Sporadic running hot, please help!!!

Greetings all,

I am new here and in dire need of help/guidance...

As for background I bought a 2002 Jimny(JB43, DOHC 16v 1328cc) used in 2016 in Greece. At purchase it had 115,000 km. I have maintained it religiously, since purchase and have accumulated a total 270,000 kms as of this writing. A year or so ago, the fan clutch failed, it did not overheat, I had it replaced, all was well. Last summer it began running hot again, diagnosis revealed a 90% clogged heater core. I flushed it successfully. All was well. At this time, due to time/mileage, I replaced the radiator, upper and lower hoses, thermostat and radiator cap. Flushed the cooling system and replenished to capacity with G12 blue coolant. The cooling system was bled thoroughly, the vehicle operated well overall. The water-pump remains original and appears to be functioning as I have sufficient flow of coolant. 

The cold weather is behind us, it operated well all winter until recently.... I noticed the temperature gauge climb at the slightest incline. The overflow tank filled, there was no bubbling or severe overheat. So I began the diagnosis all over again. I performed a coolant test with the blue fluid, there are no exhaust gases present in the cooling system as the fluid remained blue. I replaced the thermostat with oem, again, tested the old one with boiling water and it appeared functional. The fan clutch locks up thoroughly and does not free play. There is sufficient hot air from the heater core. 

The running hot condition persists, I have a digital thermometer and took measurement at the various cooling system hoses. The upper radiator hose was @ 78C, the lower @34C. Both heater core hoses were @ 82C, which at least to me, shows sufficient coolant flow through heater core. The radiator cap, replaced again with new oem works as it allows coolant overflow to and from the reservoir. The temperature gauge points to just below the H, I can hear the condensor fan come on, AC is OFF as a preventative measure. I started the car cold at home, allowed to reach operating temperature and then held the throttle at 1500 rpm for 1 hour. There was no drama, all worked well. I then allowed it to run another hour at idle, nothing happened. I did this as I was told that the head gasket is breached between the fire ring and coolant passage and/or the cylinder head is cracked. If this were the case, why then did the engine not exhibit any running hot during the testing in the driveway?

This morning with an ambient temperature of 20C, I climbed a small incline the temperature gauge rose and the condensor fan kicked in. I immediately pulled over and observed the overflow tank to be almost full, there was no boiling. I turned around and headed back down the hill in third gear, the gauge dropped to just below the half mark and remained. There was consistent heat from the heater....

At this point, I am completely at my wits end. The car is my daily driver, it is 100% stock, off road use is limited to dirt roads with most driving in town. THANKS IN ADVANCE FOR ANY GUIDANCE as I must cure the running hot problem ASAP!

Welcome to the forum. You are going through the exact same set of symptoms as my last jimny. Eventually I replaced the head gasket and was found to have a very slight warp. Unfortunately. So yeah I think it sounds like time for yours.

You say the expansion tank fills up- does the level go back down to correct when cold?

Is there any residual pressure in the radiator if you drive the car then let it cool down completely without touching anything? (Listen for a slight hiss when you undo the radiator cap, also the level will be down in the radiator, when cold it should be completely full right to the top with no air space.)

If the answers are no and yes in order then you have a leaking head gasket.

(on the other hand, if the rubber hoses are flattened when cold and there is a partial vacuum in the radiator the cap is faulty)

I was suspicious of a head gasket, but fought to avoid the inevitable.

To answer your questions... the overflow tank does not return to level, there is a hissing when opening the radiator cap.

Yesterday, I pulled the cap/hose from the overflow, inserted it into a plastic bottle filled with water and started the motor cold. Within the first few moments I observed bubbles, once the thermostat opened additional bubbles. I did this as I do not have access to a pressure tester or blue fluid which turns color for checking Co2 in the cooling system.

As for background I am an American living in Greece for professional reasons and do not have my relationship with shops here that I had back home. So I will begin my research for a "trustworthy" shop.

A few questions for those who have gone down this road with the Jimny.... and overall historical references from those herein...

a) Is the cylinder head prone to warping or cracks? I ask as the motor is healthy and strong, no oil consumption, runs very smooth. Although it ran hot a few times, I did not allow any severe overheating or extended driving.

b) What head-gasket was used, OEM or aftermarket?

c) Was the head overhauled or reinstalled after checking and cleaning? My experience has been to simply clean the head surface and reinstall in most cases.

Thanks much for your replies and guidance!

Considering the mileage, you've done well for the head gasket to last this long. They fail in a very unconventional way on these engines so I can relate to your frustrations trying to pin-point it.
I don't know of any M series heads cracking. That was an issue on the earlier engines but not these.
On the few I've done I've had the head tested and skimmed as a precaution.
I've used cheap eBay head gasket kits and not had any problems.
At that mileage I'd definitely recommend replacing the timing chain while you have it off.

Agree with above comments. The m series is really rather good and reliable right up to the point it's not which is usually fixable relatively quickly and cheaply and then it's back to being reliable again. They can be a bit fussy about after market sensors but that's not unusual given what passes for quality control these days.