Highway overheating - slow temp rise

My 2003 Outback Sport has had an overheating problem since the weather has warmed up (2 months/ 3k miles). At highway speed (60+) after 10-15 minutes of driving the temperature will slowly creep up. I can reverse this creep by simply turning the interior temp gauge to hot, but it drops faster if I turn on the blower as well. The car had a dealer replaced radiator back at 90k miles (rotted from lack of coolant change), it’s had a timing belt done (but not sure the water pump was done), driver?s side head gasket (just after radiator), and total 150k miles on it. There are no coolant leaks, and the reserve is full. I’ve also made sure the engine oil level is normal.

It sounds as though either the radiator doesn’t have enough cooling capacity or not enough air is flowing through the radiator. When you turn up the termperature gauge you are adding a second radiator, the heater core. Turning on the blower helps remove the heat from this second radiator. The first thing I would do is take a garden hose and squirt water from the engine side of the radiator through the grille. The honeycomb of the radiator might be clogged with bugs and debris. The next step would be to warm up the engine and feel all over the radiator to see if the temperature is uniform. If it isn’t, the radiator has a flow problem. If these two steps don’t solve the problem, it is possible that the cooling fan for the radiator does not pull enough air. Finally, a radiator hose could be weak and the higher speed of the engine causes the hose to collapse and impede the coolant circulation.

Thanks - I read some other threads about feeling your way through the cooling system after the engine is up to temp to see if there’s anyplace (hose, etc) it gets cold… Luckily I have the PDF set of service manuals - so I’ll figure out where the coolant flows and give that and your ideas a try :slight_smile:

Don’t forget the thermostat. If it’s not opening all the way, you’ll get the exact symptoms you’re describing. It’s generally an easy part to replace.

Additional data - I’ve been watching this over the last few weeks and have observed a few more things:

  • After an hour of highway driving I have observed two different scenarios:
    • The top (inlet) hose was hot, but the lower (outlet) hose was only warm (this would be “normal”?).
    • On another ocasion, both top and bottom hoses were the same hot - almost too hot to touch.
  • I used a bluetooth OBD II connector to monitor the temp at highway speed. It runs 212-214F without the cabin heat/blower on, and if it’s warm outside (65F+), and I leave the cabin heat off it slowly rises into the 240F range which pushes the temp gauge up close to red.
  • both cooling fans do work, but they appear to be coming on only when the coolant is extremely hot (haven’t checked coolant temp yet with the OBD II ).
  • when the temp gauge is above half (hotter than normal) at highway speed, if I hit the clutch (droping RPM from 3500 to 800 idle), the temp gauge will dip cooler, then rebound to the previous reading.


  • how do I test for a bad thermostat?
  • how do I test for a failing water pump?
  • what sensor(s) should I be checking?

Make sure that both fans are running all of the time when the A/C is on. (including the DEF position.

Checking an erratic thermostat can be difficult to do and in my opinion a thermostat is a maintenance item that should be changed every 3-4 years whether faulty or not. It’s cheap insurance.

I do have an issue with a repair that was done. You state the driver’s side head gasket was replaced. On a Subaru BOTH should be done at the same time and a new thermostat should be part of the repair. Changing only one hesd gasket is not a very smart move to put it politely.

This cannot be allowed to continue as 240 degrees is way too hot; and especially so on an all-aluminum Subaru engine.

Pressure wash the front of the radiator to clean all of the bugs and road debris off. If that doesn’t do it, I think you need a new radiator.

Thanks for the advice - interesting side note: A/C hasn’t worked in a couple years. Symptom is compressor clutch doesn’t seem to be engaging when you push the cabin button, but I haven’t tested with volt meter at compressor…

I found the temperatures in the service manual I’m going to test next with the OBD data:
Thermostat opens: 169-176F (partial) 198F (full)
Main fan trigger: 203F

If the radiator is the problem it will be evident by checking the temperature of the lower hose when the problem is apparent with the engine running at fast idle. The lower hose should be at least 20* cooler than the upper hose. Of course, it the radiator fan is not operating the same condition will occur when stopped.

After starting the engine, I monitored the coolant temp, and when it was 175F or so, the lower (output) hose seemed pretty cool to the touch. Think I should try a thermostat? :slight_smile:

Temp sensor and main fan operate correctly - the fan engaged at 205F as expected - and shut off when the coolant temp got down to 195F or so.

