Overheating at motorway speeds/high revs for long periods of time

The subject pretty much says it all. Basically for whatever reason when I’m at high revs usually motorway driving for more than 20-30mins at a time all of my coolant is pushed back into the reservoir which is obviously causing the engine to overheat as there is very little coolant to run through the system I have done a full coolant flush not long since my mate seems to think it could be the head gasket seal and the combustion gases are pushing the coolant out of the radiator but that doesn’t make sense why it would only happen when motorway driving for longer periods of time as it is totally fine any other time and even fine if I only get on the motorway for 15min so I’m confused. Any suggestions before I get it block tested as I’m currently on holiday?

It sounds like your head gasket has failed. I would be testing for this, pressure test or chemical test for gasses kin the coolant.
Only saying as pushing the fluid into the bottle is a symptom and head gasket failure is a Jimny thing.
 

When the coolant gets hot it expands and the excess is pushed into the reservoir. Normally you only get a bit of coolant in the reservoir, but if it is filling it up and overflowing then it’s more than just expansion, it’s getting pressurised, the common culprit is a head gasket.

If you get the engine hot, open the bonnet and rev the engine, look for bubbles in the reservoir or do a pressure test.

Another common symptom is a spark plug, sometimes one will be washed clean whilst the others have the usual deposits on them.

In really bad cases you can sometimes see the steam in the exhaust,or white smoke.

Head gaskets are usually a fairly easy ob for your local garage.

Robin

It sounds more like a clogged rad to me.  Can you control the temperature by using the heater on full blast?  Does the level in the coolant reservoir return to normal overnight when the engine is stone cold?

Head gasket failure would show up as permanent loss of coolant, steam from the exhaust even in warm weather and with a hot engine, mayonnaise under the oil filler cap, and (normally) an odd smell from the oil.

Most cars don't get serviced according to the schedule after the warranty expires, and the "mileage OR time, whichever comes first" rule is often ignored.  So many Jimnys will be on their original factory fill of coolant.  That's why things block and corrode.
 

Well I thought that might be the problem to but after a very thorough coolant flush the problem still persisted

I don't think a flush really does much as the radiator passages are so small and, if they're already blocked, they don't get flushed.

Overheating only at high loads does suggest poor cooling rather than anything else.  As well as the rad itself, is the thermostat opening properly?  How hot is the bottom of the radiator when it overheats?  Is the fan belt slipping?

Speaking from experience you will find that when you have replaced every other part of the cooling system because you don't want to face the prospect of a failed head gasket that is what it will be. The m13 is not prone to overheating but when it does it will take the head gasket with it. It will also probably need a skim too. Been there, got the T-shirt.

They don't have a massive reputation for blown head gaskets - it's not a Rover K-series* - so it needs some careful diagnosis first.  There's another thread on here where it turned out to be a clogged rad (that one even went back to the main dealer for diagnosis, and thankfully they didn't leap at the chance to do a £2000 job!).  A few other people had had the same experience. 

*usually in the MGF, because the location of the engine hides a small coolant leak that they are prone to

 

Will any garage be able to do a proper diagnosis and how much would that cost roughly. As at this point I’m questioning whether it’s even worth the hassle of fixing.

It's always worth fixing.  Have you seen the price of Jimnys now?  Only severe rust can write them off.

A blocked /  faulty radiator is easy enough to eliminate. Go for a run and get up to temperature, stop and feel the radiator top to bottom (engine off). It should be a uniform temperature to the feel, any cold spots are a blockage or delamination of the cooling fins.
Unfortunately blown head gaskets on the Jimny is a reoccurring theme in temperature regulation problems, much like swivel oil leaks are the Kingpins. It's hard to say if the head gasket is the cause or the victim of overheating (chicken or egg), but as Lambert said most end up fixing the head gasket. So eliminate this first as it's high in the probability charts.
Any garage should be able to test, you can even get DIY chemical test kits online to check for exhaust gasses in the coolant.
Is it worth fixing ?
Depends how good the rest of the Jimny is and how much you want to keep it. It's not a complex job, again most garages should be able to provide this service and it's easy enough to do yourself if you enjoy working on vehicles. If you don't pay someone.
Have a feel of the radiator and good luck.

Here is a DIY test kit. Shop around, cheaper exist.
Test kit

Oh and it's not a 2000 pound job. I had the head gasket and clutch done at the same time with the engine removed and that was only about 850 pounds. But in the preceding 3 years that I spent nursing the car along thinking that the next step would fix it I probably spent most of the 850 on parts and labour trying to replace radiators,  water pumps, viscous fan units, thermostats and hoses and numerous flushes and bleeding attempts. And then I still had to do the gasket.  Now I had had that one from new and at the time had no intention of shifting it so it made sense to preserve with it so I saw the whole thing through,  most people would not be keeping it that long so would put a new radiator in deciding that should fix it and then sell it on without ever finding the full extent of the problem.  But believe me once they start having temperature control issues they will need a head gasket.