Join Date: May 2003
Location: Houston, Texas
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Re: Coolant circulation diagram
I'll start at the water pump:
Pump discharges to 3 parallel circuits: left (driver's side) bank, right (passenger side) bank, and oil cooler/EGR cooler. The oil cooler and EGR cooler are plumbed in series. All circuits come back to the crossover integral to the top of the front cover. The heater is supplied from the passenger side of the crossover, the thermostat is at the driver's side.
The heater return is T'd into the degas bottle return -- there is no check valve there.
When the thermostat is closed, the coolant travels though the bypass passage within the front cover back to the w/p inlet. When the thermostat is open, this passage is closed and coolant goes to the radiator. There is a small line off the radiator that goes to the degas bottle, the degas bottle return is back to the w/p inlet along with the heater return (see above).
The heater circuit is in parallel with the thermostat/radiator circuit. They both return to the w/p inlet, and the cycle starts again.
Some common cooling system myths:
- The water pump makes pressure. Nope. It makes flow. The resistance to that flow is what produces pressure.
- The water pump is responsible for the system pressurizing. Nope. The pressure rise measured across the pump is equal to the pressure drop of all the circuits between the pump discharge and the pump inlet. System pressure is primarily a result of thermal expansion of the coolant.
- The thermostat controls coolant temperature. Nope. It's not really a thermostat like the one on the wall of your house, it's a thermally-controlled flow distribution valve. Between its start-to-open (STO) temperature, 89C/192F on a 6L, and its full-open temperature (about 202-205F), it is true that flow is proportionally divided between the bypass and the radiator, but the actual stablilized temperature of the system is reached when the heat flux in (from engine and EGR) equals heat flux out (via radiator and heater) (Q-dot in = Q-dot out). This depends on coolant flow rate, engine load, airflow rate, and ambient temperature and the thermostat doesn't enter into it.
Former Cooling Guy
'99.5 F350 Lariat CC SRW 4x2, 7.3L 4R100
'99 Mustang Cobra, SCCA Solo2 E Street Prepared
'97 Dodge Neon, SCCA ITA/STU/EP Club Racing
Now I do subsea ROV tooling.