Overheats when driving

https://www.2carpros.com/articles/symptoms-of-a-bad-water-pump

There's a whole bunch of causes of overheating. The water pump could cause it, but that would be way down at the bottom of the list of suspects. First you have to look at the other clues and observations. The way a water pump would cause this is if the impeller is spinning freely. In rare cases, such as with VWs, the impellers corrode away. The clue is the hot coolant isn't circulating to the radiator, but it also isn't circulating to the heater core. You'll get cold air from the heater. The radiator, radiator hoses, and heater hoses will be cool.

A plugged radiator, internally, is another possibility. Stop the hot engine, then feel the radiator from side to side. It should be hotter at the top, cooler at the bottom, and there should be no cool spots as you run your hand from side to side. A cool section tells us those tubes are plugged. No heat is being given up by those tubes. In this case, the engine will tend to overheat at lower speeds, and may run cooler at highway speeds where there's more natural air flow through the working tubes.

Given the age of the vehicle, and especially if you live in a state where they throw a pound of salt on an ounce of snow, corroded cooling fins on the radiator are a good possibility. Volume of air flow isn't the issue here. The cooling tubes will be hot, like they should be, but without the fins, there isn't enough surface area to dissipate the heat. The engine will run at normal temperatures in the city and at lower speeds. It will run hot at highway speeds and when ambient air temperature is higher. To identify this, the fins will crumble like a rotten chocolate chip cookie when you run your fingers over them. Even though they appear to still be there, they've lost all effectiveness and aren't giving up any heat, regardless of air volume. Since air volume, (road speed), isn't the issue, overheating occurs when more heat is generated, (engine speed and load). Another clue, when this happened to my '88 Grand Caravan, is the coolant temperature came down rather quickly at highway speeds when I ran the front and rear heaters on one of their higher speeds. The two heater cores acted like little radiators.

The radiator can also be plugged externally. Look for a butterfly collection, leaves, mud, or anything else blocking air from flowing through the cooling fins. The clue when the radiator isn't doing its job is you'll get really hot air from the heater.

Always consider the possibility of a leaking cylinder head gasket, but that is very uncommon on the 318 c.i. engine. Combustion gases that sneak into the cooling system can pool under the thermostat causing it to remain closed. Thermostats need hot liquid to open. Hot air won't do it. Here's a link to an article that shows how to test for a leaking head gasket:

https://www.2carpros.com/articles/head-gasket-blown-test

You might get more ideas from this article:

https://www.2carpros.com/articles/engine-cooling-system

If you still have an overheating problem after checking all of these common things, we have to look at the more elusive causes. The radiator has a maximum capacity of the amount of heat it can disperse. Is the engine developing more heat than it is supposed to, or is the cooling system incapable of dissipating the amount of heat it was designed to do? If both are normal, most likely there's not enough air flowing through the radiator. Most people don't realize how much of the vehicle's design is attributed to engine cooling. The most common modification is many owners remove the shroud around the radiator fan, thinking that will improve air flow. In fact, it does just the opposite. The shroud is there to force the fan to draw air through the radiator only and nowhere else. Without it, the air will just go back around the fan blade on the sides and run through it again without fresh air being pulled through the radiator.

You'll also see a rubber seal from corner to corner either under the front edge of the hood or on top of the core support. That is needed to prevent air from flowing over the top of the radiator, thus bypassing it. Also look for anything missing that was bolted to the core support. A common item used to be the charcoal canister. Those often sit right behind a large hole in the sheet metal panel. If removed, the exposed hole lets air flow through it rather than going through the radiator. Even small holes are blocked at the factory. If you look at the AC lines or transmission cooler lines that pass though the core support, you'll usually see foam seals to block air from flowing through there.

If this problem only starts right after parts were replaced, be aware the fan and / or the water pump can run backward on some engines. That happens when newer models switched over from V-belts to the flat, multi-ribbed serpentine belts. To accommodate the belt routing, some pulleys run in the opposite direction as on older models. For fan blades and water pump impellers, they have to be reshaped to do the same job when running the other way. An older water pump will bolt onto the engine, but will try to circulate cool coolant from the top of the radiator into the engine where it runs into the closed thermostat. We've also read stories of people being able to accidentally misroute the belt causing the water pump to run the wrong way.

Let me know what you find, then we'll figure out where to go next.