Overheating issues - all suggestions welcome!

Hi folks, looking for some wisdom to help me get to the bottom of a problem. Apologies in advance for the length of the post but i need to share some background to hopefully get the best advice ...

I have a 2007 Jimney 1.3 with the 82HP VVT engine which i have owned for a few years. The car is LHD version bought in Greece, it spends most of the year in Greece but I bring it back to the UK occasionally in winter. Its currently here.
The heater on the car has never worked, i guess it has always had a faulty thermostat but otherwise the car ran fine and I don't need heaters in Greece.
In September last year (in Greece) it developed a leak on the metal pipe connecting the water pump to the engine block. No OEM part was available so my mechanic in Greece had a part fabricated and fixed the problem. All seemed well, apart from the fact that the viscous fan seemed to kick in more regularly than it had before the leak - but that could have been my imagination. I continued using it in Greece without problem through September & October.
I then drove the car through Greece, Italy and into France. At which point it started running much hotter than i was comfortable with. It was losing coolant but with no external leak. I nursed it back to the UK.

At this point it was showing all of the classic signs of head gasket failure so i flushed and cleaned the cooling system and tried Steel Seal to see if it would cure the head gasket problem. It seemed to work, coolant level now stays the same, white smoke has stopped and the oil seems clean. I can drive the car in urban conditions at 30-40 mph without any problems. If i take onto a dual carriageway and get up to 60 mph, however, the temperature quickly climbs towards red and I have to back off the power to get the needle to drop. If i continue to push it, the car will overheat with a back pressure problem forcing coolant into the expansion tank. As the car cools down the coolant is not drawn back from the expansion tank into the cooling system.

The car is now at my local garage. My mechanic was initially convinced it would still be a head gasket problem but has done a chemical test and there is no evidence of combustion gasses in the coolant.
Before he sets off stripping everything down to try and identify the cause (probably at great cost) are there any common issues that we should be looking to rule in, or out, based on your collective & vast knowledge? I have read some of the previous threads on common causes of overheating but wanted to give the full story of my car in the hope that you can guide me in the right direction.
I've heard that Steel Seal can solve one problem and create others. I welcome your thoughts on this along with any useful pointers on what else may be the cause and how to narrow down the search for a solution.

Apologies again for 'chapter and verse' and thanks in advance for any suggestions.
 

Hi,

Unfortunately Steel Seal and all of the variants should be regarded as a temporary fix. When a Jimny over-heats you end up with a head gasket problem and the same the other way round, head gasket problem then over-heating.

Along with the head gasket, the head probably needs a small amount of skimming.

The most common fault is a leak from combustion chamber to coolant and then the coolant gets pressurised, but you can get coolant leaking in to the combustion chamber, in this instance your test kit might not see the problem. If this happening one plug will be washed clean.

The heater is likely to be another problem, definately not related to the thermostat, if the thermostat wasn't opening your Jimny would over heat in a few miles, or possibly before the end of the street in a hot country.

There are other issues which can cause over-heating, E.g. running very lean, so pull the spark plugs and have a look.

Robin

Thanks Robin, really appreciate you taking the time to respond and specifically your insight into why the chemical test may have come back negative. I'll investigate further.

Worth your mechanic looking at both the water pump & the radiator, to make sure it's pumping & to make sure it's not blocked, if it runs okay at lower speeds it could just be a flow issue.

FYI, for the heater not working it can be caused by a mineral build up inside of it.

I ended up replacing mine, 130 or so quid and too me two days, dashboard out jobby, well worth it tho.