Engine Overheating

Sorry I missed your replies. I was out of town at the nation's second largest old car show swap meet.

1400 ohms is acceptable for the coolant temperature sensor but the 5.0 volts is not unless you measured that while it was unplugged. If you back probe the connector while it's plugged in, you should find between 0.5 and 4.5 volts. The higher the temperature, the lower the voltage. By the way, are you on a two-wire sensor? If it only has one wire, you're looking at the sensor for the dash gauge.

The fan should not be needed at highway speeds. If turning on the heater fan helps, check the radiator for corroded and crumbling cooling fins.

Caradiodoc.

Inspect the cooling fins on the radiator to see if they are badly corroded. That will prevent them from giving up the heat to the air. The clue is running the heater reduces the overheating. The heater core is a small radiator.

A leaking cylinder head gasket is always a good suspect too, but you would be losing coolant, and you did not mention that observation.

You said the problem does not occur when the heater is on. That eliminates the thermostat and water pump. Everything is working properly at that point.

The only other thing you did not mention is if the radiator fan is turning on. Your model should still have two coolant temperature sensors. One is for the dash gauge. That one will not cause an overheating problem. It only reports coolant temperature. The other sensor is for the engine computer. Both of them are extremely reliable because they have just one part inside them, but corrosion on connector terminals, or other wiring problems can cause it to report a temperature that is too low, then the computer will not turn the fan on. The clue to that is the overheating will never occur at highway speeds where natural air flow is sufficient.

Have you actually check that your engine IS hot with a thermometer?

It could be that you have a bad temperature gauge, a bad sensor, or a bad connection in the wiring or something.

How is your coolant level? Maybe you have a leak somewhere (internal/external). I suppose that's the first thing you checked though.

Other possible problems:
*you serpentine belt is slipping on the pulley resulting in the water pump no working properly (check you tensioner). Speaking about pulley, your water pump pulley is the original one right? (Or at least an original replacement, same size etc.).
*bad hoses
*your electric fans don't come on
*bad radiator cap
*glogged water jackets in your engine

Since you've replaced the water pump, radiator and thermostat, these are the other reasons I can think of.

I suggest you pressure test your cooling system to check for leaks.

That was not a typo. Check the cooling fins, not the fans. If you can get to the front of the radiator without the AC condenser being in the way, rub your fingertips over the fins and see if they crumble like a rotten chocolate chip cookie. This is especially common if you live in a state like I do where they throw a pound of salt on an ounce of snow.

This happened to my 1988 Grand Caravan when it was twelve years old. The temperature gauge would start to creep up at anything over sixty mph and sixty degrees. I drove it from Wisconsin to Denver, but it was mostly night driving when the temperature was cool, so I did not have a problem. By two yeas later, it even ran too hot when it was thirty five degrees. The fan turned on like it was supposed to, but the air flow did not do anything.

I used that van to drag around a tandem-axle enclosed trailer that was bigger and heavier than the van, and with that radiator, I could keep the temperature down by running the front and rear heaters on high. Problem was I was taking that trailer to the nation's second largest old car show swap meet which is always the first weekend after the Fourth of July, so it is always hot outside. Running the heaters is not what you want to do on a hot summer day! I finally had to get a new radiator. I patched so many holes in it that I was spending more time at that than sleeping! With the new radiator, the temperature stayed normal, even when pulling that trailer on a hot day.

Next I would perform a chemical test at the reservoir to check for a leaking cylinder head gasket. That involves drawing air from the reservoir, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

It may be less expensive to have your mechanic do this test. You might be able to borrow the tester from an auto parts store that rents or borrows tools, but they typically make you buy your own bottle of liquid That is because the liquid is rendered ineffective if it is allowed to freeze or if it becomes contaminated with coolant. That would cause it to provide incorrect results for the next person who borrows it. The test usually only takes a few minutes. A bottle of the special liquid may cost more than the cost of the test.

Take the radiator out and get the core cleaned a flush may not be enough if you have blocked core tubes, also test the thermostat in a pot of boiling water just to make sure that it is opening.

The very first thing you probably should learn is, don't let the car overheat. Ever.
If you drove it hot to the point of losing power and by the sounds of it you did it more than once, you have likely done serious internal engine damage now such as a blown head gasket or cracked head.
You may have had a small problem to start with but driving it hot has developed into something far more serious.