FellowTraveler
Well-Known Member
Later GMT400 w/6.5 diesel cooling system design, modifications and FACTS.
GM never put heater hose on/off valves on this configuration however a heater hose restrictor is installed at the thermostat housing so coolant always flows through both heater cores then to cool side of radiator spilling onto internal transmission cooler.
While here is much debate about installing an on/off valve in the heater hose circuit the GM C/K (GMT400) up fitter manual clearly states when there is a heater delete or on/off valve installed in heater hose the heater port coolant flow must still be routed to the radiator by a loop circuit or in the case of an on/off valve via a tee fitting or damage would occur to engine/transmission there is no further explanation.
Upon examination of the cooling circuit I have found that the heater hose is also the ‘steam vent line’ on the 6.5 diesel allowing steam to escape through the heater circuit into the cool side of radiator/accumulator where it will eventually vent to atmosphere. Steam is always created in a 6.5 from cavitation around valves and cylinder walls when ethaline glycol & water (EGW) coolant is used so closing off that vent will cause other problems. GM knew of this engine damaging steaming from cavitation early on and even incorporated steam vents in the LS series engines “to vent the engine damaging steam caused by cavitation.”
The bypass at the dual thermostat housing to water pump is full flow no restrictors two types of thermostats can be used the conventional hi flow and a drilled full units.
The conventional hi-flow thermostats are used in two configurations first is as equipped by factory and second is w/restrictor in the bypass between the thermostat housing and water pump.
The full flow thermostat has three 3/8” holes drilled into its body x 2 = +- the flow of the bypass between the thermostat housing and water pump so completely blocking off bypass flow between thermostat housing and water pump is now possible so most of the coolant flows through the radiator however the heater hose steam vent must remain when using EGW and with any waterless coolant because of any condensation build up that may occur in the waterless coolant which will be steamed out shortly after warmup and it needs to vent.
NOTE: My tests using refractometer show water entering into waterless coolant from condensation is not a worry the amount water egress is never an issue and it will quickly boil/steam off after engine warmup and vent as it should providing the factory heater hose steam venting is intact. In fact large amounts of water in EVANS coolant can be removed by removing the coolant from the cooling system and boiling off water via a big pot and stove.
GM never put heater hose on/off valves on this configuration however a heater hose restrictor is installed at the thermostat housing so coolant always flows through both heater cores then to cool side of radiator spilling onto internal transmission cooler.
While here is much debate about installing an on/off valve in the heater hose circuit the GM C/K (GMT400) up fitter manual clearly states when there is a heater delete or on/off valve installed in heater hose the heater port coolant flow must still be routed to the radiator by a loop circuit or in the case of an on/off valve via a tee fitting or damage would occur to engine/transmission there is no further explanation.
Upon examination of the cooling circuit I have found that the heater hose is also the ‘steam vent line’ on the 6.5 diesel allowing steam to escape through the heater circuit into the cool side of radiator/accumulator where it will eventually vent to atmosphere. Steam is always created in a 6.5 from cavitation around valves and cylinder walls when ethaline glycol & water (EGW) coolant is used so closing off that vent will cause other problems. GM knew of this engine damaging steaming from cavitation early on and even incorporated steam vents in the LS series engines “to vent the engine damaging steam caused by cavitation.”
The bypass at the dual thermostat housing to water pump is full flow no restrictors two types of thermostats can be used the conventional hi flow and a drilled full units.
The conventional hi-flow thermostats are used in two configurations first is as equipped by factory and second is w/restrictor in the bypass between the thermostat housing and water pump.
The full flow thermostat has three 3/8” holes drilled into its body x 2 = +- the flow of the bypass between the thermostat housing and water pump so completely blocking off bypass flow between thermostat housing and water pump is now possible so most of the coolant flows through the radiator however the heater hose steam vent must remain when using EGW and with any waterless coolant because of any condensation build up that may occur in the waterless coolant which will be steamed out shortly after warmup and it needs to vent.
NOTE: My tests using refractometer show water entering into waterless coolant from condensation is not a worry the amount water egress is never an issue and it will quickly boil/steam off after engine warmup and vent as it should providing the factory heater hose steam venting is intact. In fact large amounts of water in EVANS coolant can be removed by removing the coolant from the cooling system and boiling off water via a big pot and stove.
Yeah, purging these systems after repair is a PIA that results in runaway ECT if not done properly! The Thermostat location etc. all made this a requirement as they are no longer the highest spot on the engine. The changes do allow a steadier temperature in the engine with less cold shocks on t-stat opening.