So I replaced the water pump, thermostat, thermostat housing and radiator cap. Fully flushed (and flushed and flushed) the system - filled up with coolant and took her for a test drive around the block. I noticed when I got back that both the upper AND lower radiator hoses were hot hot hot. Before replacing all this the lower radiator hose was cool-ish to the touch - I figured this was the radiator doing it's job.

What would cause my lower hose to be so hot? Air in the system perhaps.

(unfortunately, I don't have a temp gauge to see how the engine is faring - just a dummy light).

I only have a 2.5L 4cyl so I don't know from experience but have read some posts here about the 4.0 and I believe there are two different water pumps, which are either clockwise or counter clockwise rotation. If you got the wrong one it could be trying to suck from the top hose and push the hot water back through the bottom hose. If that is happening then you should see the top hose contract as you rev the engine. Just something to check to make certain.

Unless the motor is overheating, it doesn't matter how hot/cold the BOTTOM rad hose is. Verify motor temp by aiming a point-n-shoot infrared temp gun at the t-stat cover.

Why would it do that? A water pump only moves water, it doesn't create or reduce pressure.

Thanks for the thooughts. Mine is the 2.5L 4cyl as well -- the water pump is a reverse flow (I was sure to get the correct one). I just went for a longer drive and noted that both hoses were hot, but the top hose was quite a bit hotter - I'm hoping this is fairly normal. I don't really trust the dummy light (or even know at what temp it would supposedly trigger).

You wouldn't see that even if it was the wrong pump! If you did you would have much bigger problem then a pump spinning in the wrong direction.

My first vehicle was a 71 F100 with a 302 in it and I had to put a wire coil into the lower radiator hose because when you revved the engine the hose would collapse. The water pump creates suction which does create a pressure differential between each side of the water pump, otherwise there would be no flow. Even in a pressurized system there will be a pressure differential between one side of the pump and the other, but if the engine is still cold when it is first started it hasn't built up pressure in the system yet, and you could easily see the suction in the hose then.

You are confusing suction and discharge in a closed system with increased or reduced pressure.

Once again, a water pump, or even an oil pump, doesn't create or reduce pressure.

Correct!
The water pump only circulates the water it does not create positive or negative pressure.

Does the dummy light turn "on" when the motor is started, then turns "off" after the motor starts? If it does, u r probably good to go....if it never lights up (when u start the motor), u have an issue that needs to be fixed......burned out bulb, bad ground, etc. There have been some posts saying "I didn't notice the needle was in the red"......I've never seen a post saying "I didn't that red light on the dash".

We've momentarily seen 230* on our ScanGauge in our XJ and the dummy light never even so much as flicked. FWIW, mathematically, the left edge of the red mark on the factory gauge is 241.25*.

Ah - interesting. Just checked and the temp dummy light does NOT turn on when I turn the ignition (oil pressure does). Guess that's next on the list - anyone change out those dash bulbs before?...

I've never changed them on any of my older XJ's but I have on my 98. Pretty simple job. Just pull the cluster and change the bulbs. To to hard to do. Also if you want this would be a bood time to replace all your bulb with led's if you want to go that route.
BTW, Good catch for djb for mentioning about the bulb.

Maybe that collapsing hose is a Furd thing.

The only way to make fluid move is to create a pressure differential. That's what the pump does. On some vehicles, the pump can create enough suction to collapse the lower hose. Depends on the system design.

Ok by the definitions here, if the pump is moving water out of the hose, and there is a restriction to the water entering the other end, then that restriction has caused a drop in pressure, which is caused indirectly by the pump moving the water.

I bet though if you place your hand over the radiator end of the lower hose, then rev the engine you will feel a suction on the skin of your hand, and that by definition is a drop in pressure. Also if you spin a centrifugal pump and no more water is allowed into the pump the impellers cause a cavitation in the water inside the pump which is essentially an area of vacuum where only water vapor exists and that is also a drop in pressure.

Even oil pumps move oil because of pressure drop, when the cavity opens between the gears in the pump, they create a void into which oil is pulled. That void is not filled by magic, it is filled by the pressure differential between the vacuum of the void and the atmospheric pressure outside the pump on the other side of the oil. Flow takes place when there is more pressure on one side of a fluid than there is on the other, it will move from the point of highest pressure to lowest, otherwise it will be stationary.

The pressure in a pressurized closed loop cooling system comes from the expansion of the coolant due to increased temperature, and when it equilibrates at operating temperature the pressure cap regulates it to 14psi or whatever it is set to, but if you were to place a pressure gauge just before and just after the water pump you will read slightly more pressure on the discharge side, even if it is a fraction of a psi. Water pumps my only create flow, but flow creates the pressure, either positive or negative.

If you dont believe me, then do the experiment yourself, place a thin walled hose between the bottom of the radiator and the inlet of the water pump and see if it doesn't collapse when you start the engine. There is a reason lower water hoses are made of a stiff reenforced material while you can get away with using the thinner more flexible hoses on the return line.