What makes early toyota pickup truck engines so reliable compared with American competitors?

Only the 4 cylinder Toyota trucks were reliable because of a well designed and possibly over engineered basic engines. The V6 pickups had their own set of problems.

And do remember this: the engines are reliable and that's about it. You'll still have to replace small parts like water pumps, pulleys etc. but the engines will last forever as long as you TAKE CARE of them. They do hold up to abuse well but run any engine with no/bad oil and you will shorten/ruin it's lifespan. There isn't much to go wrong with on the 4 cylinder Toyota trucks, even the current Tacoma 4 cylinder.

Extremely tight manufacturing process controls (compared to many competitors at the time) meant that all the internal moving parts were at least an order of magnitude (if not several orders) closer to all "being the same" compared to many other auto manufacturers at the time. When all your moving parts are the closer to the exact measurements/mass etc., they're designed to be, there's less "slop" and everything fits and moves together better from one engine to the next. In an engine turning at thousands of RPM its whole life, that lack of variation in part size/mass etc., makes a difference in long-term durability.

Another thing those old engines also had going for them was that they were pretty simple. They were understressed for their displacement (they didn't produce a lot of power compared to similar sized engines today), they were very well designed in terms of oil delivery and cooling capacity and the more lax emissions requirements of the time allowed them to be less complex compared to today's engines in that regard too. The result is that there isn't/wasn't as much to go wrong with them on top of being very well built and designed. All together, that was a recipe for a long lasting engine design.

They were simply just built like tanks. I've worked on one with a 22RE in my automotive maintenance class and wow it seemed really solid. Toyota knew they had an advantage back then when the American automakers just made junk. HOWEVER, those old American diesel trucks were no joke either. They just keep going and going no matter how much you beat them. I've seen them from experience.

Better design and engineering.

80-90s American cars where pretty poorly built as the car companies tried to keep costs down. The Japanese manufactures by comparison invested in new designs and materials and made more durable engines, which as you say, many are still running today.

The early ones with the 18R, 20R and 22R were all pretty much the same engine and were a bullet proof design. A little maintenance and they would last forever.... Simple to work on

Hi well built, attention to detail is why unlike an american truck which is thrown together by some monkey.

The ones you saw were just a fluke.