Cooling System Pressure Test

Perform a pressure test on the cooling system to identify leaks in the radiator, hoses, water pump, heater core, and engine block or head gaskets.

Warnings

Tools required

Parts

• Radiator cap (if found defective) × 1 — 16 PSI rating for 390ci V8

Preparation

1. Ensure engine is completely cold to the touch
2. Park vehicle on level surface and engage parking brake
3. Verify cooling system is filled to proper level before testing
4. Visually inspect all accessible hoses, clamps, and connections for obvious damage or deterioration
5. Have replacement coolant available in case system needs topping off after repairs

Procedure

1. 1

   Remove and inspect radiator cap
   Remove the radiator cap from the radiator neck located at the top driver side of the radiator. Inspect the cap gasket for cracks, hardening, or deformation. Check the pressure relief spring for proper tension and inspect the vacuum valve for debris or damage. Clean the radiator filler neck sealing surface with a rag.
2. 2

   Test radiator cap pressure rating
   Attach the radiator cap to the pressure tester's cap testing adapter. Pump the tester to build pressure and observe the gauge. The cap should hold 15-16 PSI without releasing pressure. If the cap releases below 14 PSI or fails to release above 18 PSI, replace the cap.
3. 3

   Install pressure tester adapter
   Select the correct adapter from the pressure tester kit that seals properly on the AMC radiator neck opening. Install the adapter onto the radiator filler neck, ensuring it seats completely and forms a proper seal. Thread or lock the adapter according to the tester manufacturer's instructions.
4. 4

   Connect pressure tester
   Attach the pressure tester pump assembly to the installed adapter. Ensure all connections are secure and the gauge reads zero PSI before beginning the test.
5. 5

   Pressurize the cooling system
   Slowly pump the tester handle to build pressure in the cooling system. Watch the gauge and stop pumping when pressure reaches 15-16 PSI (the system's rated pressure cap value). Do not exceed 16 PSI to avoid damaging components.
6. 6

   Observe initial pressure hold
   Monitor the pressure gauge for 2 minutes without pumping. The system should hold steady pressure with minimal loss (no more than 1-2 PSI drop). A rapid pressure drop indicates a significant leak that requires immediate investigation.
7. 7

   Inspect external cooling system components
   While system is pressurized, systematically inspect all external components for leaks. Check upper and lower radiator hoses, heater hoses at firewall connections, water pump weep hole (located at bottom of pump), radiator seams and tanks, all hose clamps and connections, thermostat housing, and freeze plugs on engine block sides. Look for coolant drips, wetness, or staining.
8. 8

   Check heater core circuit
   Inspect the heater hose connections at the firewall and check inside the vehicle under the passenger side dash area for signs of coolant leakage from the heater core. Coolant odor or dampness on carpet indicates heater core leak.
9. 9

   Inspect for internal leaks
   Remove the oil filler cap and inspect for coolant contamination (milky appearance). Check the engine oil dipstick for similar contamination. Remove one spark plug from each bank and inspect for coolant presence indicating head gasket failure. Look in the radiator for oil film floating on coolant surface indicating internal oil cooler leak.
10. 10

    Monitor extended pressure hold test
    Continue observing the pressure gauge for a total of 10-15 minutes. System should maintain pressure within 2 PSI of initial test pressure. Note the rate of pressure loss if any, as slow seepage may take time to identify visually.
11. 11

    Release pressure and remove tester
    Slowly release pressure using the tester's pressure release valve. Once gauge reads zero PSI, disconnect the tester from the adapter and remove the adapter from the radiator neck. Wipe up any coolant spillage.
12. 12

    Document findings and check coolant level
    Record all leaks found and their locations. Check the coolant level in the radiator and top off if necessary. If no leaks were found but coolant loss continues, consider performing a combustion gas test to check for exhaust gases entering the cooling system through a head gasket breach.

Reassembly

1. Install the original radiator cap (or new cap if replacement was required) and ensure it is properly seated
2. Clean any spilled coolant from engine bay and surrounding components
3. Top off cooling system to proper level if any coolant was lost during testing

Verification

• Start engine and allow to reach normal operating temperature while monitoring for leaks at previously identified or repaired areas
• Verify temperature gauge reads normal and no coolant is being expelled from overflow
• Confirm heater operates properly and produces hot air
• Re-check coolant level after engine cools and top off if necessary
• If all leaks have been repaired, system should maintain proper coolant level during normal operation

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