Engine 1987 5.0 overheating at both idle and driving

If it's not the radiator,which is highly suspect with that mileage you probably have air in the system. Get a Lisle funnel kit and you'll never have air in it again.

If it is not the Radiator like Rick suggested, it needs burped or bled like monkeybutt says. Use the search for those terms. I have not had problems with this, but others swear having the nose up or squeezing the upper radiator hose are needed ways to get the air pockets out.
Visually inspecting the radiator inside for growths is free, as is burping the coolant system. That’s the order I’d go in, even without a cool thermometer.

I’d also recommend a no spill funnel. I finally got one this year.. it makes it easy. Not to mention no mess!

Cheapskate207 said:

I’d also recommend a no spill funnel. I finally got one this year.. it makes it easy. Not to mention no mess!

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You found something better than my large soda bottles with the bottom cut off?

Also beware that a couple people here have found brand new water pumps that were defective and didn't circulate coolant well.

I agree with proper filling and burping the system. Check the oil and coolant for a blown head gasket. There is a head gasket checker tool that most parts store will lend out. Its got to be straight water in the system and the directions need to be followed cemetery. Its not a complicated process.

Always makes me nervous to purchase a car that has a complete new cooling system. The previous owner was definitely having issues.

Another thing to check is the coolant temperature sender located in the drivers side of the lower intake...around top between the ac compressor and upper intake. If you don't have boiling over at 260 degrees then it could be giving a false reading to the guage.

When the temperature is in the middle you should feel warm air being pulled through the radiator by the fan. The fan shroud is also important and should be there and fully intact. If the air is cold coming through the radiator then its got a coolant flow issue....thermostat...low coolant....big air bubble....or water pump not circulating.

Another check is to wrap your hand around the upper radiator hose....and then around the lower hose. The upper hose should be hot...the lower hose should be warm. It the upper is hot and the lower is cold then the radiator has a restriction. If the upper is cold...no circulation...could be thermostat....air....or water pump. An extremely blocked radiator could cause a flow issue but would cause the lower hose to collapse when engine is revved....unless it still has the factory spring in the hose.

7991LXnSHO said:

You found something better than my large soda bottles with the bottom cut off?

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Yep
I don't even use the Lisle funnel kit anymore.

This is faster, removes all air pockets, and virtually no mess at all.

Have you started the car with the rad cap off, wait for coolant to come up to temp and watch for flow coming out of the rad tubes. Once thermostat is fully open there should be quite a good flow coming out of them.