U0100 prm mod03

It would have been useful to know where the hose from the tee goes and what diameter as that is new to me. 50 years as a repairer and I have never seen a top radiator hose with a tee branch.

Usually the engine heats and the thermostat opens and starts coolant circulating through the radiator, up the bottom hose into the water pump to be pushed round again.
The volume of coolant moved by the water pump, even at idle should be far in excess of that circulating through the heater matrix ensuring adequate circulation through the radiator as soon as the thermostat opens.

The top hose must be hot because the thermostat is open so why doesn't the coolant go further and heat either the branch or the rest of the hose and radiator...
Sometimes a pump with a plastic impeller will lose a vane and overheating due to insufficient circulation is likely.

Other things are more common - semi-blocked radiator tubes restricting coolant flow.
Silt build up in the cylinder block causing hot spots in the cylinder walls.
Restricted air flow through the radiator.
Electric coolant fan not working.
Excess combustion gas in the cooling system interfering with thermostat operation.
Degassing system blocked with the same result.

How To Solve An Engine Overheat Condition - EricTheCarGuy
https://www.youtube.com/watch?v=hUzOTnsWImI

Engine overheating. How to fix a overheating car engine on your car
https://www.youtube.com/watch?v=evpaTW2WJ5Y

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.

get it serviced

Section 03-01D: Diesel Engine, 7.3L V-8 1992 Bronco/Econoline/F-Series Workshop Manual
REMOVAL AND INSTALLATION Please rate highly if this helps. this is 1992, only thing I have, should be the same. Thermostat A poppet-type thermostat is used with the engine. When the thermostat is closed, coolant flows to the water pump through a bypass passage at the front of the engine. When the thermostat is open, coolant flows through the coolant outlet elbow (thermostat housing) to the radiator. CAUTION: Replacement thermostats other than the specified Motorcraft or Navistar thermostats may result in engine overheat damage. The specified thermostat does not contain an internal bypass, since the by-pass is located in the cylinder block. Therefore, whenever the thermostat is replaced, it is mandatory that only Motorcraft E5TZ-8575-C or Navistar 1807945-C1 thermostat be installed.
The thermostat is a high temperature thermostat for use with a mixture of water and permanent-type antifreeze. CAUTION: Do not attempt to repair the thermostat. It should be replaced if it is not operating properly.
Check the thermostat before installing it, following the procedure described in this section.

Thermostat Design (Acceptable vs. Unacceptable)




Removal

1. Disconnect ground cables from both batteries. NOTE: On vehicles equipped with EEC, when the battery has been disconnected and reconnected, some abnormal drive symptoms may occur while the EEC processor relearns its adaptive strategy. The vehicle may need to be driven 10 miles or more to relearn the strategy.
   

1. Drain coolant from radiator until coolant level is below thermostat. Refer to Section 03-03, Engine Cooling.

1. Loosen and remove alternator and vacuum pump drive belts.

1. Remove alternator. Position alternator out of the way.

1. Remove vacuum pump and bracket. Position pump out of the way.

1. Remove all but the lowest alternator/vacuum pump mounting casting bolt. Loosen the lowest bolt and pivot the alternator/vacuum pump casting outboard.

1. Remove the water outlet housing attaching bolts. Bend the radiator upper hose upward and remove the thermostat and gasket.

1. Inspect thermostat as outlined in this section.

Installation

1. Clean the water outlet housing gasket surfaces. Coat a new outlet housing gasket with Perfect Seal Sealing Compound B5A-19554-A (ESR-M18P2-A) or equivalent. Position the water outlet housing gasket on intake manifold opening.

1. Install thermostat in crankcase opening with the copper pellet or element toward engine and the thermostat flange positioned in the recess. If thermostat is improperly installed, it will cause a restricted flow of coolant.

1. Position the water outlet housing against the crankcase. Install and tighten the attaching bolts to specification listed at the end of this section.

1. Reposition the alternator/vacuum pump casting. Install attaching bolts.

1. Install vacuum pump with bracket and tighten to specification. Refer to Section 03-05, Accessory Drive.

1. Install alternator and drive belt.

1. Install vacuum pump drive belt.

1. Adjust alternator and vacuum pump drive belts to specification. Refer to Section 03-05, Accessory Drive.

1. Fill and bleed the cooling system. Refer to Section 03-03, Engine Cooling.

1. Connect ground cables to both batteries.

1. Operate the engine until normal operating temperature is reached, then check the coolant level. Inspect cooling system for leaks. WARNING: DO NOT STAND IN LINE WITH OR NEAR RADIATOR FAN WHEN REVVING ENGINE.
   

Overheating can seriously damage a car's engine if left unchecked. Although overheating simply means that a car's engine temperature exceeds normal operating temperatures, the causes of overheating are varied. What follows is a brief list of some of the most common causes of engine overheating.

