Hi. I got a friend with a 1980 4wd pickup, 20r engine that runs warm at highway speeds. This truck was sitting for a few years before he got his hands on it. We have gone through and replaced the waterpump, all hoses, thermostate, and installed a known good radiator and fan in the truck. All that has helped keep the truck running cool in town and at low speeds on the trails. but at highway speeds(40-55mph) the temp gauge likes to start leaning toward the hot side of things. There is no coolant disapearing, and there is no evidence of oil getting into the coolant, or the otherway around. I am starting to lean toward a headgasket starting to go. I'm hoping someone here could help give me an idea of what could be causing the overheating.
thanks

 

If a head-gasket was starting to go then you would have an oil drip.
does it get a little warmer and just stay a little warmer on the highway or does it just keep getting hotter and hotter?

 

Why?
You have a bad headgasket, compressed fuel/air from the cylinders is getting into the cooling system increasing temps. as the engine is pushed harder. Check the coolant it will have signs of exhaust gasses (hydrocarbons).

 

From whatI have been told the truck temp just increases and stedily gets warmer and warmer on the freeway. Like mentioned before, you can drive the thing all day around town and at slow speeds and it will run cool. I should also mention that a 160 degree thermostat has been put in the truck also. I giess I should also mention that before doing any work to the truck, it would run cool at speed and heat up in slow going situations. A 180 degree thermostat also came out of the truck.

 

Hey. I've got a 79 RWD Pickup with the 20R. My truck is doing pretty much the same thing. I've replaced the head gasket, fan clutch, water pump, radiator hoses, and thermostat. The radiator is free flowing. My truck doesn't seem to heat up until after 60mph. My thought is that there's something restricting one of the water jackets.

What's meant by hydrocarbons.

 

this suggest to me .. that the tubes in the rad are NOT ok .. not all of them anyway .. just enough to keep it cool under low engine load .

I think it's the rad .


.

 

Seems to me its just not getting enough current flow

Have you flushed the radiator?

 

Have the radiator cap tested to see if it holding the correct pressure. They sometimes fail and will not allow the system to make the proper pressure.

I have had the same symptoms and after replacing almost all of the cooling system I found out it was the radiator. Go get a infrared thermal gun and shoot it at the radiator in different places and see if it's consistant hotter at the top and cooler the lower it goes. If you find a section that is not consistant with the rest of the radiator, plugged tubes.

With a blown head gasket you should have water being pushed out of th radiator into the overfill tank.

 

try a 180 degree thermostat, maybe the coolant isnt getting enough time in the radiator to cool? Just a thought.

 

I have a 1983 4x4 that I have owned since new. I had the same problem you describe when I first purchased it in Florida. I had a 4 core radiator installed and after that I never had the problem again in 26 years of driving it. The 1979-1983 Toyota radiators were not the best design, circulation, fin spacing, etc. I would install a 3 or 4 core radiator.

 

radiator. another thing you could try is to split one of your hoses going to your heater core, put a tee in it and flush it with a hose. I had an early 90s p/u and it would never heat up. did that, pop*, brown thing came out and never a problem again. it's also an easy way to flush your system yourself.

 

i would also say you need to flush the whole system with a cleaner. i just went though the same thing, the truck sat for a few years so it built up a lot of crud. it was fine until i had a long highway trip, then it cooked on me and i almost lost a headgasket. i tried to flush the radiator but it was too far clogged. i just put in a new aluminum radiator and it should be perfect now.

i would not change to a cooler thermostat, that only bandaids the true problem.

 

Usually, heat buildup (when it's not due to RPMS) is either an oil, air, or coolant issue. If you know the oil system is ok, you look at the coolant system, and then air system. Check thermostat, coolant, and hoses - completely. Then check the fan. If you drive with the heater on and it cools your engine down, it's usually a dead give a way that it's the coolant system.

Good luck