The colder it gets outside, the hotter my car gets...

Ever since my rebuild, my car was fine. Now that it's getting colder, my car is starting to overheat on long-ish drives. It just snowed HARD here and I went for a drive, only to see the temp gauge jump 4 ticks. Normally when it's cold, it just jumps 1 or 2. But since it's more cold now that before, I think there's some kind of correlation.

Something else to scratch your brain: it only jumps when I drive. When I come to a stoplight, the gauge goes right back down just below operating temp. So when there's cold air going through the radiator, it overheats. When it's idling in it's own heat, it's fine.

My first thought was the thermo because it's on the outside of the engine (kinda) and the cold could be making it close up in spite of the actual engine temp. Then when I stop, no cold air is going past the outside of the thermo and the engine temp opens it again. Does this sound plausible or is there something I'm missing with this whole thing?

Thanks guys.

TL;DR: Car overheats in the cold, not when idle. I suspect the thermo to be the culprit. Thoughts?

Car info:
1998 Legacy Outback Wagon with EJ25D (HG rebuild about 2500 miles ago). No performance modifications whatsoever. I drive below 3500 rpm because I need the car to last.

Bleed the coolant AKA "burping"

Or, in other words.. Purge all the air from the cooling system.

You shouldn't have to do this is you use the air cap (screws out of the passenger side of the radiator), and have some patience and SLOWLY fill the coolant. If you see it fill up to the radiator neck youre not done. Wait for it it go down. This will start to happenpre and more frequently. It takes more than just a few minutes. After it's pretty much full and you can't get anymore in at all, close the cap. Then FILL the overflow bottle to the top. Drive the car until it gets up to normal op temp. Then shut the motor off until it cools ALL the way down. This will allow the system to depressurize and create a vacuum. This will pull coolant out of the overflow bottle. Keep an eye on it. It will either get really low or empty out depending on how patiently you poured coolant in the radiator. Fill it up here and there. After driving a few times it should stop pulling any extra antifreeze at all and start to normally fill and drain.


Okay, so here's what I want you to do. As soon as the temp rises up while driving, shift down until the motor is revving ABOVE 3500 (4000 RPM at the MOST) RPM. This will flow enough coolant through an OPEN thermostat to cool an engine with bad head gaskets or a bad thermostat. Any other issues, and it won't cool down. If it doesn't work at first, try a few more times. Keep the heat ON and at full "defrost." drive like this for a while. Does it cool down? And how long does it take? What does it do with the heat and everything completely off?

After you start driving like normal, how long does it take to overheat again (while drving) if it cooled.

Why speeds does the overheating occur?

Which water pump do you have? The one where the thermostat is mounted vertical or horizontally?

How long does it take for it to start overheating when you FIRST drive it for the day?

How long does it take to overheat when you drive it after you've driven once or a few times?

Have you replaced the thermostat before?

Originally Posted by broknindarkagain

Bleed the coolant AKA "burping"

I did that when I put the coolant in and it ran fine until it got cold. Would the cold weather suddenly cause air bubbles?

Originally Posted by Kennyfvholla

Okay, so here's what I want you to do. As soon as the temp rises up while driving, shift down until the motor is revving ABOVE 3500 (4000 RPM at the MOST) RPM. This will flow enough coolant through an OPEN thermostat to cool an engine with bad head gaskets or a bad thermostat. Any other issues, and it won't cool down. If it doesn't work at first, try a few more times. Keep the heat ON and at full "defrost." drive like this for a while. Does it cool down? And how long does it take? What does it do with the heat and everything completely off?

I drive the car below 3500 for a reason... I'm almost certain the HGs are fine and that it's the thermo (if anything). My tranny (4EAT) is going bad so I don't want to put any more strain on it by downshifting just to test the cooling system.

Nothing that I do with the HVAC or electronics does anything whatsoever. I've tried with my brights, fogs, and stereo all the way up, heater full blast (defrost and non-defrost), and the opposites of both. I thought the heater would help because that's what you do in a car that's heating up: turn on the heater vents. Also, my cabin air circulation is off. I check every time to make sure.

After you start driving like normal, how long does it take to overheat again (while drving) if it cooled.
On a cold start, it will take probably 10-20 minutes of driving time to start over heating. (in this weather, anyway. I didn't have a problem in the warm months.)

Why speeds does the overheating occur?
Yesterday, it did it while I was going 40. Today, it didn't do it while I was going 25 or 75. I'm pretty sure it has more to do with the duration of driving time and the temperature.

Which water pump do you have? The one where the thermostat is mounted vertical or horizontally?
I have a GMB water pump that was replaced with the rebuild. No additive or any kind of gunk could have (or SHOULD have) clogged it or impaired it. I'm not sure if it mounts vertically or horizontally because I replace it on an engine stand. My guess would be horizontally.

How long does it take for it to start overheating when you FIRST drive it for the day?
How long does it take to overheat when you drive it after you've driven once or a few times?
On a cold start, it's usually 10-20 minutes. Sometimes when I get back in the car after going into a store or something it will start to heat up and sometimes it won't.

Have you replaced the thermostat before?
I have replaced it with a Duralast part from AutoZone. It was cheap and I was in a bind. If I replace it again, I'll be sure to use the factory part from a dealer.


One more thing: I've noticed that on long stretches of road with minimal stoplights it tends to heat up faster than in city/stop and go traffic. This has nothing to do with the speed either.

Originally Posted by taylormac1993

Have you replaced the thermostat before?
I have replaced it with a Duralast part from AutoZone. It was cheap and I was in a bind. If I replace it again, I'll be sure to use the factory part from a dealer.

That may be your problem. Only use OEM Subaru thermostats. They are only $23 from the dealer. All other thermostats are garbage.

You can take out your thermostat and drive around and see if it still overheats to check if it is a bad thermostat.

Driving below 3500 RPM in a lower gear isn't going to place any more stress on the trans. I understand your worry, but you will actually put less load on the trans by downshifting into 3rd at whatever speed. It's going to help diagnosis, but suit yourself.

I'll post up a picture of the water pumps and what I am talking about. But if you used a brand new water pump I'd say it's safe to assume that its the better flowing one with the beefier thermostat.

Get an OE thermostat as soon as you can. Something is causing the engine to not cool properly when operated for extended periods of time. I'm thinking thermostat.

I also don't want to put more stress on the ENGINE which is my main concern. Anywho, I think what I'm gonna do is replace the thermo with the OEM part and replace the coolant with an exact 50/50 mix so I'm sure it's good to go. Would it be worth it to get some radiator hose insulation too or is that a waste of money?

Revving the engine up (for less than a minute) to where its making almost peak torque, to COOL the motor puts WY less strain on it. There's no load, and lots more coolant flow to cool the motor. Over heating puts more strain and wear on all the bearings than anything else. You'd be doing your engine a favor. I'm trying to help diagnose/fix your issue. I would never te you to do anything that's going to harm your car or make any other issues worse.

Do what you said and let us know how it goes.

Hose insulation is a waste of money.

you didn't drive thru lots of leaves recently, did ya?

Originally Posted by HDTurbo

you didn't drive thru lots of leaves recently, did ya?

I might have.... I'll make sure there's no shit stuck around my radiator too. Didn't think about that.

Check the radiator.

Replace that damn thermostat. STAT (no pun intended).

It has been very well documented that anything other than an OEM thermostat on these cars is bad.