M16a headgasket

I had a garage do my last one as it needed a clutch too so I haven't had one apart to offer direct experience. From the account of the garage it was a fairly simple job.

'The rad level is low everytime when checked and its always exactly the amount that is in the expansion bottle but hasn't returned...'

In comparison to the radiator the expansion tank doesn't hold much water so if the loss from the radiator is the same as the excess in the expansion tank it suggests you aren't loosing any water, it is just getting re-positioned. I wouldn't have thought that re-positioning of the water from rad to expansion bottle would cause an over-heat.

However, you also mentioned water sloshing in the heater matrix, which does suggest you are loosing water somewhere.

It could be a head gasket, pressurising the water system, do you see any bubbles in the coolant expansion tank when running (hot)?

What do the plugs look like, any hint of a head gasket failure?
Or, the oil, any hint there?

You seem to have checked all of the pipes etc, but what about a heater matrix fault, you might not notice a leak in there?

Usually when the heater matrix dies it mists up the windshield big time and won't clear. That's what my last jimny did and my old polo did it too.

The remaining quantity of coolant in the rad and the expansion bottle indicates there is no leak anywhere else.
I've have just had a good rummage under the dash and there is no sign of a leak there...

The coolant normally heats up, pressurises the system, opens the rad cap and expands into the expansion bottle.
My theory is if the head gasket failing is pressurising the system, it opens the rad cap and pushes out coolant. When you turn the engine off it immediately closes the rad cap as the pressure goes, therefor not allowing the water to return into the rad as it cools..
Might be wrong but, having replaced most of the cooling system I can't think what else it could be... as I said I'm happy to do it, just need to order the right bits!

This is exactly what mine was doing.

wrote:

The rad level is low everytime when checked and its always exactly the amount that is in the expansion bottle but hasn't returned...
 

See that line there?  If that is the only symptom on which your diagnoses is based, then you're quite likely to have the same symptoms on reassembly.  The chances of that being a leaking head gasket is slim to non existent.  If you have enough combustion pressure leaking into the cooling system to force coolant out of the pressure relief valve, it will be enough to fill and overflow the reservoir

For some reason your cooling system is unable to develop a vacuum as it it cools - most likely a leak between the neck of the radiator and the overflow reservoir (and yes, that includes a cap that doesn't seal properly)  I've had this issue and traced it to a crack in the overflow reservoir cap - when the coolant in the system expanded it would flow over to the reservoir but when it cooled, instead of the vacuum lifting the coolant out of the reservoir, it would suck air in through the crack.

The radiator cap is not as simple as you might think - it has three separate points at which it can leak - the pressure relief valve that opens to allow flow outwards is the first, the return valve that allows flow inwards is the second, and the third is where the cap seals to the neck of the radiator.