Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSFORD1987BRONCO II 4WD V6-177 2.9LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTTECHNICAL SERVICE BULLETINSBY SYMPTOMCUSTOMER INTERESTENGINE - ROUGH IDLING CONDITION
1987 Ford Bronco II 4WD V6-177 2.9L
Engine - Rough Idling Condition
1987 Ford Bronco II 4WD V6-177 2.9LSECTION Engine - Rough Idling Condition
89ford34
^ FUEL SYSTEM-EFI ENGINES - AIR BY-PASS VALVE REPLACEMENT PART INOPERATIVE
^ IDLE - ROUGH-REPLACEMENT AIR BY-PASS VALVE INOPERATIVE
Article No. 89-24-4
FORD: 1983-88 ESCORT, EXP, THUNDERBIRD 1984-88 MUSTANG 1986-88 CROWN VICTORIA, TAURUS 1988 TEMPO
LINCOLN-MERCURY: 1983 LN7
1983-86 LYNX 1983-88 COUGAR 1984-86 CAPRI 1986-88 CONTINENTAL, GRAND MARQUIS, MARK VII, SABLE, TOWN CAR 1988 TOPAZ
MERKUR: 1985-88 XR4TI 1988 SCORPIO
LIGHT TRUCK: 1985-88 BRONCO, F-150, F-250, RANGER 1986-88 AEROSTAR, BRONCO II, ECONOLINE 1987-88 F-350 1988 F SUPER DUTY
ISSUE: A rough idle created when the air by-pass valve is replaced may be caused by a revision made to the valve's wiring harness.
Vehicles built before the 1989 model year had a diode in the air by-pass valve wiring harness. The new replacement valves have a 3D marking on the plastic portion of the solenoid, above the connector cap, Figure 1. They have a diode in the air by-pass valve.
On the air by-pass valves used on vehicles built before the 1989 model year, the positive and negative leads are not important to the operation of the valve. However, the polarity on the new replacement air by-pass valve is important because if the wiring harness is reversed the air by-pass valve will not work.
ACTION: Reverse the wires in the air by-pass valve connector and retest the valve. Refer to the following EFI Engine Application Chart for engines using these air by-pass valves.
OTHER APPLICABLE ARTICLES: None
WARRANTY STATUS: INFORMATION ONLY
OASIS CODES: 620800
^ FUEL SYSTEM-EFI ENGINES - AIR BY-PASS VALVE REPLACEMENT PART INOPERATIVE
^ IDLE - ROUGH-REPLACEMENT AIR BY-PASS VALVE INOPERATIVE
Article No. 89-24-4
FORD: 1983-88 ESCORT, EXP, THUNDERBIRD 1984-88 MUSTANG 1986-88 CROWN VICTORIA, TAURUS 1988 TEMPO
LINCOLN-MERCURY: 1983 LN7
1983-86 LYNX 1983-88 COUGAR 1984-86 CAPRI 1986-88 CONTINENTAL, GRAND MARQUIS, MARK VII, SABLE, TOWN CAR 1988 TOPAZ
MERKUR: 1985-88 XR4TI 1988 SCORPIO
LIGHT TRUCK: 1985-88 BRONCO, F-150, F-250, RANGER 1986-88 AEROSTAR, BRONCO II, ECONOLINE 1987-88 F-350 1988 F SUPER DUTY
Figure 1:
ISSUE: A rough idle created when the air by-pass valve is replaced may be caused by a revision made to the valve's wiring harness.
Vehicles built before the 1989 model year had a diode in the air by-pass valve wiring harness. The new replacement valves have a 3D marking on the plastic portion of the solenoid, above the connector cap, Figure 1. They have a diode in the air by-pass valve.
On the air by-pass valves used on vehicles built before the 1989 model year, the positive and negative leads are not important to the operation of the valve. However, the polarity on the new replacement air by-pass valve is important because if the wiring harness is reversed the air by-pass valve will not work.
ACTION: Reverse the wires in the air by-pass valve connector and retest the valve. Refer to the following EFI Engine Application Chart for engines using these air by-pass valves.
OTHER APPLICABLE ARTICLES: None
WARRANTY STATUS: INFORMATION ONLY
OASIS CODES: 620800
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.