Cooling fans run constantly after drive

[RoughDiamond]

Before I keep chasing my tail, I just want a second opinion.

New water pump (unfortunately, Duralast brand, new/not reman), FHI Thermostat, newer radiator, new head gaskets, new ECT (not FHI brand, but OE-level quality. Resistances are within OE specs)

Had overheating issues in the past with a clogged radiator, but just this in the past 5K miles.

Car does not over heat, has good heat and no pushing of coolant into overflow, bubbles or any of the bad stuff, but both fans like to come on at a stop and run full blast until I turn the car off.

If I let the car sit and warm from idle, the fans come on, run for ~45 secs, then step-down for ~3 sec, then off. Rinse, repeat every 2 mins.

If I drive the car, it warms up pretty quickly, stays constant. When I come to a parking spot, I roll up and the fans are full tilt. Even after ~5 mins of running at idle, they stay on until I shut off the car. I CAN get them to come off if I raise the RPM to ~2500 and hold for about 1 min, the fans come off and then will cycle as normal. Temp may move a hair, but almost unnoticeable.

If I turn the A/C on full tilt, the full-power fans will come on and stay at half power until I turn the car off or the A/C off.

Upper hose on the rad is HOT. Laser thermo says ~170. Both heaters are hot as are the return lines to the pump. Lower hose is not that hot, maybe ~80-90.

Am I suffering from a clogged radiator or a sub-par water pump here? I'm almost ruling out the thermostat as it is less than 3 months old and a FHI one. I have not had the radiator professionally flushed, but the temps seem pretty consistent across the whole thing with the laser. Water pump has the stamped fins, not the cast impeller like the HD FHI ones.

Ideas? Thanks as always!

[Legacy777]

How old is the radiator?

Did you check the resistance of the coolant temp sensor at a lower temp and at a higher temp (200 deg F) when you checked the resistance?

Based on your comment about raising the rpm causes the fans to cycle properly it could be either a plugged radiator, plugged coolant passages in the block, or poor water pump. If your radiator is pretty new and not plugged and same thing with the block itself I'd probably lean towards the pump. Is the pump a stamped metal impeller?

[mdsn425]

If you have the defroster on itwill cause the fans to run on full blast.

[RoughDiamond]

Josh - The radiator is age unknown, but definitely not the original. I also had flushed the block, heater core when I did the HGs and while lots of stuff came out, I believe it is clear. The water pump DOES have the stamped fins which could just flow poorly at low rpm, and yes, I measured and plotted the resistance of the ECT vs. Temp and is was bang on with the OE diagram. Temp sender agreed as well.

Mdsn- the fans do this with the HVAC controls in either off, vent or bilev. In AC, defrost or MAX AC, the fans will run at half speed.

[Legacy777]

At this point without pulling the radiator or pump to either check or replace it's anyone's guess.

The cause is lack of cooling at idle, either by lack of radiator performance or pump performance.

I think I'd probably pull the radiator, maybe flush it to see if any junk comes out and to see how it looks. If it's good, it's probably the pump.

[squibbiez]

I know this is an old thread...

My 1993 legacy awd automatic exhibits exactly the same symptoms RoughDiamond mentioned. The fan comes on after idling or driving and coming up to temp like normal. After a drive, the fan doesn't ever seem to turn off. It was a 1-owner car before me and service records and visual check suggest it's 100% original - radiator, wp, etc.

Coolant temp at the top radiator hose is 180-200 degrees, temp gauge stays dead center all the time. Heater blows hot. I just did a coolant flush with some flush product. The flush went very smoothly, and the original coolant drain and subsequent flushes all seemed quite clean.

Is it at all reasonable to think the coolant temp sensor might be bad and sending a high reading when the car gets warmed up? The only reason I hesitate to suggest this is all my searches on the forum indicate CTS problems are connected with running and starting issues, not continuous cooling fan operation. The Legacy starts and runs like a champ in all weather.

[Legacy777]

Is the AC or defrost buttons on the HVAC system turned on? If so the fans will run.

[squibbiez]

I fixed it with a new Subaru thermostat after flushing the rad, engine, checking temp sensor resistances, etc. did nothing to help. Thanks for the advice!

[Legacy777]

Thanks for letting us know.