Reservoir Style Heat Exchanger Cap

[gts1544]

We recently installed a coolant reservoir on our 1986 Perkins 4-108 with a hose running off of the side of the heat exchanger filler neck, however the coolant does not seem to return to the engine while the engine is cooling after use. The heat exchanger cap was not replaced by the installer, CRC Engines of Virgin Gorda, BVI's. Knowledgible mechanic friends of mine up here in the USA tell me that the original cap will not work and that I need a "reservoir style" cap to allow the coolant to return to the system. The original cap is a spring loaded pressure type cap the holds a seal with the its seat, which is below the reservoir line. It appears to me that this action would block any return flow. Any ideas out there? gts1544 / George

[NCboatrx]

You need a pressure cap with an extra rubber seal where the top of the cap seals on the outside of the filler neck. These caps have a little valve in the stem which allows the coolant to come back into the heat exchanger (radiator) when the engine cools down. They are readily available from any NAPA store.