They should come on full blast with the A/C on. And once you're moving, the ambient air coming in through the grill will overpower the fans, even at full blast. So unless you're sitting in stop and go traffic, with the A/C off, it won't make much difference.

How hot is it running? The temp in our '04 was creeping under load on hot days last year, and a thorough cooling system flush brought it back down. I don't think I saw more than 195 last year (haven't had the scangauge in it this year, but the factory gauge hasn't wiggled above the middle). Now, I'll admit I don't live in a climate that's very strenuous on cooling systems, but we see a few 100* days, with nasty humidity, and it still didn't struggle.

 

Like I said, I drive it in Death Valley in the summer. It blew the headgaskets. The ambient temperature outside is frequently 115 degrees + and the grades are steep and long, on dirt.

 

I'm in the Phoenix area and understand the heat. Once it's above 115 my car will NOT cool down. Fans going and all that. I've also noticed that cleaning my radiator of dirt and debris in the fins help a lot. And I don't have to worry about temps so much. It'll still get hot like 212 but it won't overheat either.

 

I keep the radiator clean, but H6's run hot in general and have notably less space between the fans and the timing cover vs. the NA H4 like the one you have. A while ago I was reading a build thread of someone who drives his EZ30D swapped earlier-gen Legacy/Outback in similar conditions and I completely understood why he had a pile of 5 EZ30D's with blown headgaskets in his garage...

 

As long as the temp doesn't hit 235, it should be okay.

Turning the fans on High when driving at speed will not do anything. Clean radiator, good coolant, run the lower temp thermostat. You may want to make sure the radiator core is not restricted. Same with the heater core.

My VDC, the boosted one, runs high engine temp under boost in 100+ temps, but it never goes over 220. You cannot tune the fans on your car. You can tune the ECM; change timing and fueling. I have definitions for.my H6 that may work on yours if ECUFlash is not reading your ROM and you want to make changes to control combustion temps.

 

Thanks for the tips/info, everyone.

As long as the temp doesn't hit 235, it should be okay.

I've had the car well above that, granted, the headgaskets were already leaking by then.

Turning the fans on High when driving at speed will not do anything.

Agreed - most of the desert driving I do is at or below 45 MPH. On my last Mojave Road run, I was cruising along at about 25-35 max in most places. Would definitely leave it on ECU control at highway speeds.

Clean radiator, good coolant, run the lower temp thermostat. You may want to make sure the radiator core is not restricted. Same with the heater core.

Should be all good there. Replaced the radiator back in 2021, coolant is regularly changed, putting in the lower temp thermostat in the JDM replacement engine. Heater core works fine and is unrestricted - no goop anywhere in the cooling system, thankfully.

What kind of coolant temps are you reaching?

With a blown headgasket? North of 245*
Before that? In the 220s on occasion but that was also when the car was bone stock.

Did you do any cooling upgrades after doing the larger tires? This does make the transmission heat up more, and the factory cooler is in the rad.

A few months after throwing on the wheels/tires, I drove out to my buddies place in Utah and swapped transmissions (lower mileage unit) and in the process, the factory rad cooler line got a pinhole leak in it that wasn't noticed until 1+ gallon of fluid was lost - no issues with the trans, thankfully. Sooo I bypassed the in-rad cooler and installed a Hayden 404 cooler mounted in front of the condenser. Drove home okay in 103 degree heat with a mildly blown headgasket without having to make noteworthy stops, just altered driving style to reduce load (which had been the norm for years by that point).

My '03 beater is a similar build. 29" tires, 3.5" lift. I installed a new Denso radiator, bypassed the internal trans cooler and run an external, using a new OEM thermostat, STi hood scoop. My outside temps are usually 100-110F during the summer months, it's rare that i see above 210F.

Mods are almost identical to mine (yours has a little less lift but is otherwise the same) - that's good to see the combo working. I think at this point I should be pretty good then with the plan for the new engine - lower temp thermostat & a manual fan speed control switch that allows me to have stock fan function or manually command high speed using a 3PDT switch:

 

What kind of coolant temps are you reaching? Did you do any cooling upgrades after doing the larger tires? This does make the transmission heat up more, and the factory cooler is in the rad. My '03 beater is a similar build. 29" tires, 3.5" lift. I installed a new Denso radiator, bypassed the internal trans cooler and run an external, using a new OEM thermostat, STi hood scoop. My outside temps are usually 100-110F during the summer months, it's rare that i see above 210F.

 

Wanted to provide an update. I have two switches now in my possession and both should work, but I'll be trying the latter option first.
The first switch I received in the mail was a Three Pole Double Throw switch as outlined above, and the second was a Rotary Switch.

The 3PDT switch I selected is a XURUI XT-32A which follows the schematic in my previous post, but only has options to select OEM control, or manually command Maximum Fan speed:

For the Rotary Switch, the Relay side connections can't be disconnected by rotating the dial. In addition, each "pancake" level of pins is isolated from the levels above/below it. There's a vertical shaft inside of the switch with three teeth - one for each of the three levels, that when rotated, complete the connection from the Relay side to the selected 3 pins in a vertical line. So, with the switch knob turned all the way to the ECU pins on the right, there's three vertical teeth touching the inside of all the pins marked ECU, which connect those three pins to the Fan Relay connection for that specific level. Then when you rotate it to the left one click, those three teeth disengage from the ECU pins (turning the ECU side into an open circuit) and move to the next setting in line - in this case, grounding the Low circuit. The next click to the left disengages the connection for the Low Ground and then grounds Low and Medium. The following click to the left disengages Low and Medium, and grounds Low, Medium, and High.

I'll post back with pictures once I eventually get some leads and connectors wired up to this thing and ready to install into the car, which may be a few months or more. Once installed, it will probably need a small mounting bracket to help prevent this thing from rotating, as the force required at the moment might be enough to twist the whole switch assembly when panel mounted with a single retaining nut and no other fastening mechanism. Going to try greasing the tactile click plate to reduce the force required to rotate the switch, but I think a keyed bracket is going to be necessary.

In the mean-time, here's the label plate I designed. Turns out Subaru used a slightly modified Arial Black for the CRUISE button, so I carried that font style over to the design for the fan speed controller faceplate:

 

Was checking in on some of the latest Jeep stuff from NickInTime (I've also got a couple XJ's) and found this possible option which should also work great on my H6 Outback.
Having a second toggle-able fan controller with more aggressive cooling profile would be perfect for those hot days.
It'd be wired in using the 3PDT switch as the toggle to switch between aftermarket and ECU.

Trinity Fan Controller V2.2

A Universal 3 channel Automotive Fan Controller in the palm of your hand!

nickintimedesign.com

 

So I ended up ordering this. I think I'll install it as outlined above to switch between OEM and aftermarket fan control settings for those hot days in the desert.
As a side note, I was informed by a friend that he was able to view/edit/re-tune the H6 fan control temperature ranges during his H6 Baja swap project. So I have to get more info from him when possible as far as ECU revision, definition set, etc. as I wasn't able to find those settings in any definitions I looked at.

 

Yep, lots of universal fan controllers out there. That one looks pretty cool!

I've always had my eye on the Dakota Digital one. Nearly pulled the trigger for my 1UZ 4Runner project, might have done it if I'd been using an OBDII engine.

Electronic Fan Controller 70 Amp with Bluetooth Control

Dakota Digital manufactures digital instrumentation and accessories for the automotive, motorcycle and car audio enthusiast.

www.dakotadigital.com

 

Looks like the outputs on the Dakota Digital one won't directly line up to the existing fan relays on any of my current project cars, but it's pretty neat! Never thought Bluetooth control would be a thing for fan controllers.