Pressure test the cooling system to identify leaks in the radiator, hoses, water pump, heater core, and block/head gaskets using a specialized pressure tester.

Warnings

Tools required

Parts

• Replacement coolant (if system requires draining) × 1 — Use OEM specification

Preparation

1. Ensure engine is completely cold to the touch
2. Park vehicle on level ground and set parking brake
3. Place drip pan under radiator area to catch any coolant spillage
4. Verify coolant level is at proper fill line in radiator when cold (remove radiator cap slowly to check)
5. Inspect all visible hoses, clamps, and connections for obvious damage before testing

Procedure

1. 1

   Remove radiator cap and inspect
   With the engine completely cold, slowly remove the radiator cap by pressing down and turning counterclockwise. Inspect the cap's rubber seals for cracking, deterioration, or damage. Note the pressure rating stamped on top of the cap. Original 1966 Corvette systems typically use a low-pressure cap (often around 7 PSI); verify the actual rating stamped on the cap and use that figure as your test ceiling. Set cap aside in a clean location.
2. 2

   Install pressure tester adapter
   Select the correct radiator neck adapter from your pressure tester kit that fits the 1966 Corvette radiator filler neck (typically a GM standard neck adapter). Install the adapter onto the radiator filler neck, ensuring it seats fully and creates a seal. The adapter should lock or clamp securely into place similar to how the radiator cap attaches.
3. 3

   Attach pressure tester to adapter
   Thread or clip the pressure tester pump assembly onto the installed adapter according to your tester's design. Ensure the connection is secure and the tester gauge reads zero PSI before beginning.
4. 4

   Pressurize the cooling system
   Slowly pump the pressure tester handle to build pressure in the cooling system. Monitor the gauge continuously and stop pumping when pressure reaches the rating stamped on the radiator cap (verify the actual rating; many original 1966 Corvette caps are around 7 PSI). Do not exceed the cap's rated pressure.
5. 5

   Observe initial pressure hold
   Watch the pressure gauge for 2 minutes without pumping. The system should hold steady pressure without dropping. A slow, steady drop indicates a leak somewhere in the system. A rapid pressure loss indicates a significant leak.
6. 6

   Inspect radiator and hoses
   With the system pressurized, visually inspect the entire radiator surface (front, back, top tank, bottom tank, and side seams) for coolant seepage or drips. Check all radiator hoses (upper and lower) along their entire length, paying special attention to hose ends where they connect to the radiator and engine. Look for swelling, cracking, or coolant weeping. Inspect all hose clamps for tightness and proper positioning.
7. 7

   Inspect water pump
   Examine the water pump housing and the weep hole located on the bottom of the pump body (between the impeller and bearing). The weep hole is designed to leak slightly when the pump seal fails. Look for fresh coolant drips, coolant residue, or rust staining around the pump housing, pulley hub, and weep hole. Use the inspection mirror and shop light to check all sides of the water pump.
8. 8

   Inspect heater hoses and core
   Follow both heater hoses from the engine (typically one from the intake manifold and one from the water pump) through the firewall. Check for leaks at connection points and along hose length. Inside the vehicle, check the passenger side floor carpet for dampness and smell for sweet coolant odor, which indicates heater core leakage.
9. 9

   Inspect engine block and cylinder heads
   Check the freeze plugs (expansion plugs) on both sides of the engine block for coolant seepage or rust staining. Inspect the head gasket areas where the cylinder heads meet the engine block on both sides for coolant leaks. Look for coolant in the valley between the cylinder heads on top of the intake manifold. Check the intake manifold gasket surfaces where the manifold meets the heads and block.
10. 10

    Check for combustion gas contamination
    While maintaining system pressure, observe the coolant in the radiator filler neck (if visible through the adapter) for bubbles rising steadily, which indicates exhaust gases entering the cooling system through a blown head gasket or cracked head/block. Also smell near the radiator for exhaust odor.
11. 11

    Monitor pressure over extended period
    Continue observing system pressure for a total of 10-15 minutes. Re-pump to restore test pressure if needed. Document any pressure drop rate. A system losing more than 2 PSI in 15 minutes indicates a leak requiring repair.
12. 12

    Release pressure and remove tester
    Release pressure from the cooling system using the pressure relief valve on the tester. Wait until gauge reads zero PSI, then disconnect the pressure tester from the adapter. Remove the adapter from the radiator neck and reinstall the original radiator cap, pressing down and turning clockwise until it locks into place.

Reassembly

1. Reinstall radiator cap securely with proper engagement
2. Top off coolant if any was lost during testing procedure
3. Wipe up any spilled coolant with shop rags and dispose of properly

Verification

• Radiator cap is properly installed and locked into position
• No coolant puddles or drips under vehicle after test completion
• All leak locations have been documented for repair
• Start engine and allow to reach operating temperature, then visually recheck any suspected leak areas identified during pressure test

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