Driving, no issues with overheating. Noticed the coolant res. was low so added coolant, now my 2004 Disco HSE runs hot.....makes no sense to me does it you?

 

Makes perfect sense.

1. You were low on coolant.

2. So low that the tip of the coolant sensor (top of the intake section of the engine) was not touching liquid. It was taking a steam bath. And just like pasta on the stove, that steam is hot, but a lot cooler than sticking your hand in the boiling pot.

3. The heat gauge showed normal during all this, because Rover thinks you don't need to know about it until the the truck melts down. Gauge shows 50% normal 130-240F.

4. So you were overheating before AND had low coolant. Now you are still overheating.

5. Runs hot equals what position on the gauge? Red light ever come on (284F)?

6. Have you looked at real coolant temp with an OBDII connected scaneer?

 

Yes, when I jumped in it fresh yesterday morning on the way to work, after topping off, the issue that really got my attention was the fact the red light came on. Immediately pulled in parking lot shut her down cooled all day after work, then nursed her home before the light popped back on or got overheated........

 

Well, there could be more to it.

The gauge shows 50% from 130-240F. You should be running in the 190-205 range, maybe 210 at drive thru line.

Without a scanner you don't know the temp. Running at 235 for long periods kills the head gaskets. 284 is where the light turns on to warn you that mechanic will be able to buy that new boat....

Take a look through the post listed below for cooling problems. You have them, but you have not gotten enough info yet to narrow it down.

 

Ouch.
You don't want to drive with the read light ever on.
You need to bleed the cooling system.

You probably hear the water fall sound? From the heater core when you take off from a red light?

That means now you need to bleed the cooling system to get all the air out and then you will need to add more coolant.

Hopefully that is all that is wrong.

 

One way to avoid this and other surprises in the future is to develop a weekly ritual on the day of your choice. On that day, the "I love this beastie" celebration, before cranking up, open the hood, and check all fluids, battery wires for tightness (just wiggle), inspect fan belt, hoses for cracks and leaks, etc. Walk around an look at tires, use your air gauge, etc. Then you spot problems when they are much smaller. Don't have to be a mechanic. The owner manual tells you to check oil level every 250 miles.

For current issue once you bleed the system, you still have to locate what was leaking, it may not stop by itself.