans come on at 93 and high fans at 97 degrees, so there is nothing wrong with the thermostat or fan activation unless it has given up the ghost in the last few weeks.
It's not the absolute temperature reading of 95 that worried me, it's the fact that it was completely out of character (it has never gone that high before) and that I know the dashboard gauge always reads lower than "true", so a reading of 95 means actual temperature was probably 105-115, and if it boiled a 50/50 mix of coolant with the cap released it must have been at least 108.
So I can only conclude that it was an air-lock in the system that took 4 days to find its way to somewhere it could do some harm, or possibly there is an intermittent fault with the low fan speed (which would also fit the symptoms) but checking the fans fairly carefully they seem to be working OK on both low and high, so I don't think it's that, especially if it was a coincidence!
The moral of the story though, is on this engine when checking the opening temperature of the thermostat and the cut in and cut out temperature of the low and high fan speeds, it should be compared with the engine temperature reading taken from a Lexia, not from the gauge on the dash.
I really don't understand why this has started after Forte was added - after 2 1/2 years of absolutely no problems with overheating. At least I know one thing it is not - the fans. The fact that they were blasting on high while the radiator was stone cold proves that, I also double checked the cut in and cut out temperature of the low fan speed afterwards (93 and 90 respectively) and it was absolutely fine.