I don't think measuring the temperature of the coolant inside the radiator, with the cap off, is a reliable comparison.

What is the instrument panel temperature gauge reading when the fans come on?

I presume your car is a 1999, 2.5. If so, it has two sensors related to coolant temperature. Both are in the water (crossover) pipe, as shown in the attached diagram. One is for the instrument panel temperature gauge. The other goes to the engine control module and is used for fuel/emissions and radiator fan control. (This is what your scan gauge is reading when connected to the OBD connector.)

The radiator fans should come on somewhere in the range of 200-210F, so I would not initially suspect the sensor, especially if the instrument panel gauge is also reading at the normal point.

Did the fans come on at 200F before the thermostat was changed, or only after?

Did you use a genuine Subaru thermostat?

 

panel reads a smidgen below half when warmed up. Couple days ago i noticed the fans were on. Changed the thermostat with a Sub one.

the top radiator hose gets hot,but the lower does not.

 

Instrument panel temp gauge is indicating normal, so the coolant temperature is not unusually low, such as 90F, or too high.

The fact that the upper radiator hose is hot, and the lower one is cool, can be misleading -- the function of the radiator is to remove heat from the coolant, so the return line to the engine at the bottom should be significantly cooler than the hose coming from the engine to the radiator at the top.

Also, the lower hose will be cool if there is limited flow through the radiator, which will happen if the thermostat remains closed. The thermostat can remain closed if the engine is able to dissipate heat directly (from the engine block) and through the in-cabin heater core, as might be the case if you are in a cold climate.

Does your car have air conditioning? If it does, are you sure that the controls are not set for defrost and that the fan speed control is Off?

It's possible that the engine coolant temperature sensor has become too sensitive, and is triggering the fans to come on a bit too early. But they are coming on pretty close to the normal point so there isn't a strong case to suspect the sensor. In any event, the sensors can be tested by measuring their resistance (Ohms) at known temperatures, and comparing the readings against specifications; unfortunately, I don't have the specs for your generation of Outback.

 

And you have to be careful using an IR thermometer to measure the temp of a liquid. Use it on the outside of the radiator=OK, but not on liquid directly.

Research "emissivity" if you want to know more.

 

I have got similar problem, but in my case, the temperature gauge doesn't move at all. I have been in quite a few 1-2 hours trip, but still the gauge so no movement. I don't think the gauge is broken, yet i cannot think of what's causing this. I have got Subary Legacy Outback Limited 1999. I would be very happy if you could help me in any way.

 

Perhaps start by checking the wiring to the temperature sensor -- perhaps the connector came off the sensor, or the wire is broken.

 

I'm going to jump in here, with a very similar problem. I've recently had the head gaskets on our '04 OBW 2.5 done, along with the timing belt, while the engine was out, including all the stuff that goes along with the belt service-pump, T-stat, etc. I picked the car up last week, and the fans were running on a 36 degree day. We've had the car since new, and only heard the fans when the compressor runs. I called the shop, and the mechanic suggested running it at a slightly raised idle to burp the system. After about twenty minutes, I got a fair amount of air to bubble out, and the fans have started to behave. But, according to my Ultra Gauge, there was an 'impending' code, P0483, Fan Rationality Check (!). What is this, and how do I cure it? The code is only impending, and the Check Engine Light is not on. Since the burp, the temps are 190* to 204*, right where it should be, where as before, it was swinging between the low 180s to about 220. If it's now finally burped, hopefully the code will go away.

Has anyone ever heard of this?...Thanks.

 

P0483, Fan Rationality Check (!). What is this, and how do I cure it?

This code is triggered when the coolant temperature does not drop after the fans are turned on.

The ECM controls the fans (i.e. switches them on and off); however, it has no direct way to determine if the fans are actually turning; instead, it compares the fan switch state (On or Off) to changes in the coolant temperature. If the fans are supposed to be running, yet the coolant temperature does not drop, then that's considered "irrational" in terms of the logic built into the OBD system. If this occurs while idling (typically over a 5 minute period), the system will trigger a "pending" code, and if it occurs a second time (another drive cycle), a full code is recorded and the Check Engine light is turned on.

The pending code will clear from memory automatically after 40 drive cycles without the same symptom appearing. It won't affect operation, or turn on the CEL. It can be cleared using an OBD reader that has code-clearing capability.

 

Plain OM-Thank you for the explanation in hobbyist/shadetree/frustrated mechanic speak. I have an Ultra Gauge, that I bought a few months ago, and it usually resides in my daily driver, a '96 Cherokee. There are eight areas on the screen, six of which can be changed, and two permanent, a 'loop', designating closed loop operation, and a pictogram of a heart, which beats large to small every two seconds. When there's impending doom, the heart beats different, the top separating from the bottom. It did this for about a week in my JEEP, until it died one day a few weeks ago. It alarmed when the car's computer set a code, and I could not clear it, as there was a real malfunction-the failure of a less than a year old Cam Position Sensor. It knows when trouble is coming, and I was too unfamiliar with it to heed the warning. Now in the Subie, the little icon is 'broken hearted' again, but the fans seem to be acting rationally!

