Intake Manifold Gasket

Replace the intake manifold gaskets on the 390ci V8 engine to restore proper sealing between the intake manifold and cylinder heads, preventing vacuum leaks and coolant/oil seepage.

Warnings

Tools required

Parts

• Intake manifold gasket set × 1 — Use OEM specification for AMC 390 V8
• Intake manifold end seals (front and rear) × 1 — Use OEM specification
• RTV silicone gasket maker (high-temp) × 1 — Red or black RTV rated to 500°F
• Carburetor base gasket × 1 — Use OEM specification
• Coolant × 1 — Ethylene glycol based, appropriate for cast iron

Fluids

• Engine coolant (50/50 mix) — 4 qt

Preparation

1. Ensure engine is completely cold to the touch
2. Disconnect negative battery cable
3. Place drain pan under radiator and drain coolant by opening petcock or removing lower radiator hose
4. Remove air cleaner assembly and set aside
5. Label all vacuum lines with tape and marker for reinstallation reference
6. Photograph or diagram vacuum line routing, ignition wire routing, and linkage connections
7. Relieve any residual fuel system pressure by loosening fuel line at carburetor slightly
8. Have clean workspace prepared for intake manifold storage during gasket replacement

Procedure

1. 1

   Remove carburetor and linkages
   Disconnect throttle linkage, kickdown linkage (if automatic transmission), and choke cable from carburetor. Disconnect fuel line at carburetor inlet using two wrenches to prevent twisting the line. Disconnect all vacuum lines from carburetor ports. Remove carburetor mounting nuts (typically 4) and carefully lift carburetor straight up off manifold. Plug intake manifold openings with clean rags to prevent debris entry.
2. 2

   Disconnect electrical and vacuum connections
   Disconnect distributor vacuum advance line. Disconnect coolant temperature sender wire. Remove PCV valve from valve cover and disconnect PCV hose from intake manifold. Disconnect brake booster vacuum line from intake manifold. Disconnect any remaining vacuum accessories such as heated air intake control or emissions equipment.
3. 3

   Remove distributor
   Mark the position of distributor rotor relative to distributor body with a marker. Mark the position of distributor body relative to intake manifold. Remove distributor hold-down bolt and clamp. Carefully pull distributor straight up and out of engine. Do NOT rotate the engine crankshaft with distributor removed or timing will be lost.
4. 4

   Disconnect coolant hoses and bypass
   Disconnect upper radiator hose from intake manifold outlet (if equipped). Disconnect heater hoses from intake manifold fittings. Disconnect coolant bypass hose at front of intake manifold. Have drain pan positioned to catch residual coolant.
5. 5

   Remove intake manifold bolts
   Starting from the outer bolts and working toward the center, loosen all intake manifold mounting bolts in a crisscross pattern. The AMC 390 typically uses 12 bolts securing the intake manifold. Remove all bolts completely and store in organized manner. Note that some bolts may be different lengths.
6. 6

   Remove intake manifold
   Carefully pry intake manifold loose from cylinder heads using flathead screwdriver in designated pry points (typically at front or rear). Do not pry between gasket surfaces. Once loose, lift manifold straight up and away from engine. The manifold is heavy; use proper lifting technique or get assistance. Place manifold on clean work surface gasket-side up.
7. 7

   Remove old gaskets and clean surfaces
   Remove and discard old intake manifold gaskets and end seals. Use gasket scraper and razor blade to carefully remove all old gasket material from cylinder head surfaces and intake manifold mating surfaces. Be extremely careful not to gouge aluminum or cast iron surfaces. Clean all surfaces thoroughly with brake cleaner and lint-free rags. Inspect valley cover for damage or warping. Use shop vacuum to remove all debris from valley area and intake ports. Surfaces must be completely clean and dry.
8. 8

   Inspect intake manifold and valley cover
   Inspect intake manifold for cracks, warping, or damage. Check manifold mating surfaces with straightedge for warping (should be flat within 0.003 inches). Inspect valley cover for corrosion or damage. Replace valley cover if compromised. Check all coolant passages for blockage or corrosion buildup.
9. 9

