I have a 03 400ss with 496ho motors. The port motor has me scratching my head, it loses coolant while sitting. If I top it off and run 100 miles it is at the same level upon return, but after two month of sitting it has lost 1/2 gallon of coolant. Bilge doesn't show signs of coolant, oil is clean. Where the h3ll can it be going?

I have read the circulating pump, but I am back to the bilge shows no signs.

I topped it off again today and plan to fire motor tomorrow then place rags under the pump to see if it is a slow drip over time.

Any other thoughts?

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The heat exchanger rubber seals are known to leak as they age. You will see coolant in the bilge. Water pump will leak from the weep hole. You will see this.

Rent a cooling system pressure tester. Very easy to use. I was loosing pressure from a cracked tube in heat exchanger. Coolant was going out exhaust.

THIS!
Those rubber seals heat and swell and don't leak, but they cool and contract and then they leak.

This is a good time to replace your coolant as well, as it makes a mess when you pull it a part. You should replace your coolant every 6-7 seasons anyhow, or whenever it starts to turn dark/metallic.

Order up the new grommets AND order up new heat exchanger end cap rubber seals. Replace the two grommets and the two end cap seals as well as the coolant in both motors.