Exhaust Gasket - Rear

This procedure covers replacement of the rear exhaust gasket(s) on the 2021-2024 Ford F-150 3.5L EcoBoost V6, located between the catalytic converters and rear exhaust section.

Warnings

Tools required

Parts

• Rear exhaust flange gasket × 1 — Use OEM specification
• Exhaust flange bolts (if corroded) × 4 — M10 x 1.5 x 35mm

Preparation

1. Park vehicle on level surface and engage parking brake
2. Place wheel chocks at front wheels
3. Allow exhaust system to cool completely (minimum 2 hours after last operation)
4. Raise rear of vehicle with floor jack and support securely on jack stands at frame rails
5. Spray all exhaust flange bolts and hanger bolts with penetrating oil and allow to soak 15-30 minutes

Procedure

1. 1

   Locate rear exhaust flange connection
   Position yourself under the vehicle near the transmission area. Identify the rear exhaust flange connection where the catalytic converters connect to the rear exhaust pipe section. This is typically located just behind the transmission crossmember. The flange will have 4 bolts connecting the two sections.
2. 2

   Support rear exhaust section
   Position a floor jack or exhaust jack under the rear muffler section to provide slight support. This prevents the exhaust from hanging on the flange and makes separation easier. Raise jack until it just contacts the exhaust but does not lift it.
3. 3

   Remove rear O2 sensor (if necessary for clearance)
   If the rear oxygen sensor interferes with flange access, disconnect the electrical connector by pressing the release tab. Use a 22mm O2 sensor socket to remove the sensor. Turn counterclockwise while supporting the sensor wire. Set sensor aside in a clean location.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
4. 4

   Loosen exhaust hanger bolts
   Locate the rubber exhaust hangers supporting the rear exhaust section within 2 feet of the flange. Use a 13mm socket to loosen (but do not remove) the hanger mounting bolts. This allows the exhaust to move slightly for easier flange separation.
   Torque spec

   Hanger Bolts23 Nm (17 lb-ft)
5. 5

   Remove exhaust flange bolts
   Using a 15mm socket, remove all 4 bolts from the exhaust flange connection. Work in a crisscross pattern, loosening each bolt gradually rather than removing one completely before moving to the next. This prevents binding. If bolts are severely corroded, apply additional penetrating oil and allow to soak, or carefully apply heat with a propane torch while avoiding damage to nearby components.
6. 6

   Separate exhaust flange
   With all bolts removed, gently lower the jack supporting the rear exhaust section approximately 2-3 inches. The flange should separate. If stuck, use a pry bar carefully between the flanges to break the seal. Do not pry against the catalytic converter body. Tap lightly with a rubber mallet if needed. Once separated, lower the rear section enough to access the gasket.
7. 7

   Remove old gasket and clean flange surfaces
   Remove the old exhaust gasket from between the flanges. Use a wire brush to thoroughly clean both flange mating surfaces, removing all old gasket material, carbon buildup, and corrosion. The surfaces must be clean and flat for proper sealing. Inspect both flanges for cracks, warping, or damage. Replace components if flange surfaces are not flat.
8. 8

   Install new exhaust gasket
   Position the new exhaust gasket over the studs on the front flange (catalytic converter side). Ensure the gasket is oriented correctly with any directional markings facing the proper direction. The gasket should sit flat against the flange surface without being forced.
9. 9

   Reconnect exhaust flange
   Raise the rear exhaust section using the support jack, aligning the bolt holes in the rear flange with the studs or holes in the front flange. The gasket should remain in position between the flanges. Align all four bolt locations before proceeding.
10. 10

    Install and torque flange bolts
    Apply anti-seize compound to the threads of the exhaust flange bolts. Install all 4 bolts finger-tight, ensuring the gasket remains properly positioned. Using a torque wrench, tighten the bolts in a crisscross pattern to the specified torque. First pass: 20 Nm, second pass: 45 Nm (33 lb-ft) final specification. This ensures even gasket compression.
    Torque spec

    Exhaust Flange Bolts45 Nm (33 lb-ft)
11. 11

    Reinstall exhaust hangers
    Adjust the rear exhaust section position so the rubber hangers align properly with minimal stress. Tighten the hanger mounting bolts to specification using a torque wrench. Ensure the exhaust is not twisted or under tension.
    Torque spec

    Hanger Bolts23 Nm (17 lb-ft)
12. 12

    Reinstall O2 sensor (if removed)
    If the oxygen sensor was removed, apply a small amount of anti-seize compound to the threads only (avoid contaminating the sensor tip). Thread the sensor in by hand, then tighten to specification using the O2 sensor socket and torque wrench. Reconnect the electrical connector until it clicks.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)

Reassembly

1. Remove support jack from under rear exhaust section
2. Visually inspect all connections to ensure proper alignment and no interference with driveshaft, frame, or body components
3. Verify exhaust system hangs level with approximately 1 inch clearance from all surrounding components

Verification

• Lower vehicle from jack stands
• Start engine and allow to idle for 2-3 minutes while listening for exhaust leaks at the rear flange area
• Check for visible exhaust gas escaping from flange connection
• Rev engine to 2000 RPM briefly and verify no leaks or rattling sounds
• After 5 minutes of running, carefully feel around flange area (without touching hot components) for escaping exhaust gas
• Test drive vehicle and verify no exhaust smell in cabin and normal exhaust sound
• Re-inspect all fasteners after first 50 miles of driving to ensure proper torque retention

Related trouble codes on this vehicle

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