Gen3 (2010) Jimny Overheating Saga

Hello!

I have a Gen3 Jimny which has been causing me no end of overheating issues.
It has a M18A swap (the 2nd one of these, the 1st succumbed to overheating and died) which probably doesn't help but there has to be more going on.

Monitoring the temps through bluetooth OBDII shows:
- Idling from cold? Stays around 84c.
- Idling while driving? If I don't rev the engine (~2-3k rpm) it'll shoot past 110-120c.
- City driving? 85-100c when moving.
- Highway driving? At 100kph eventually it rises above 100c and can't cool itself back down.

Turning the cabin heater on helps a bit, and running the AC (while on hot mode) to try and run the electric fan doesn't seem to do much. Obviously above 100c is boiling and so the overflow fills up, and steams off the rest requiring constant topping up.

I've replaced the radiator with a bigger 3-row, which did seem to help. I've moved the winch controller out of the way of the front grill (vertical bars) and also moved the licence plate out of the way to try and get more airflow. I've replaced the water pump. All of which helped, and I thought solved it but still no luck.

Apart from a couple more things to try, like removing the winch to get more underflow, and possibly looking at the viscous fan hub (seems fine to me though), and also possibly looking into the thermostat, is there anything else I can do?

Are there any upgrades? I tried to fit a electric fan to replace the viscous, but it fouls the front of the engine/have no clearance. I could remove the AC system but it gets bloody hot in Australia so ideally no. Can you upgrade the thermostat perhaps? 

Do these symptoms indicate a specific problem such as coolant flow, fan issues, air issues - is there any specific test I can do to narrow things down? I've flushed the fluid with rad-flush twice but happy to try again.

I'll attach any photos as required when I get a chance.

It won't be boiling at 100c as its under pressure. It will however probably have a bad head gasket and will be pushing combustion gasses into the water jacket displacing the coolant which then sublimated upon reaching atmospheric pressure. The initial over heat was probably because the old radiator was impaired which is why the new one helped but didn't cure the problem. It took me 3 years and every part of the cooling system to work out the head was done on my last one.

Lambert has worst-case experience, worth reading his other messages in the forum on the topic.

If it were me, I would test to confirm that the indicated temps are correct. Use an IR thermometer and/or plumb another sensor into the coolant circuit. Assuming the temps are confirmed, >110dec G means there is a problem at "idling while driving" condition, although I don't really know what you mean by that. The other operating modes' temps sound fine to me.

Do you experience coolant temps >110degC only on hot days, or all the time? I've heard that it can get rather warm in Oz sometimes, so if you are only hitting the highest coolant temps on the hottest days you might just need to go farther down the path of improving airflow. Ventilating the bonnet will do wonders.

On the other hand, it is certainly not unheard of for a Jimny to blow a head gasket and start running hot. There is a way to test for combustion byproducts in coolant. If you want to be sure you don't have the problem Lambert had, get that test done-- as I recall, it is a chemical one adds to the coolant that changes color in the presence of exhaust gases.

Edit: I just noticed you didn't mention flushing the block.  I assume you did that when you swopped the motor, but if not, it is worth doing.

I've had similar experiences to Lambert on 3 different occasions now and from my experience, if an M Series engine is overheating, it needs a new head gasket. The head gasket might not be the original cause and you may well have fixed the cause in the mean time, but once the engine has overheated the head gasket will be compromised and start leading combustion gases into the coolant.

I've ordered a combustion gas tester so we'll see how it goes, thank you all.

On the head gasket, is it worth doing yourself? I'm not much more than an amateur mechanic with only mid-level tools.

Just my opinion: having done a few head gaskets on OHC 4cyl engines, it is probably not something that I would recommend someone leap into unless they are very confident of their abilities. It sounds like maybe you are not feeling that confidence, so I would say find a pro tech you trust to do the job.

On the other hand, it is not rocket surgery. What it really needs is someone who is patient -- not a tool-throwing hothead -- and tidy.  Follow the manual slowly and carefully, and you can't go wrong.  And there are a lot of fellows here with specific experience to guide you. It is kind of a steep hill to climb, though, if you have never done it before.

Put it this way: your second head gasket job is pretty easy.