Faulty Radiator
1. A car that overheats will often have a faulty radiator. A radiator is responsible for cooling hot engine coolant that picks up heat from inside a car's running engine. A radiator "radiates" the heat from engine coolant out into the outside air. A faulty radiator loses its "radiating" effects and allows engine coolant to become overheated, thus rendering it ineffective at adequately cooling and engine.
Faulty Water Pump
2. A faulty or malfunctioning water pump prevents adequate engine coolant flow and can cause a car to overheat. A water pump serves to pressurize and propel engine coolant throughout a car's engine and radiator to increase the heat-reducing capabilities of engine coolant. A faulty water pump loses its ability to adequately pump and propel engine coolant, and can cause a car to overheat.
Coolant System Leaks
3. A leaky engine coolant system reduces the level of circulating engine coolant, which increases engine temperature and leads to engine overheating. Radiators, water pumps, and coolant system hoses and seals--all of these coolant system parts can develop leaks, which can result in low coolant levels and engine overheating.
Faulty Thermostat
4. A car thermostat regulates the flow of engine coolant. A thermostat is a heat-sensitive valve that opens when a car engine reaches a set operating temperature and closes when a car engine is cold and warming up. If a thermostat gets stuck in the closed position, coolant will be prevented from reaching the engine, which will quickly lead to engine overheating and potential engine damage.
Low Engine Oil Level
5. Engine oil, in addition to lubricating an engine's internal parts, helps to keep engine operating temperatures reduced by eliminating friction within the engine. If engine oil levels are low, friction and heat build up inside an engine, a condition that causes increased engine operating temperatures and can lead to engine overheating.

I am not aying that this is what I think it is but I found this on your year BMW

Some BMW, Volkswagen and Audi cars have a common problem that the water pump impeller becomes dislodged from the input shaft of the water pump leaving the water pump looking ok from the outside, but failed on the inside. If your car experiences this problem it will over heat rapidly much like a thermostat failure. For exact specific repair procedures for this repair visit our online repair manual page.




Failed BMW, VW, Audi Water Pump Troubleshooting Procedure

• Check Water Pump Operation - To troubleshoot this problem first drain about a half a gallon out of the cooling system, next remove the thermostat from the engine. With the thermostat out start the engine (stand back) coolant should be forced out of the thermostat port, if not the impeller has dislodged itself from the water pump input shaft and needs to be replaced. To replace the water pump finish draining the remainder of engine coolant, remove the serpentine belt and coolant hoses. Next remove the water pump mounting bolts and remove the water pump. Clean gasket surfaces and reinstall new water pump and reassemble. Refill with coolant and recheck system, top off engine coolant after it has re-cooled.

Yes, it could be the thermostat, but overheating may also be caused by many other things. Ensure engine tune or other mechanical disfunction is not contributing to the overheat problem.
Replace the thermostat, but if overheating persists I suggest you carry out the following checks:-
1) Check coolant system is not leaking fluid due cracked/perished hose or hose fitting, leaking water pump etc,
2) Check engine cooling fan/s are operating as required when engine temp rises above cut-in threshold - check fan control relay is ok,
3) Check for collapsing radiator hose on suction side of water-pump when motor revs raised above idle.
4) Check coolant reservoir pressure cap is serviceable,
5) Check coolant system is properly filled - carry out any bleed off procedure specified to clear any air locks- (ensure heater core coolant flow is turned on).
6) Check condition of radiator core:-
i) for blockage of air flow to (bugs/grass etc),
ii) for core internal blockage to coolant flow,

Your water pump impeller has probably come apart, you will have to do a timing belt job to get a new water pump in there. If you are not comfortable with that, take it to a shop. But yes, to check for water flow, pull the return line on the coolant bottle (little one on the top) and hold it so any water that would come out would go into the bottle, and start the engine (the engine has to be cold to try this test) there should be an almost immediate flow of water, if no water comes out, even if you give it a little gas, (should be a decent flow, not a trickle) than your water pump is in fact dead, very very common problem on 1.8 & 2.0 Jetta's, Beetles, and Passats.

you probably have air trapped in the system. Have dealer or radiator shop bleed system.

I assume your cooling fan is working.

what engine v6 or v8?

SECTION 303-03: Engine Cooling 2001 Lincoln LS Workshop Manual
DESCRIPTION AND OPERATION Engine Cooling The cooling system components include the:

• block heater (optional)
• cylinder head temperature sensor
• fan blade, fan motor and fan shroud assembly
• radiator
• pressure relief cap
• degas bottle
• radiator draincock
• water pump
• oil cooler (optional)
• water thermostat

The water thermostat:

• controls the engine coolant temperature.
• allows quicker engine warm-up.

The degas bottle:

• provides a location for system fill.
• contains coolant expansion and system pressurization.
• provides air separation during operation.
• replenishes the engine coolant to the system.

The fan blade draws air through the radiator to help cool the engine coolant.
The fan motor:

• operates only when the engine is running.
• will not operate when the engine is off.

The engine coolant flows:

• from the lower radiator hose to the water pump.
• from the water pump to the engine block and the cylinder heads.

A closed water thermostat returns the engine coolant to the water pump. An open water thermostat allows the engine coolant to flow to the radiator.
Unsatisfactory coolant materials:

• Alcohol-type antifreeze does not provide adequate water pump lubrication.
  • has lower boiling point
  • reduced antifreeze protection
• Alkaline brine solutions will cause serious engine cooling system damage.

The cylinder head temperature sensor provides a signal to the temperature gauge.

• will invoke failsafe cooling.

The optional block heater:

• electrical heating element is installed in the block cooling jacket.
  • uses a standard 110V (220V in Europe) electrical supply
• keeps the engine coolant warm during cold weather.

The auxiliary water pump (3.9L only):

• provides heater coolant flow boost.
• has a secondary function of providing engine-off cooling.