Overheating, but not overheating?

[TravellingEnt]

Had an experience on Monday that I can't explain...... a repeat of an experience 4 years ago.
We took our Petrol V6 to the south coast for a long weekend. 200 miles there absolutely fine. On Monday, after a 20mile run when we got back to base there was a bubbling from under the bonnet. Rather worried, I looked and the de-gassing tank was bubbling away and I could hear the same from behind the bulkhead eg engine drivers side. I checked and the cylinder head temperatures were normal (87 degrees), dashboard temp normal (11o'clck) and the low coolant alarm hadn't triggered. I left it for the night to cool down.
Next morning there was no water visible underneath, so no obvious leak. The LCA did trigger - the pick-ups were now dry as the water level was below the minimum mark.
Checked the engine bay and gave the big water pipe a good squeeze water moved around in the top tank, so I put a couple coffee mugs full of water into it to the max line(ok I know I shouldn't do this but.....)
Ran the engine for a while. All seemed ok. Drove a while - all ok. Drove home - all ok. The temp gauges fluctuated a little just as expected.

Previous owners had the water pump and belt replaced in 2014 and the pipes look like new ones too.

I've ordered a UV test kit, but as we drove home without issue, I suspect that'll not show anything. I'm stumped where to look next as I think whatever happened, really should be fixed permanently. Would love some thoughts and pointers where to concentrate my efforts.

[g8dhe]

A quite common event is pin-hole leaks, where a pipe develops a very small hole that opens when the system is up to temperature and pressurised, this then emits a fine stream/jet of coolant, then when the system cools it sucks in air to replace the lost coolant this then gets trapped around the pin-hole or in the engine, so the LCA doesn't always detect this as coolant has been replaced with air so level is the same. After a few cycles of this there is enough of an air-lock to cause over heating, and then the entire cycle starts again The UV dye may well detect this as it sprays nearby locations and it dries off, another way is to watch when dark with the a thin beam of light for the spray/steam from the leak but this needs to be done at the full working temperature.

[TravellingEnt]

Thanks for the info Geoff. I'll report back after the uv is delivered and i've done the test.
I've done some research and came across 'waterless coolant' http://www.evanscoolants.co.uk/Coolants ... c_cool_180. Although not cheap, if it lasts for 20 years (longer than me I think) and doesn't pressurise the system, it seems interesting. Have you or anyone any knowledge of it?

I'm pretty sure I'll be draining, fixing something, flushing and refilling the bongo. With the V6 is the special technique required to expel air, or is it just the diesels that need it? I can't quite decide from the various posts.

[Bob]

The V6 needs to be bled.

[g8dhe]

Yes "waterless" coolant is mentioned in the forum, try a search. Problem comes if you have a problem out in the field ... what are you going to fill up with ? The only thing is water, in which case you back to an expensive cleanout, water removal process and an expensive refill. I think only one person went for it and he had it done cheaply for the review he gave.....

[TravellingEnt]

Well I've done the UV test and as far as I can see there're no leaks in the system. Have done some reasonable journeys and then run the engine until the cylinder head is reading 100degrees, hot enough to pressurise the system and I assume to open the thermostat, but can't be sure.
Notwithstanding my failure to find the cause of the bubbling, I'm going to have a thorough flush and refill done and probably new thermostat so that I know the system is circulating without obstruction and has good coolant for the winter.
I'd like to have got to the cause of the mystery, but some things are not to be...........