Honda S2000 (AP1/AP2) Coolant Drain, Refill and Air Bleed

The S2000 is known for trapping air when the coolant is changed. An air pocket shows up as a temperature gauge that climbs past its usual three bars, or as a heater that never gets properly hot. The cure is patience. Drain through the radiator drain cock. Then refill slowly, using the air bleed bolt at the front of the cylinder head to let air out, and keep repeating the fill, squeeze and bleed cycle with the heater on full hot until the heater blows really hot. Use premixed Honda Type 2 coolant. S2KI owners also mention a second bleed point, a capped rubber nipple on the metal coolant line along the firewall, and an engine block drain bolt next to the oil filter. They disagree about the block drain: one says it is hard to reach, very tight and not needed because the radiator drain recovered the full amount, while another says the block should be drained too. Adapted from a community writeup by darcy on S2KI (https://www.s2000.club/OM/How%20to%20change%20your%20coolant%20for%20dummies-%20-%20S2KI%20Honda%20S2000%20Forums.pdf), with details from the S2KI coolant thread (https://www.s2ki.com/forums/s2000-under-hood-22/coolant-change-thread-740482/).

1. 01

   Step 1: Start with a cold engine

   Do this only when the engine has sat long enough to be cold. Opening a hot cooling system can spray scalding coolant.

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2. 02

   Step 2: Set the heater to full hot

   Turn the ignition on and set the heater temperature to maximum heat so the heater core circuit is open to drain and refill. Switch the ignition off again.

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3. 03

   Step 3: Get access to the radiator drain

   Raise the front of the car just enough to reach the radiator drain cock at the bottom of the radiator, and support it safely. Place a container under it.

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4. 04

   Step 4: Open the radiator cap and drain cock

   Remove the radiator cap. Open the plastic drain cock about halfway by turning it to the left; it does not need to come out. Let the coolant drain. Measuring what comes out tells you roughly how much to put back.

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5. 05

   Step 5: Open the front air bleed bolt

   While it drains, open the 10mm air bleed bolt at the front lower driver side of the valve cover area. This lets air in so the system drains more completely.

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6. 06

   Step 6: Optional: use the firewall bleed point

   There is a second bleed point on the metal coolant line along the firewall: a short closed rubber nipple held by a spring clamp. The writeup used it to blow out a little more old coolant with gentle shop air. If you open it, close it again before refilling.

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7. 07

   Step 7: Clean and refill the reservoir

   Close the radiator drain cock snugly. Slide the overflow reservoir up and out, wash out any sediment, refit it and fill it to the MAX line.

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8. 08

   Step 8: Fill the radiator slowly

   With the front bleed bolt open, pour Honda Type 2 coolant slowly into the radiator. When coolant runs out of the bleed bolt without air, close the bolt. Squeeze the upper radiator hose to push trapped air along, then open the bleed bolt again and add more coolant. Repeat until no more air comes out. Close the bleed bolt only snugly. The two S2KI sources disagree on how tight it should be (one says barely snug, the other about 8 lb-ft at most), so use the factory service manual value if you have it and never force it.

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9. 09

   Step 9: Run the engine to warm up

   With the heater on full hot and the blower on high, start the engine and let it idle. One method leaves the radiator cap off; another sets the cap at its first detent so it is not pressurised. Watch the radiator; if coolant starts to overflow, shut the engine off.

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10. 10

    Step 10: Bleed again with the engine off

    With the engine off, open the bleed bolt to let trapped air escape, add coolant, squeeze the upper hose and close the bolt when coolant comes out free of bubbles. Never open the bleed bolt while the engine is running.

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11. 11

    Step 11: Repeat until the heater blows hot

    Keep repeating the run, stop, bleed and top-up cycle until the heater blows properly hot rather than lukewarm and the gauge settles at its normal three bars.

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12. 12

    Step 12: Final top-up and check

    Let the engine cool, top the radiator off to the neck, fit the cap fully and set the reservoir to its cold mark. After the first drive and cool-down, check both levels again and top off as needed.

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