Jimny specialist

Because of experience. I have in my time replaced the entire cooling system piecemeal starting with the radiator cap and ending with the head gasket. By all means question the various parts of the system but when an m13a starts over heating, which they don't generally as an engine do, then eventually the head with have to come off. You can nurse them along for years with it but eventually you will have to intervene.

With any suggestion as to the cause of a fault you can never be 100% sure, but experience will indicate the most likely cause and you eliminate this first. 
In my head I've got these as the top causes of issues, just based on experience and listening to others on this forum-

Overheating / coolant bottle overfill when hot    - Head Gasket failing
Unable to select 4X4                                          - Wire on Transfer case (PB Select) or Vacuum pipe rot
Steering Wobble                                                 - Kingpin Bearings
Gearbox issue selecting first                               - The 2009 gearbox bearing issue
Cuts out when hot but then starts again              - Crank sensor
My fan belt keeps snapping                                - Crank pully is failing
Brown colour underneath                                    - They fizz in contact with water
Anything else                                                      - Previous owner mods or lack of maintenance 

So its wise to eliminate the known problems first, and that's just based on experience.

Hang on, aren't these things supposed to be reliable?  That's a long list,  anyone want to add any more ? 
 

We don't have enough information. How many miles has it covered? Does it tow a lot? How often did it overheat both before and after the rad change? And as to coolant getting into a valve, Pardon!
Changing the rad. cap is the first step as it is cheap and quick. Then you can remove and check the thermostat. Two simple and quick checks that you should do anyway even if you finish up removing the head.

Roger

Change the thermostat on an m13a? Quick and easy? Only if you want to take the inlet manifold off. Which is much easier to type than do for the record.

This doesn't make any difference. First you have overheating and this can cause the head gasket to blow. A properly built engine does not blow a head gasket without reason and overheating is the reason and one item that must be checked is the thermostat. If it requires the manifold removing then so be it. Either way it must be checked/replaced before a satisfactory repair can be achieved. In much the same way that you change gaskets that have been disturbed or head bolts that are one use only, thermostats must be tested and approved for further use. The fact that this item is difficult to get at makes it even more imperative that the correct course of action is adhered to.

Many motorists are fair game for unscrupulous garages that change things for no good reason and come up with farcical comments about what they found, "coolant getting into a valve" is one such.

I asked questions earlier and will await the answers.

Roger

My other car (not a Jimny) boiled it's coolant last summer on a really hot day in standing traffic. It turned out to be the expansion bottle cap (they don't have a rad cap), basically it wasn't sealing when the engine was hot and under pressure.

The local garage who service it thought it could have a cracked cylinder head as the water pump had just gone, but the plugs looked normal, no smoke in the exhaust, no oil in the water, just boiled the water.

It turns out with the rad cap leaking it wasn't holding pressure so could boil at 100C and had also sucked air in to the water system. A new cap was all it took and several attempts at 'burping' the coolant to get all the air out.

Next time it happens check around the rad cap, hoses etc for mosture, it will be fairly obvious if it has a leak. Check the oil for water, the water for oil and remove the plugs, one of those will be wrong if it is a head gasket.

Running very lean can also cause over-heating, check the plugs for this.