My 90 with a 4.0 has one, just aft of where the pipes come togather, ez to get to. Have you tuned it up, plugs, and wires? Also on mine the HCT sensor was bad, making it run in choke mode, burnning alot of fuel and took out a new O2 sensor.
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Allen D
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64 Galaxie 500 with 428 FE
66 F-100 4x4 With 67 GT 390 Hi Pro and Toyota Power Steering
67 F-100 2wd for parts
68 F350 crew truck
80 Bronco with a 460 engine
90 Aerostar With 4.0
91 Aero 4x4 with 4.0
93 SHO Taurus
07 Mustang with 4.0
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Oxygen Sensor 
Linear Mode
