I cannot find the other end of a vacuum-hose routing

Check the fuel pressure regulator vacuum hose if it has fuel if so the regulator is ruptured.

EEC IV THEORY/OPERATION -1985 Ford Pickup F150
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DESCRIPTION
The center of the EEC-IV system is the Electronic Control Assembly (ECA). The ECA receives information from various sensors and switches. Based on information received and the operation program in the ECA's memory, the ECA generates output signals to control engine operation. The calibration module for EEC-IV system is mounted inside the ECA. The ECA is located in the passenger compartment, in one of the following locations: under the center console, under dash on left side of steering column, under passenger seat, or under dash behind right kick panel.
The 2.3L and 5.0L EFI models use the Bosch/Ford Electronic Fuel Injection system (EFI). The EFI system is classified as a multi point, pulse time, speed density, fuel injection system. The EEC-IV computer accepts inputs from various engine sensors to compute the required fuel flow rate necessary to maintain a prescribed air/fuel ratio throughout the entire engine operational range. The computer then outputs a command to the fuel injectors to meter the appropriate quantity of fuel.
The EEC-IV system controls 3 major areas of engine operation: Air/fuel mixture, ignition, and emission control. Additionally the system can control A/C compressor clutch operation and idle speed. The system provides self-diagnostic capabilities.
On all other models, the air/fuel mixture control is accomplished by use of a feedback carburetor (FBC). The ignition system is controlled by the ECA through a Thick Film Ignition (TFI-IV) module. Ignition timing (advance or retard) and dwell are controlled with this system to improve ignition system performance.
Emission control components controlled by this system include EGR and canister purge. These systems are normally off, but are turned on when the engine is ready to operate with the mixture change caused by EGR and canister purge operation.

Be sure to follow the procedure below to make sure your base timing is set correctly, if not performance will suffer greatly

90ford27 ENGINE/ EMISSIONS DIAGNOSIS SHOP MANUAL- 1983-1990---NOTE ADDED TO UPDATE INITIAL TIMING PROCEDURE
Article No. 90-18B-2
FORD: 1983 EXP 1984-86 LTD 1984-90 CROWN VICTORIA, MUSTANG, TEMPO, THUNDERBIRD 1986-90 TAURUS

LINCOLN-MERCURY: 1983 LN7
1983-84 LYNX 1984-86 MARQUIS 1984-90 CONTINENTAL, COUGAR, GRAND MARQUIS, MARK VII, TOPAZ, TOWN CAR 1986-90 SABLE

LIGHT TRUCK: 1984-90 BRONCO II, BRONCO, ECONOLINE, F-150, F-250, F-350, RANGER 1986-90 AEROSTAR

ISSUE: All 1983 through 1990 vehicles with TFI-IV ignition systems may exhibit spark knock/ping if base ignition timing was set after starting the engine with a remote starter. If the engine is started with a remote starter (start wire "S" terminal disconnected at the starter relay) the ignition module will remain in the start mode (timing retarded). Ignition timing will be incorrectly advanced if base timing is set while the module is in the start mode. The engine must be started with the start wire connected, using the ignition switch to correctly set the base ignition timing. The following Shop Manuals and pages are affected:

See attachment below

It's common to lose vacuum to the heater controls when under acceleration. The mode doors are typically spring-loaded to default to the defrost position so even though your feet might freeze, the windshield will be clear. First check to see if there is any type of storage canister that may have a leaking check valve. The valve / canister will be in the small hose going to the heater controls. If all you find is a tiny check valve, replace it with a larger one with a built-in storage canister. I know you can get them at the Chrysler dealer's parts department for the same symptom. Their original valve was about the size of a stack of three nickels. The replacement is about 1 1/2" in diameter and about 2" long. You can probably find them in salvage yards too.

You may find it in a junk yard. Follow the vacuum diagram under the hood to install new hoses if need be.

See and process my profile. They are available at most parts stores. Keepers for ever!

Love, Turddog