   Install new gaskets and seals
   Apply thin bead of RTV silicone to cylinder block at front and rear where end seals will contact timing cover and rear main cap. Position new front and rear end seals, ensuring they sit flush against block surface. Apply small dabs of RTV at four corners where end seals meet cylinder head gaskets to lock gaskets in place. Position new intake manifold gaskets on cylinder heads, ensuring all port openings align correctly. Some gaskets have alignment tabs that lock into cylinder head holes. Apply thin bead of RTV around coolant passage openings on gasket surface.
10. 10

    Install intake manifold
    Carefully lower intake manifold into position, ensuring gaskets remain in place and all bolt holes align. Guide manifold straight down to avoid shifting gaskets. Once seated, verify all gaskets are properly positioned by checking through bolt holes. Install all intake manifold bolts finger-tight, starting from center and working outward.
11. 11

    Torque intake manifold bolts
    Torque intake manifold bolts in three stages using a crisscross pattern from center outward. First pass: torque all bolts to 15 ft-lbs. Second pass: torque all bolts to 25 ft-lbs. Final pass: torque all bolts to 35-40 ft-lbs (consult factory specifications for exact value). Allow RTV silicone to set for 15-20 minutes before proceeding.
12. 12

    Reinstall distributor
    Ensure engine crankshaft has not been rotated. Align distributor rotor with mark made during disassembly. Insert distributor into engine, aligning distributor body with mark on intake manifold. The distributor should drop into place when oil pump drive shaft engages. If it does not seat fully, rotate rotor slightly while applying gentle downward pressure. Install hold-down clamp and bolt, leaving slightly loose for timing adjustment.
13. 13

    Reconnect coolant hoses and electrical connections
    Reconnect heater hoses to intake manifold fittings using new clamps if originals are degraded. Reconnect upper radiator hose and coolant bypass hose. Reconnect coolant temperature sender wire. Install PCV valve and connect PCV hose. Reconnect distributor vacuum advance line. Reconnect brake booster vacuum line and all other vacuum lines according to diagram or labels created during disassembly.
14. 14

    Reinstall carburetor
    Remove rags from intake manifold openings. Install new carburetor base gasket. Carefully position carburetor onto intake manifold, ensuring gasket remains in place. Install carburetor mounting nuts and tighten in crisscross pattern to 12-15 ft-lbs (hand-tight plus 1/4 turn). Reconnect fuel line using two wrenches. Reconnect throttle linkage, kickdown linkage, and choke cable. Reconnect all vacuum lines to carburetor ports according to reference diagram.
15. 15

    Refill coolant and prepare for startup
    Close radiator petcock or reinstall lower radiator hose if removed. Fill cooling system with appropriate 50/50 coolant mix. Fill until radiator is full and coolant is visible in overflow tank at proper level. Reconnect negative battery cable. Double-check all connections, hoses, and linkages for proper installation. Install air cleaner assembly.

Reassembly

1. After initial startup, allow engine to reach operating temperature and check for coolant leaks around intake manifold
2. Check for vacuum leaks by listening for hissing sounds around intake manifold base and gasket area
3. Retorque intake manifold bolts to specification after engine has completed one full heat cycle (warm up and cool down)
4. Adjust distributor timing to factory specification using timing light (typically 5° BTDC at idle for 1970 AMC 390)
5. Adjust carburetor idle speed and mixture after engine reaches operating temperature
6. Top off coolant level after first heat cycle as air pockets work out of system

Verification

• Start engine and verify smooth idle with no rough running or misfiring
• Check for coolant leaks at all intake manifold hose connections and gasket surfaces
• Check for vacuum leaks by spraying carburetor cleaner around intake manifold base while engine idles - RPM change indicates leak
• Verify proper engine operating temperature (180-195°F) and confirm thermostat operation
• Test throttle response and verify all linkages operate smoothly through full range
• Check for oil contamination in coolant or coolant contamination in oil which would indicate internal gasket leak
• After 500 miles of operation, recheck intake manifold bolt torque and retighten if necessary

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