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FORD BRONCO -> ELECTRICAL; EEC/PCM, Self-Test, wiring diagrams... -> Driveability TSB 89-12-09 for 84-89 351HO Econoline & F Series
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Driveability TSB 89-12-09 for 84-89 351HO Econoline & F Series; "...A rich running engine at high altitude may be caused by incorrect vacuum hose and electrical connections. The overly rich condition can cause stalling, hard starting, or poor fuel economy. Listed below are some incorrect vacuum and electrical connections that have been found in the field. Technicians should check for these conditions before trying further diagnosis. Spark port or EGR port vacuum supplied to 5" and 10" Hg. vacuum switch instead of manifold vacuum. Manifold vacuum signal applied to off-idle vacuum switch instead of EGR port vacuum signal. Wrong vacuum hose routing for the altitude compensator system (check vacuum schematic). Wrong electrical connections - like connecting a wire to a 7V source when it should be connected to a 12V power source. Vacuum filter is restricted or plugged by a cap or other object..."
Source: by Ford via Steve83 (Steve, That dirty old truck) at