Coolant Bypass Pipe

Replacement of the coolant bypass pipe on the 390ci V8 engine, which routes coolant between the water pump and engine block to ensure proper circulation during warm-up.

Warnings

Tools required

Parts

• Coolant bypass pipe × 1 — Use OEM specification or AMC 390 V8 compatible
• Bypass pipe gaskets/O-rings × 1 — Use OEM specification
• Hose clamps (if deteriorated) × 2 — 5/8" to 3/4" worm-drive clamps

Fluids

• Engine coolant (ethylene glycol or compatible) — 3 qt

Preparation

1. Ensure engine is completely cold to touch
2. Position vehicle on level ground and engage parking brake
3. Place drain pan under radiator drain cock
4. Remove radiator cap slowly to release any residual pressure
5. Open radiator drain cock and drain coolant into pan until level drops below bypass pipe location
6. If equipped, disconnect negative battery cable to prevent accidental starter engagement

Procedure

1. 1

   Access the bypass pipe
   The bypass pipe on the AMC 390 is located on the front driver's side of the engine block, connecting the water pump to the block. Remove the air cleaner assembly and any interfering vacuum lines or brackets for clear access. Label vacuum lines if needed for reinstallation.
2. 2

   Disconnect bypass hose from water pump
   Locate the rubber bypass hose connecting the pipe to the water pump. Using flathead screwdriver or hose clamp pliers, loosen the hose clamp at the water pump end. Twist the hose gently to break the seal, then slide the hose off the water pump fitting. If hose is stuck, carefully cut it rather than risk damaging the water pump neck.
3. 3

   Remove bypass pipe mounting hardware
   The bypass pipe is typically secured with 1-2 bolts threading into the engine block. Apply penetrating oil to bolt threads and allow to soak for 10-15 minutes. Remove the mounting bolts using appropriate socket or wrench. Keep hardware organized for reinstallation.
4. 4

   Disconnect pipe from engine block
   The bypass pipe inserts into a bore in the engine block with a gasket or O-ring seal. Gently wiggle and pull the pipe to extract it from the block. If pipe is corroded in place, apply more penetrating oil and use a gentle twisting motion. Do not pry against aluminum or soft metal surfaces.
5. 5

   Remove old gasket material
   Using gasket scraper and wire brush, clean all old gasket material and corrosion from the engine block mating surface and bypass pipe mounting area. Ensure the bore in the block is clean and free of debris. Clean the water pump connection point as well.
6. 6

   Inspect mating surfaces
   Examine the engine block bore for corrosion, pitting, or damage. Check that the bypass passage is clear and not blocked by debris or corrosion. Inspect the water pump neck for damage if reusing existing hose. Light surface corrosion can be cleaned with wire brush.
7. 7

   Prepare new bypass pipe
   Install new gaskets or O-rings onto the new bypass pipe according to manufacturer instructions. Lightly coat gasket surfaces with coolant or appropriate sealant if specified. Ensure O-rings are seated properly in their grooves.
8. 8

   Install bypass pipe into block
   Carefully insert the bypass pipe into the engine block bore, aligning the mounting holes. Push pipe firmly into place ensuring the gasket/O-ring seats completely. The pipe should sit flush against the block mounting surface.
9. 9

   Install mounting hardware
   Thread mounting bolts into the engine block by hand first to ensure proper alignment and avoid cross-threading. Tighten bolts evenly in a crossing pattern to manufacturer specifications. Without specific torque values, tighten securely but do not overtighten - typically 15-20 ft-lbs is appropriate for this application.
10. 10

    Connect bypass hose
    Slide the bypass hose onto both the pipe and water pump fittings. Ensure hose is pushed fully onto both connections. Position hose clamps approximately 1/4 inch from the end of each connection and tighten securely. Replace hose if it shows cracking, hardening, or deterioration.
11. 11

    Refill cooling system
    Close radiator drain cock. Fill cooling system with appropriate 50/50 mixture of coolant and water through the radiator cap opening. Fill until coolant is visible at the top of the radiator. Squeeze upper radiator hose several times to help purge air bubbles from the system.
12. 12

    Initial leak check
    With radiator cap still off, start engine and allow to idle. Watch bypass pipe connections closely for any coolant leaks. Add coolant as level drops during warm-up. When thermostat opens and coolant begins circulating through radiator, top off coolant level.

Reassembly

1. Reinstall air cleaner assembly and any removed vacuum lines or brackets according to labels or diagram
2. Reconnect negative battery cable if disconnected
3. Top off coolant overflow reservoir to FULL COLD mark

Verification

• Run engine until fully warmed to operating temperature and verify no leaks at bypass pipe connections
• Check that heater produces hot air, confirming proper coolant circulation through bypass system
• Allow engine to cool, then recheck coolant level and top off if necessary - air pockets may cause initial drop
• Inspect bypass pipe area after first drive cycle for any signs of seepage or wetness
• Monitor coolant level and temperature gauge over next few days to ensure system is functioning properly

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