So. quite possibly, it may take about 40 key cycles to satisfy the code, if in fact it's behaving? Could I hasten the process by disconnecting the battery for a while, 're-booting' the computer? Thanks for your patience with my long-winded novel!

 

The "broken heart" syndrome!

Yes, it will take 40 driving cycles (that's the info I have on this particular code) for the pending code to self-clear. And yes, I believe it can be forced to clear by disconnecting the battery for a while (to allow all the powered systems to discharge). I don't normally favor this approach, however, because this also clears "learned" memory in the engine and transmission control modules, and this in turn sometimes leads to odd symptoms when the control modules revert to their factory defaults and have to "relearn".

But can't the Ultra Gauge be used to clear the codes as well?

 

The ECU never actually threw a code, to turn on the CEL. The Ultra Gauge has an alarm to warn of an impending code, and if you ask it to, it will show you what may be on the way. The code was never set, therefore it didn't need to be cleared. I guess it's a sort of early warning. And yes, the UG will clear a code, once it's been set. Unfortunately, the UG does not give a description of the problem, just the code number. I used the book that came with my Actron code scanner to decipher the description. There's a sticky at the beginning of this forum listing all the codes which a Subie may be afflicted with. It's going to get printed...

 

I suspect that the UG isn't more intelligent than the OBD system, but it's "smart" enough to use what the OBD already has to trigger the broken heart, and that is the pending code.

As I mentioned earlier, the P0483 requires the symptom to appear in two driving cycles. The first time, a pending code is "set", but that does not turn on the CEL. The second time the symptom is detected the OBD assumes the first time the problem appeared wasn't a random anomaly, and it then sets the permanent/hard trouble code and turns on the CEL. (In both cases, the ECU is "throwing" or "setting" a code, but in the first instance it's not a full code, just "pending".) The UG is using the pending code to provide the broken heart early warning.

i would think you should be able to clear the pending code with the UG if it is showing it; in fact, most OBD readers (I have a real basic unit) should do so. Have you tried it?

 

When my wife comes home from work Saturday, I'll see if the little heart is beating odd. If it is, I'll see if Ultra gauge still has the impending code. If so, I'll try to clear it. Or, get the Actron out, and see if it will pick anything up. I guess in a way it's a blessing that I don't know more about the Subie than I do. In the eight and a half years we've had it, it has never asked for more than routine maintenance. Thanks for stickin' with me, plain OM!

 

Well, an update to the ongoing saga (hopefully nearing the end). My wife set out Saturday morning on a trip to Philly, to see her parents, about 120 miles. Six miles into the trip, the Ultra Gauge is showing north of 230*, and the dash gauge was climbing toward the red. She pulled over and called me, and the temp started to go down some. The temp came down to around 205, and she came home. The Subie dealer's parts department is open Saturday, so we went and bought a 'genuine' T-stat. I put it in, refilled the system, let it burp, and it's been fine ever since! I drove it Sunday on the Interstate about 30 miles, took the back roads home, up and down a few steep mountains, and the temp never got past the high 180's. I've been down this road with the JEEPS, and now the Subie just re-enforces the fact that we should use only dealer T-stats.

I've included shots of the two stats, the beefier one on the right being the Subie item. Check the difference in the size of the spring! I'm hoping that a cheapo thermostat was the source of the misbehavior.

Now, to replace the noisy 'new' serpentine belt...

 

That's good news. Yes, the need to use a genuine Subaru t'stat is often mentioned in cooling system threads here. I missed that point . . .

(See http://www.subaruoutback.org/forums...-illustrated-guides/26238-draining-replacing-coolant-thermostat.html#post381340 )

 

Does the same sensor drive both fans on 1st gen outback?

Sorry if this ends up in the wrong place. I have searched for a wiring diagram for a first gen (97) outback with no luck. I am probably using the wrong lingo.

I want to find out if the temperature sensor (not the one that sends to the dash) drives both fans. My car is overheating (needle near the max), and the right fan (passenger side) only comes on.

There was a link on this site that pointed to a shop manual. Unfortunately, as sometimes happens, the link is dead.

I think my overheating is caused by a head gasket problem, but that's not the issue of this posting. I simply want to examine the wiring and do a trace.

 

There's two sensors mounted on the back of the water (crossover) pipe; one is for the instrument panel temperature gauge, the other goes to the engine control module (ECM). The ECM controls both radiator fans, depending on the need, through a series of relays.

Might find an applicable service manual here: Directory /Auto/Japan/Subaru/Legacy Outback/

 

Service manual

Thanks very much for this link to the service manual. Is it indexed in any way to facilitate searching? The page that I needed was at the end of a long list of pdf's (with no descriptors) that pertain to the cooling system despite the fact that I started looking in the electrical section.

 

Not other than what's on the web-page.

The pdfs, once downloaded, can be searched using Adobe Acrobat reader, and perhaps other similar tools.