Your symptom of overheating at high speeds or on hills indicates the overheating problem occurs under high load/high RPM.
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The causes of this generally fall into one of two categories:
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1) An issue with the cooling system not allowing it to dissipate enough heat. Given your observations and the
repairs you've already tried, this seems unlikely.
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2) An issue with the engine causing it to generate more heat than it's supposed to, and the cooling system can't keep
up the above-normal heat load.
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The most likely reasons your engine would run hotter than normal are running too lean (not enough fuel/too much air),
improper ignition timing, or poor exhaust flow. It's possible you might have a small head gasket failure allowing exhaust
to push into the engine water jacket, but that usually wouldn't be load-dependent.
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If I had to take a wild-a** guess, a restricted exhaust is the most likely culprit. Another commenter he fixed the same issue
by replacing the catalytic converter. You can have a mechanic test it by putting an exhaust backpressure gauge in place
of the oxygen sensor. Or he can check the temperature difference across the inlet and outlet of the converter. If the temperature
doesn't change then the coverter is not working and is likely partly clogged.
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Good luck,
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Buzz L.
San Jose, CA
Former '86 Dolphin 500, '89 HiLux Longbed, and '96 Tacoma owner
