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White Exec
Good quality antifreeze/coolant mix is good quality antifreeze/coolant - Right?

Well, I thought so until this year, when I had to drain and refill my 2.5TD's system several times, in order to sort out another problem. After an engine-out and new radiators session, my local workshop had refilled the system with a good quality readymix, OK down to -18C. I had some more of the same stuff at home, and later bought some additional mix elsewhere, which was also green, readymix, -18C, "for all types of engine" etc. etc.

So the XM acquired a mix of seemingly identical coolants. Fine - and many of us have done something similar for many years.

But after a week or two, the header tank was found to contain not clear green mix, but what can only be described as Thick Grey Snot. Dark grey, and the consistency of thick honey - only just pourable at 20degC !

I removed the header tank, and removed almost 2 litres of the grey goo from it. Worse, the goo was not water soluble, even hot water, and required dishwasher or bio detergent to shift it. It was not solvent-soluble either.

Fortunately, below the header tank, all was clear green coolant. So I replaced the plastic header, topped up . . . and . . . a few days later . . . another 2 litres of goo in the tank.

A search on the web ('coolant contamination') threw up masses of information, and some of it you could easily lose sleep over.

Basically, there are now two main types of coolant mix; both are ethylene glycol based. There is the traditional type, with a life of around 2 years, and there is the newer, more expensive, "long life" OAT (organic acid technology) type. Many car manufacturers fill their new vehicles with the latter, but don't label it as such. Colour is no guide whatsoever. Either type can be any colour; there is no standard. Few containers specify what they contain.

Mixing types can cause all sorts of problems, including lack of corrosion inhibition, failure to protect gaskets and seals, shortened life of the mix, and in some cases crystallization and the formation of gel or goo.

For a start, read this:

Another rule is NOT to use neat coolant (where it is sold as a concentrate), but always to dilute with a minimum of 25% water, preferably distilled/de-ionised. Citroen's own Dynagel 9103 is meant for dilution by 50%, and provides protection down to -35degC. That's good enough for most of us, even in Altnaharra.

Chris Gregory

Mixing types can cause all sorts of problems, including lack of corrosion inhibition, failure to protect gaskets and seals, shortened life of the mix, and in some cases crystallization and the formation of gel or goo.

So is this what happened in your case?
White Exec
Hi Jan,

What I got was the formation of thick, grey goo - which filled the coolant expansion/header tank. The goo apparently forms where the mix-of-types comes into contact with air. So, in a carefully-bled system, that's in the header tank. Remove the goo, and it reforms in the same place. Thank goodness it was there, and not deeper down in the system, nor, in my case, in the top of the radiator.

The other effects are some of those listed on various web sites, including Bluecol's.

A question I forgot to ask: Has anyone else seen this, or anything similar?

Hi Chris

Yes, seen it but did not recognise it.
You may well have solved one of my minor problems.
Car has been SORN for nearly 2 years.
Not been driven more than a few hundred yards every few months.
Just enough to polish the brake disks.
I noticed the grey goo in the header tank last spring.
No matter what I tried the stuff came back.
Decided the oil cooler or the head gasket was gone.
Put the grey colouring down to the oil being churned in the round header tank when the car is revved.
So stopped with the MOT prep work.
Before last winter I tested the coolant and decided to increase the concentration.
Drained a quarter and just topped up from the current anti-freeze stock.
No idea what the PO had it filled with.
Will have to have another look at it sometime when the weather improves.

White Exec
Hi John,

Sounds exactly the same.

My first reaction when the grey goo appeared was also head gasket (but this job had just been done), or oil-cooler (which was then jumped out-of-circuit). So neither of these, but an easy mistake.

Goo is grey, thick, immensely sticky, and resistant to being diluted or washed off with water (strong detergent needed). It looks nothing like the brownish oil-water foam/meringue/emulsion that oil-in-the-water produces. It also doesn't seem to have anything to do with the car being in regular use or laid up. It's just a chemical reaction between the two antifreeze types.

So, if you see it:
1. Remove the header tank and clean it out.
2. Check the goo is ONLY in the header tank; if it's visible below the header tank as well, then top of rad, and heater matrix (!) might need cleaning too. It seems that the goo only forms where coolant is in contact with air, though.
3. Flush whole system thoroughly, and refill with a trusted mix.
4. LABEL the rad top with what's in it, and use more of exactly the same.


Surely this 'grey goo' is oil leaking into the coolant system. I had copious quantities in the header tank and the only cure was the blanking off of the pipes and connections to the water/oil heat exchanger surrounding the oil filter. I have a replacement oil cooler to fit but as the car has been fine without it in circuit, I have yet to fit it. Nothing to do with anti-freeze mixes!
White Exec
Absolutely not. When my system was producing grey goo, there was no oil in the coolant. I've had oil in water (prior to replacing head gasket and removing oil cooler from circuit), and it looks quite different: black and thin and oily, and floating on top of the water in the expansion tank. The grey goo is just that - grey and the thickness of just-pourable honey.

So far as mixing types of antifreeze is concerned, my warning is simply based on reading many reports of accidental mixing on the web. Bluecol's site is a good place to start, but there are endless others reporting the problems.
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