my Jeep overheated suddenly and it was not the thermostat. Do you think the next step is the water pump?

First off, we need to know about what you got. What year is it and what motor and acessories you have?

Since it has 200,000 miles on it, for the following discussion, I am assuming it is an older Jeep Cherokee with the 4.0 Liter 6 cylinder engine. This engine has two fans: A small belt driven one and a larger electric fan. You jeep will overheat in hot weather if the fans do no work. In cooler weather, the electric fan hardly ever turns on geting enough cooling from the small belt driven fan.

The belt driven fan has a viscous clutch that is temperature controlled. By default, this fan is fully energized at startup nad if its temperature is low, i will begin to disengage with time. After about 5 minutes, it should be at idle condition if cool, that is, the fan will not be at full speed. This fan turns all the time, but when it is not needed, it runs slow. When needed, the fluid clutch comes into play and the fan comes up to speed. It is quite noisy when at speed.

If the small fan is working, at startup it should be full on and making fan noise. A few minutes later, it will quiet down if not needed. If the viscous clutch is not working and this fan is not coming on when things get hot, that may part of the problem. It supplies about 30% of air flow capacity to the radiator. The other 2/3 is supplied by the large electric fan.

The large electric fan is supposed to turn on when the coolant get hot. Does it turn on? A temperature switch controls this function. I'd have to check my manuals, but this switch may not directly operate the fan; it may signal the engine controller (computer) and the controller operates fan relay. The relay handles the subtantial current draw of this fan. There are 3 items in this chain of control: the temperature sensor/switch, the engine controller, and the relay. The item most likely to fail is the temperature switch, next is the relay, next is a fuse, and then the fan itself. The final failure parts is the engine controller.

The big fan is usually off when traveling at highway speeds except if in extreme desert like temperatrues.

The big fan comes in to play in stop and go traffic on hot days.

A clogged radiator is possible. According to your post, the problem began while traveling at highway speed. Another possibility is a collasping lower radiator hose. The hose is different in construction to the upper hose. The lower hose must contain features to prevent collaspe. Some hoses have a coiled spring wire in the hose while other contain anti-collaspe construction.

The thermostat: If the thermostat failed and closed, overheating follows quickly. You said that a new thermostat did not fix the problem. Two possibilites: the new thermostat was installed backwards. A thermostat installed upside down will not open as quickly as it should. The sensing element should face toward the hot side. Consult your manual. If the new thermostat was installed correctly, then look elsewhere (assuming the new thermostat is a good unit). Was it tested in a pan of hot water to see if it works?

How do you know that the water level was ok? Looked at the overflow bottle? The radiator can be short of water with a normal fluid level in the overflow bottle. The coolant system is 'closed' in that it is designed not admit air. Again, I make another assumption that you have a proper radiaro cap in place. it has 2 seals in it. The outer seal fits on the top of the fill neck and seals that opening from the atmosphere. The second seal is on the spring loaded pressure valve. This valve limits system pressure. When the set point pressure is exceeded, coolant forces it way past the valve and flows through a hose to the overflow bottle. After the system cools down, a partial vacuum deveops and coolant is drawn from the overflow bottle back into the radiator.

If there is an air leak anywhere between the radiator fill neck and the bottle, air is drawn into the system on cooldown instead of fluid. After a few thermal cycles, the engine coolant system is short of fluid. If the fluid level iin hte bottle si not behaving as expected, inspect the path from the bottle to the radiator for leaks. Old oveflow hoses have been known to develop splits on the underside.

A water pump is a possibility. it can fail by eroding the impeller until it is a poor pump. I have been drivng a 6 cylinder 4.0 liter jeep engine since 1993 (on my second one) and have not seen an impeller failure.

I'd check these items first:

1. Both fans.

2. Radiator cap.

3. Wind blown trash in and across the radiator core. You did remove that piece of cardboard that was put in there for winter driving? If you have an air conditioner, clean its core of trash also. It sits in front of the main radiator and can block part of the air flow.

4. Lower radiator hose.

If the temperature gauge shows overheating but everything seems nornal and an external probe measures normal engine temperature, then suspect the temperature sensor for the gage.

You could, of course, take it to a shop.

Let us know what you find.