I recently decided to try the 6 rated NGK stock plug, instead of the 5. I live in the tropics and noticed that the Honda manual recommends colder heat range for hotter climes. Now the car overheats when hill climbing or at sustained highway speeds. Could it be because a colder plug transfers heat more quickly to the cylinder head it is overwhelming the cooling system?

 

Yes, a colder plug transfers heat better to the head. But if THAT causes it to overheat, your cooling system was marginal. Check the fins on the radiator for corrosion - if they're crumbling to the touch, get a new radiator. Look for any other issues with your cooling system.

Have you kept after the coolant (fresh antifreeze) to avoid buildup of corrosion inside the water-jacket?

 

Coolant is fresh (consistently kept that way) and the radiator is just about 3yrs. old (still LOOKS new). I do however agree that the cooling system appears marginal. On the highway, if I keep the speed just under 100kph, everything is ok. Once it touches 100 it begins to heat up. Hill climbing, at whatever speed, triggers overheating. The manual claims that a grade 6 plug is optimal for my ambient conditions but I did not have these issues with the grade 5 plugs.

Still, rather than just changing plugs, it does appear the cooling system requires servicing. I will give the fin cleaning suggestion a shot. BTW, the T-stat is missing, so internal blockage seems very likely. Is there a flush product that you can recommend?....something that won't aggravate the problem?

 

No thermostat? Try putting one in.
The resistance of the thermostat helps balance the coolant flow to the different places it has to go. Maybe you get too much flow to the block & not enough to the head? The head is where the temperature is measured.

And internal blockage is more likely in the narrower coolant channels. Keeping the coolant clean & fresh is good. When you open the thermostat housing, look whether there's any coating of scale inside. And the new radiator is good. If the outside of the radiator isn't filled with bugs or dust (or whatever?) then clean it up.