looking at this thread and others of the same nature on different forums, the 1KZ-TE engine does suffer overheating problems causing head damage on all the various vehicle applications that Toyota fitted them.
all of our vehicles are the best part of twenty years old. on that i think we can all agree. ive heard of overheating issues on trucks which have done big milage and some which have done very little. this has been apparent in many different countries from some of the forum content i have read, indicating age is an issue. the whole problem hasnt be helped by the in accuracies which exsist in the trucks temprature gauge. this has been addressed and solved in some threads by the replacement of some very cheap resistors.
my thoughts are, and please believe me im no expert, that due to the age of our trucks , is it that due to their age, the coolant systems are giving up due to the collective failure of some of its components?
ive found that with no oil in the fan, a little wear in the water pump, thermostats almost twenty years old not opening enough/on time and radiators collecting over a decade of shit between the fins, engines can run a little warm. this, together with dodgy gauges and cheap/rubbish parts (chinese radiators) means alot of cracked heads?
im using question marks alot as i want this thread to be debated more as this is still a problem with this truck.
just a few thoughts. cheers.
I'm no expert but I don't think a little wear in both the thermostat or the water pump will have any marginal affect. Age relevance is correct any mainly linked to the radiator which doesn't cool as well as a newer unit. Over time the fins get damaged, less air flow through the rad, and dirt builds up in between the rad and AC rad. The Viscous fan does help a lot in cooling the engine down, especially in stop and go traffic. I've put 180,000 on my truck, and all I've replaced is the Radiator, nothing else in the cooling system other than the actual coolant and the truck keeps on going.
My "main" theory on why these engine's end up with cracked heads is due to excessive heat. Firstly, I believe having the feed at the bottom of the engine is a poor design as the top of the engine tends to get hotter than the bottom and you end up with a delay when the top needs to be cooled down and there's no cool water there yet - Problem 1. Secondly, these trucks put out high EGT standard which doesn't help cooling down a turbo'd engine. I've seen actual facts and temperature read out's of how much lower a after market down pipe/Exhaust system helps lower those EGT. An inter cooler helps massively in this area as well. Lastly, these trucks tend to run more on the leaner side from factory and from what I've read a leaner diesel engine burns hotter than a richer diesel engine, and thus excess heat for no real reason!
However even after writing all that, I still don't 100% agree with it. Why? Because when I bought my colorado 15/16 years ago it was couple years new, with about 30,000 easy miles on it. Ever since buying it I babied it, engine oil change every 3000 miles, kept the fuel system clean, filters changed regularly. Garaged all her life. Never towed. Yet one day driving on the motorway I noticed all of a sudden a slight rise in my temp and pulled over on to the hard shoulder. Bubbles coming out from the expansion tank..hmm puzzled. Let it cool down and drove home, temp ok. From this point on, it was using water, sipping it on my daily commutes to work and back but I still drove it with little knowledge of what was actually going on. Eventually it got worse, and you guessed it, I had a cracked head! 65,000 miles on the motor, no hard driving, I was puzzled. Must have been built on a friday? Until, few years down the line I joined the forum and realized I wasn't the only one! Anyway, for those of you wondering, I drove the car with the cracked head for about a year not knowing what damage it was actually doing. Eventually I had the head off, and realized the chambers were starting to corrode and pistons were rusting. Low millage Engine in from Japan and not a problem since. And I'll be honest with you, I've worked this engine a lot harder. Towing more than I should be towing, maxing out my speedometer, increased fueling and boost and she hasn't broken a sweat. I'll jump inside and drive half way across the world with her tomorrow!
So really, I'm a little lost. I know for a fact, high heat = cracked head. However I also know these heads can crack whenever they want to as well and I'm living proof. On the other hand I've seen loads of abused cruisers with the 1zkt/e engines going strong with a lot of miles on them. Same goes for the 2.4 which were also known for cracked heads.
looks like another for my to-do list 
https://www.youtube.com/watch?v=3ACx7GTCn7Y
But hay, you don't have to believe me. It's not like I've stripped every bolt off these engine multiple times and rebuilt them or anything, over the past 16 years of owning a cruiser! 
).
.
