Dear Ole Mechanic;
I have an old 1977 Chevy pickup with a 350 (I think) V8 and a four-speed manual transmission. I brought the truck last August to use as a ranch truck for our acreage. We have a couple of horses and feed, fencing and manure that need to be hauled. I hadn’t driven the truck very much until I used it a few days ago to run to town. The engine temperature gauge was starting to read HOT by the time I got over there. It had a chance to cool down for almost an hour before I headed back home. I checked the radiator level before I started back and it was OK. By the time I got home, the radiator was steaming. I hosed down the radiator to cool it down, and then I added a little water to fill it back up. Figuring that the thermostat was the problem, I removed it. Now I can’t even go twnety miles without the truck starting to overheat. What could be wrong?
I suspect that your problem is that you have no thermostat in the cooling system. Even when the thermostat open, it acts as a restrictor to slow down the flow of coolant through the radiator. Without some restriction in the system, the water pump can push coolant through the radiator so fast that it doesn’t have time to cool down. The first thing that I suggest is to get a good thermostat back in where it belongs. If you can’t afford that, then find a washer with a 5/8th's inch hole and the same outside diameter. I have used the washer in race-car cooling systems and that way I didn’t worry about a sticking thermostat.
While you have the thermostat housing off to replace the thermostat, fill the engine-block with water right up to the top of the thermostat housing mating surface. Remove the fan belt(s) so that the water pump cannot run. Then, start the engine and let it idle while you look to see if any bubbles start coming up through the water in the thermostat opening. With the block full of water, you can run the engine at idle for a couple of minutes without boiling the water. If you see bubbles coming up through the water, then combustion gases are getting into the cooling system. The bubbles indicate that there is a leaking head gasket or that there is a crack in the engine. The crack could be in the block or more likely in one or both heads. The engine is going to need work because combustion gases in the cooling system will displace coolant and the engine will overheat. It may take some miles to do so, but no if's, and's or but's--it will overheat!
If you don’t have any bubbles and the engine still overheats even with the thermostat or washer in place, it is time to look at the air-flow through the radiator and the water-flow through the radiator. First, look at the fan. If the fan is mounted backwards, it will move some air in the correct direction--towards the engine--just not as much as it should. Also, someone could have put a newer fan on the older engine and it will be running backwards, trying to push air forward through the radiator. The newer fans run backwards compared to the old ones. That could cause overheating out on the highway. While you are looking at the fan, don’t forget to check the hoses for kinks. Also, squeeze the lower hose to check the internal spring that keeps the hose from collapsing from the suction caused by the water pump. If the fan and hoses are OK, then put a trouble-light behind the radiator and look through the fins from the other side. You should see some light. I have seen radiator fins completely plugged with bugs and dirt. Flush or blow out the crud with water or air pressure from the engine side. Do not use a high pressure washer! Some of them have enough pressure to tear the tubes and ruin the radiator. If everything is OK up to this point, then it is time to check the water-flow through the inside of the radiator. A garden hose will work OK. Reverse the flow through the radiator; in other words, run the water in the lower radiator hose opening and let it come out the top hose opening. Unless you want water all over the engine, take the upper hose loose at the engine and direct the water down so it splatters all over your feet. Now that your feet are wet, go ahead and make sure that water will go through the engine-block by directing water down through the thermostat opening. If both the radiator and block will run a full garden-hose flow of water through them, then it is time to look elsewhere. What is left? If the engine ignition-timing is retarded too much, it can cause overheating, but then I would bet that you would be complaining about a severe lack of performance, too. Also a kinked exhaust pipe could cause overheating, but, again, performance would be horrible.
If everything mentioned above is OK and the truck still overheats, trade it off quick. Good luck.
Herr Professor Nuzanbolts