I would check for a vacuum leak first. you can start the vehicle and spray starting fluid on the vacuum lines. If the Idle raises then you have a vacuum leak. NOTICE: do not spray starting fluid around or on hot engine parts or electrical elements, starting fluid has a very low flash point and will ignite very easily.
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If you lost a vacuum line then the heater would let hot air out the vents with or without the fan motor running. Do this test with the engine up to operating temperature Touch both hoses with the heater on and temp control to the hottest position. Now check the temperature of both hoses. They should be the same. If not that indicates a blockage of water to the heater core. Test the water flow valve too. It can be plugged with rust or not moving to let water into the core. The core itself can be plugged too. If you really need a hose diagram, a Chilton's manual will be your source of information. The engine won't idle well with a vacuum hose off.
It will be an intake manifold vacuum tap. Check all the taps from the intake manifold. The fpr hose may be teed off from another hose to another device. Just trace them all down, and you should find it. There would be a vacuum diagram scheme on the emissions label under the hood- see if it is still legible.
In 1989, all vehicles sold in the US were required to have vacuum routing stickers placed in the engine compartment. There should be a vacuum diagram sticker on the fan shroud. If it has peeled off, you'll need to find a shop manual and figure out which of the many possible diagrams apply (not an easy task).
For 2000 Lincoln LS 3.9L SFI DOHC
8cyl... Radiator - Removal & Installation
Drain the engine cooling system.
Remove the upper radiator sight shield.
Remove the air cleaner outlet tube.
Remove the six bolts and the two radiator upper support brackets.
Remove the upper radiator hose.
Remove the bolt and position the receiver drier aside.
Disconnect the dual flow coolant valve electrical connector and
the A/C line from the fan shroud.
3.0L engines only:
Disconnect the Throttle Position (TP) sensor and the Idle Air
Control (IAC) valve electrical connectors.
Remove the bolts.
Remove the bracket.
3.9L engines only:
Remove the bolt and position the electric water pump aside.
Disconnect the high pressure cooling fan bracket and line.
Inspect the seal and install a new seal if necessary.
Disconnect the return hose from the cooling fan and shroud.
Separate the return hose from the fan shroud and position aside.
Remove the two bolts and the fan shroud assembly.
Remove the A/C condenser.
Remove the two bolts and position the multi-cooler assembly aside.
Remove the bolts and the condenser support brackets.
Remove the radiator.
Install the following:
Condenser support brackets
Fan shroud assembly
Connect the return hose to the fan shroud
Connect the high pressure cooling fan bracket and line.
3.9L engines only:
Replace the bolt and position the electric water pump aside.
3.0L engines only: Connect the Throttle Position (TP) sensor and the Idle Air Control
(IAC) valve electrical connectors.
Replace the following
The upper radiator sight shield.
The air cleaner outlet tube.
The six bolts and the two radiator upper support brackets.
The upper radiator hose.
The bolt and position the receiver drier aside.
Hope help with this (remember rated and comment this).
A vehicle that old will suffer from brittle and cracked vacuum lines. I would say there is a bad, or several bad lines leaking away vacuum from the intake manifold. Don't forget to check the hose going to the PCV on the valve cover. A vacuum leak will also cause the engine to stumble and miss intermittently when idling.
rough-idling engine can signal a number of positive crankcase ventilation (PCV) valve problems, such as a clogged valve or a plugged hose. But before beginning the functional checks, double check the PCV valve part number to make certain the correct valve is installed. If the correct valve is being used, continue by disconnecting the PCV valve from the valve cover, intake manifold, or hose. Start the engine and let it run at idle. If the PCV valve is not clogged, a hissing is heard as air passes through the valve. Place a finger over the end of the valve to check for vacuum.
With the engine at idle, vacuum should be felt at the PCV valve.