Remove and replace the exhaust downpipe connecting the exhaust manifolds to the intermediate exhaust pipe on the 390ci V8 engine.

Warnings

Tools required

Parts

• Downpipe assembly × 1 — Use OEM specification or aftermarket equivalent for 390ci V8
• Exhaust manifold to downpipe gaskets or donuts × 2 — Use OEM specification
• Downpipe to intermediate pipe gasket or donut × 1 — Use OEM specification
• Exhaust flange bolts and nuts × 8 — 5/16 inch or 3/8 inch grade 8

Preparation

1. Ensure engine and exhaust system are completely cold to the touch
2. Park vehicle on level, solid ground and engage parking brake
3. Place wheel chocks behind rear wheels
4. Raise front of vehicle with floor jack to provide adequate working clearance underneath
5. Support vehicle securely on jack stands positioned at factory jacking points on frame
6. Spray all exhaust flange bolts and nuts with penetrating oil and allow to soak for at least 15 minutes

Procedure

1. 1

   Access downpipe area
   Position yourself under the vehicle with adequate lighting. Identify the downpipe running from both exhaust manifolds that converges into a Y-pipe configuration before connecting to the intermediate pipe. Inspect all connection points for rust, corrosion, and accessibility.
2. 2

   Disconnect downpipe from intermediate pipe
   Locate the flange connection where the downpipe meets the intermediate exhaust pipe, typically near the transmission crossmember. Using a 9/16 inch or 15/16 inch wrench depending on hardware, remove the two bolts securing this flange. If bolts are severely corroded and break, use a reciprocating saw or cutting wheel to separate the connection.
3. 3

   Remove driver side exhaust manifold flange
   Locate the flange connection between the driver side exhaust manifold and the downpipe. Remove the two bolts and nuts securing this connection using appropriate sockets. Keep track of any conical washers or spring washers that may be present. If fasteners break, drill out remaining studs or use a stud extractor.
4. 4

   Remove passenger side exhaust manifold flange
   Locate the flange connection between the passenger side exhaust manifold and the downpipe. Remove the two bolts and nuts securing this connection. This connection may be more difficult to access due to steering linkage and front suspension components. Work carefully to avoid damaging surrounding components.
5. 5

   Separate downpipe from hangers
   Check for any exhaust hangers or support brackets holding the downpipe assembly in place. The 390ci V8 typically has at least one hanger near the Y-pipe junction. Remove any retaining hardware and slide rubber isolators off their mounting points. Support the downpipe with one hand to prevent it from falling.
6. 6

   Remove downpipe assembly
   Carefully maneuver the entire downpipe assembly out from under the vehicle. The Y-pipe configuration may require rotation and angling to clear the frame, crossmember, and steering components. If pipe is stuck at slip joints, use a pipe spreader tool or carefully apply leverage with a pry bar.
7. 7

   Clean all mating surfaces
   Use a wire brush to thoroughly clean the exhaust manifold flanges and the intermediate pipe flange. Remove all old gasket material, carbon deposits, and rust. Inspect manifold flanges for cracks or warping. Clean the flange surfaces on the new downpipe assembly as well.
8. 8

   Install new gaskets
   Install new exhaust gaskets or donuts at all three connection points. For the manifold connections, the conical or donut gaskets typically fit into the recessed area of the exhaust manifold outlet. Ensure gaskets are properly seated before proceeding.
9. 9

   Position new downpipe assembly
   Carefully guide the new downpipe assembly into position under the vehicle, starting with the rear connection to the intermediate pipe. Align all three flange connections simultaneously, which may require patience and minor adjustment. You may need an assistant to support the pipe while making final alignment.
10. 10

    Install intermediate pipe connection
    Install the bolts at the downpipe to intermediate pipe flange. Thread bolts by hand first to ensure proper alignment. Snug the bolts finger-tight but do not fully tighten yet to allow for final alignment adjustments.
11. 11

    Install manifold flange hardware
    Install bolts and nuts at both exhaust manifold flanges. Apply a small amount of anti-seize compound to threads if available. Thread all fasteners by hand first, ensuring gaskets remain properly seated. Snug all hardware finger-tight.
12. 12

    Tighten all connections
    Working from front to rear, tighten all exhaust flange connections. Start with the manifold flanges, alternating between bolts on each flange to ensure even seating. Typical torque for exhaust flange hardware on this application is 25-35 ft-lbs, though no specific torque specification is provided. Finish by tightening the intermediate pipe connection. Verify all connections are secure.
13. 13

    Reinstall exhaust hangers
    Reinstall any exhaust hangers or support brackets that were removed. Ensure rubber isolators are properly seated on their mounting points and that the exhaust system hangs with proper clearance from frame, body, and drivetrain components.

Reassembly

1. Double-check that all fasteners are tight and no tools or parts have been left under the vehicle
2. Verify adequate clearance between downpipe and all surrounding components including steering linkage, frame rails, and crossmembers
3. Lower vehicle from jack stands carefully and remove jack stands before lowering completely

Verification

• Start engine and listen for exhaust leaks at all three flange connections while engine is cold
• Check for visible exhaust smoke or puffing at connection points
• Allow engine to reach operating temperature and recheck all connections for leaks
• Perform a short test drive and listen for any abnormal exhaust sounds or rattles
• After test drive, re-inspect all connections for signs of leakage or loosening

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