It sounds like you have a partially clogged radiator or underperforming water pump. When the car is driven continually, it slowly uses up the ‘reserve’ capacity and begins to overheat because it just can’t shed the heat fast enough.

Of course, given the propensity of Subarus to have head gasket problems, I’d look into that as well.

It could be low pressure caused by a bad radiator cap. After the car is warmed up is the radiator hose extremely firm? if not you have a coolant pressure issue.


  • Water Pump - I have suspected this for a while but mechanic today indicated Subaru water pumps tend to fail loudly and catastrophically (they?re run by the timing belt).
  • Hose firmness - at idle, after the car has warmed up, they are a bit soft (easy to deform .25-.5" by squeezing with two fingers). Definitely firmer than when the engine is off.
  • Head gasket ? if this was the problem wouldn’t I be burning coolant? In 3 months of watching this issue I haven’t lost a drop of coolant. Checked with mechanic today and there is also no discoloration or debris in the overflow.

First attempt ? failed:

This car has an A/C condenser that completely covers the radiator (they overlap 100% and the condenser is in front). Mechanic cleaned a fair amount of debris from the condenser by separating the two and using compressed air. Radiator is pristine, however the condenser has a large percentage of its fins crushed which may restricting the airflow to the radiator.

Took it on the road tonight and it?s still having the problem. I’m inclined to check the coolant system pressure next, and get this condenser replaced (which should have the side-benefit of getting my A/C working again as it?s likely that?s got a leak in it :).

I’m not saying that a head gasket is the cause of this overheating problem; only that the proper procedure is to always replace both AND the thermostat at the same time.

A large number of crushed condenser fins can cause overheating but do not expect the condenser replacement to be a 100% fix for the A/C problem. Most A/C leaks are around the compressor shaft seal and any A/C repair at all after this length of time will mean that the system should be flushed, the drier changed, etc.
Point being, consider the compressor angle before spending bucks on the condenser and expecting it to be a cure-all.

Turned out to simply be a radiator that was partially clogged from failure to change the coolant frequently enough (my guess). Simple test for this - use an infra-red laser thermometer to take temperature samples from various radiator veins after producing the overheat condition. The temperatures varied by 40-60 F from hottest to coldest. With the new radiator I’m getting a nice even temp all over, and on the highway the temp stays below 190F even held at 3000 RPM. Also replaced the thermostat + gasket while I had it all apart - although I tested the factory one and it worked fine…

  • how do I test for a bad thermostat?

The best way is to remove the thermostat, put it in a pot of water with a thermomter, and record what temperature the thermostat opens. Or simply replace it with a new one. they only cost $10. 7 years and 150,000 miles is more than enough. I replace these every coolant change, about 3-4 years. Same with the radiator cap. Cheap insurance.

  • how do I test for a failing water pump?

I doubt the water pump is a problem. The heater core trick works, and would not if the water pump is suspect. Typically these only fail when the impeller shaft seal goes or the shaft bearings fail. YOu’ll notice a dripping of coolant from the weep hole under the pump and/or a horrible grinding sound.

  • what sensor(s) should I be checking?

The sensors that trigger the cooling fans to come on. But, at highway speed, the cooling fans don’t need to work. There is more that sufficient air blowing through the radiator.

At this point, I suspect the thermostat or the radiator. Your due for another service. Get the cooling system flushed out (clear water only, no chemicals), replace the thermostat and radiator cap, the radiator hoses and heater hoses (7 years and 150,000 miles is pushing it), and refill with the proper mix. You should be good to go for another 4 years.

I had the exact same problem. Slow temp rise at higher speeds. One mechanic thought I was crazy since he didn’t drive it far enough and it was a cool day. I had 2 other parts replaced but it turned out to be my radiator. The clue was that the heater cooled it down. When you turn on your heat, the heating core acts as a second radiator which tells you that your main one has an issue. I think mine was clogged from a previous owner dumping who knows what in there. It looked like calcium build up or something. Anyway it’s fixed now. It’s August, driven it 10 hours and back with the AC on, and no problems since.

Hi, I have the same issue. I’m curious how to best flush the system and if I just refilled the radiator 4 months ago should I just put new in or can I put the old back ?

I have heard of using a garden hose but I am not sure I can get access to one.


Flushing shouldn’t be necessary, just add new coolant after replacing the radiator.