cooling fan not shutting off - Subaru Outback - Subaru Outback Forums
 
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post #1 of 4 (permalink) Old 06-16-2011, 07:58 PM Thread Starter
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cooling fan not shutting off

1996 auto, (ej 25)

new here, I got this car a couple of years ago w/ a head gasket bad. I refurbished the motor myself, I did everything, including water pump and valves. the car has been flawless until of late I noticed that the cooling fans (both) would not cycle normally. I replaced the thermostat, bleed the system, it works good after you let it cool down, then I start it up, it heats up, I feel the thermostat open, radiator gets hot then fans turn on for just a short period of time then shut off, (normal) well after driving it in the city for 30 minutes, I get back to the driveway and I listen. fans don't turn off, I ran the AC while driving but when I hit the slow roads I turn it off. heater blows hot, radiator seems to be clear and un obstructed(water hose flows freely through it) no leaks, not using any coolant, the HG's I used were the updated style. and I do run the conditioner (subaru) I was thinking the coolant sensor in the coolant pipe but like I said earlier, when I feel the bottom radiator hose get hot, the fans turn off right away, after the 30 min use, the bottom hose is cool (no circulation) and radiator feels cool, fan stays on. again, it does not over heat, or even fluctuate when circulation stops. I know I should pull the sensor out when fans stay on, but the lower hose is cool, so the engine is hot, that's why the ecu is telling the fan to stay on, why is in not circulating??
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post #2 of 4 (permalink) Old 06-16-2011, 09:14 PM Thread Starter
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so I just let it sit and cool, opened top rad vent, topped rad off, started and idled/1500rpm until fan turned on, before fan turned on lower hose felt hot, then fan on, then off, perfect. I turned AC on, fans turned on, then I turned it off, fans turned off. I noticed a possible slow drip from lower hose, I tightened hose clamp slightly, could it be sucking air or loosing coolant forming a air pocket when hot? right now i'm thinking, the car must just be hot, i just can't understand why the lower and rad feel cool to the touch when the fans are "stuck" on??
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post #3 of 4 (permalink) Old 06-16-2011, 10:33 PM
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i just can't understand why the lower and rad feel cool to the touch when the fans are "stuck" on?
It's quite possible. The sensor for the ECM fan control is in the water pipe at the top of the engine. It's measuring the temperature of hot coolant as it comes out of the block.

The bottom hose of the radiator is the "cool" return to the engine. Coolant flows from the water pipe to the radiator where it is cooled, especially when the fans are on. If the outside air temperature, flow rate through the radiator, and the fan speeds are all right, the coolant can well be cooled down to close to ambient temperature and would feel cool to the touch in the lower hose.

The cooled coolant goes from the lower hose to the thermostat and then to the pump where it is returned to the block. But the thermostat sensor is on the engine side, and in the path of coolant going through the continuous flow bypass circuit (water pipe, cabin heater core, thermostat). The bypass will still be providing hot coolant which will keep the thermostat open at least partially. Even though the lower radiator pipe feels cool, there might still be coolant flow through it.

So, the coolant at the top of the engine, in the water pipe, is too hot. This coolant passes through the radiator and is cooled by the fans. The cooled liquid continues to flow because the thermostat sensor is responding, in part, to the hotter coolant coming through the bypass circuit.

Quote:
could it be sucking air or loosing coolant forming a air pocket when hot?
If the coolant is low due to a slow leak, the remaining coolant has to absorb the same heat, so it will tend to be overheated. This will cause the ECM sensor to turn the fans on. The fans and radiator might cool the liquid sufficiently as it passes through the radiator, but on it's return to the block it gets extra-hot again.
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post #4 of 4 (permalink) Old 06-17-2011, 01:03 AM Thread Starter
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Originally Posted by plain OM View Post
It's quite possible. The sensor for the ECM fan control is in the water pipe at the top of the engine. It's measuring the temperature of hot coolant as it comes out of the block.

The bottom hose of the radiator is the "cool" return to the engine. Coolant flows from the water pipe to the radiator where it is cooled, especially when the fans are on. If the outside air temperature, flow rate through the radiator, and the fan speeds are all right, the coolant can well be cooled down to close to ambient temperature and would feel cool to the touch in the lower hose.

The cooled coolant goes from the lower hose to the thermostat and then to the pump where it is returned to the block. But the thermostat sensor is on the engine side, and in the path of coolant going through the continuous flow bypass circuit (water pipe, cabin heater core, thermostat). The bypass will still be providing hot coolant which will keep the thermostat open at least partially. Even though the lower radiator pipe feels cool, there might still be coolant flow through it.

So, the coolant at the top of the engine, in the water pipe, is too hot. This coolant passes through the radiator and is cooled by the fans. The cooled liquid continues to flow because the thermostat sensor is responding, in part, to the hotter coolant coming through the bypass circuit.



If the coolant is low due to a slow leak, the remaining coolant has to absorb the same heat, so it will tend to be overheated. This will cause the ECM sensor to turn the fans on. The fans and radiator might cool the liquid sufficiently as it passes through the radiator, but on it's return to the block it gets extra-hot again.
thanks for the lesson, I actually understand all that, I think I got it, I flushed the heater core and all kinds of crud came out, I guess on the ej25 when the thermostat is shut, the bypass circuit routes through the core, if it's not flowing freely it will in effect create a vacuum and will not allow the coolant to circulate, even though it needs to. another words if you thermostat is open, coolant should be circulating freely, if there's obstructed flow, it won't circulate, still not tested on the road, i'll let you know how it goes.
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