Why is my car overheating?

Get a workshop to run an ECU diagnostic as this might show where the fault is. Check the usual faults - thermostat, coolant level for leaks (cracked hoses, perforation in the radiator or leaking end tanks), fan fuses and relays and the condition of the radiator (no obstructions in the cooling fins, the core isn't blocked up with corrosion etc) and fan condition as they can burn out. Next place will be the thermo fan sender unit and any possible wire faults (damaged wiring, corroded connections, bad ground connections etc) If the car has never had a cooling system flush, now would be the time - and replace the coolant with a recommended product for your Jag.

An overheated engine can be caused by anything that decreases the cooling system's ability to absorb, transport and dissipate heat; therefore engines can overheat for a variety of reasons. Let's take a look at some of the most common causes.
Cooling System Leaks
This is the primary cause of engine overheating. Possible leak points include hoses, the radiator, water pump, thermostat housing, heater core, head gasket, freeze plugs, automatic transmission oil cooler, cylinder heads and block. Perform a pressure test. A leak-free system should hold pressure for at least one minute.
Wrong Coolant Concentration
Be sure to use the coolant recommended by your vehicle's manufacturer. The wrong type of coolant and/or mixing the incorrect concentration of coolant and distilled water can also result in engine overheating. The best bet is to perform a complete flush and fill.
Bad Thermostat
A thermostat is a heat-sensitive valve that opens and closes in response to engine temperature. Heated engine coolant passes through to the radiator when the thermostat is in the open position. In the closed position, it prevents the flow of coolant to speed up the warming of a cold engine. When the thermostat gets stuck in the closed position, coolant stays in the engine and quickly becomes overheated, resulting in engine overheating.
Blocked Coolant Passageways
Rust, dirt and sediment can all block or greatly impede the flow of coolant through the cooling system. This can limit the system's ability to control engine temperature, which may result in higher operating temperatures and engine overheating. Once again, a flush and fill is recommended to remove debris.
Faulty Radiator
By passing through a series of tubes and fins, coolant temperature is reduced in the radiator. Leaks and clogging are some of the most common causes of radiator failure. Any disruption in the radiator's function can lead to elevated engine temperature and overheating.
Worn/Burst Hoses
A hose that contains visual cracks or holes, or has burst will result in leaks and disrupt the flow of engine coolant. This can result in overheating.
Bad Radiator Fan
A fan blows air across the radiator fins to assist in reducing the temperature of the coolant. A fan that wobbles, spins freely when the engine is off, or has broken shrouds will not be able to reduce the temperature to proper level, thus possibly resulting in engine overheating.
Loose or Broken Belt
A belt is often the driving link that turns the water pump at the correct speed for proper coolant flow through the cooling system. If a belt is loose or broken, it cannot maintain the proper speed, thus resulting in poor coolant flow and ultimately, engine overheating.
Faulty Water Pump
Known as the 'heart' of the cooling system, the water pump is responsible for pressurizing and propelling engine coolant through the cooling system. Any malfunction of the water pump, including eroded impeller vanes, seepage or wobble in the pump shaft, can prevent adequate coolant flow and result in engine overheating.

No.
Leaving the thermostat out will not make the engine run hot.
Some possible causes are low coolant, faulty water pump, or leaking head gasket.

Sounds like a stuck closed thermostat. It is usually where the top hose connects to the engine block.

What you are describing is a lack of coolant flow in the engine or radiator, or lack of air flow thru the radiator.
If you are sure the coolant level is ok, and flow is good, I would check for a restriction in air flow thru the condenser and radiator.

Could be a sticking thermostat, a restriction in the air flow thru the radiator, a restriction in the coolant flow thru the radiator or block, or other issue.

sounds like the radiator might b clogged not allowing coolant to flow. remove both radiator hoses and insert a garden hose and see how much water comes out the bottom..if it flows freely then u have a blockage somewhere not allowing coolant to flow... if the hoses are rock hard when car is running and overheating then u have a cracked head and will have to change it with a new one.

Do you have any coolant flow thru radiator with cap off?

Is thermostat in correctly?

Do a Combustion Leak Test at radiator or cap,
on gases from coolant. You may have a Head
Gasket going bad

it will be cold untill the thermostat gets to 195 degrees and that lets the coolant circulate thru out the engine and radiator

Last year my car suddenly overheated while idling to the point the coolant tank exploded. I took it to a foreign car repair shop where they replaced the coolant tank and put in another thermostat. It didn't make it home before it overheated again. I took it to the Jag dealership where they felt that my hoses had expanded to much during the overheat so they replaced all of my hoses to a tune of $1000. Three months later it overheated again. I took it to another foreign car repair shop where they discovered the real culprit. An impeller had broken off of my waterpump, I had a blockage in my radiator, and I had a short in one of the coolant fans that made it only work part of the time. So about $2400 later my car has been coolant issue free for about 2 years now. Good luck and I hope this helps.