Weird car overheating question

I have a 2003 Subaru Forester. It’s a great car, but the past 2 days it’s driving me nuts.

I have a 45 mile commute to work. It’s about 10 miles to the highway, I can drive that with no problems. I drive on the highway for 30 minutes, still no problems. I start driving the side streets to the place I work, suddenly the temperature gauge is SOARING, I have to immediately crank the heater on full and pull over before it hits the dreaded “red zone”. Same thing on the way home, fine getting to the highway, fine on the highway, start driving under 40mph and ZOOM, there she goes.

Any ideas what might be causing this oddity?

Most likely that your electric fan isn’t kicking in. It could either be the fan, or more likely the temperature switch that controls it. When at speed, the radiator has enough airflow that the fan is not needed.

I’d have to agree with Kevbo, it sounds like the fan is not running when the engine is hot.

When you start out, the engine is cold. During the few minutes it takes you to get to the highway, it doesn’t heat up enough to require the fan. When you get on the highway, the higher speed of the vehicle pushes enough air through the radiator that the fan is not needed.

But now the engine is up to full temperature. When you get off the highway and have to drive slower, and possibly stop and idle, no fan and slower speed mean the engine temp now starts to climb.

Could be the fan or the sensor, or even a bad or loose wire. Easy enough to check, when you see the temperature start to climb, get out and open the hood. The fan should be running at full speed.

Thank you, I guess I should have mentioned that I checked the fans, both of them were running at full speed.
I also checked to make sure there wasn’t a plastic Wal-Mart bag or the like blocking the radiator, checked the oil for signs of coolant and checked the coolant for signs of oil. All were negative.

Subarus have a known problem with a blown head gasket, it sounds like your’s might need an expensive replacement.

Ugh, don’t say that kanicbird!

It’s actually what I fear is the most likely, even without any signs of it in the oil/coolant.

Still, isn’t it weird that it only happens after highway driving? Would a blown head gasket cause that?

Maybe there’s an air bubble in the water jacket.

Park the car nose-up on a steep incline, or jack up the front of the car. This should make the radiator cap the highest point in the cooling system, and thus where the air bubble will end up. Pop the cap to purge the bubble and fill with coolant.
If you’re willing to do that, maybe you can go farther and flush the system.

The radiator should have a small petcock to release the fluid. Refill with water and radiator flush and let the engine run with the heater on high for half an hour.

Drain and refill with fresh coolant/water 50/50 mix.

The flush dissolves mineral deposits which not only reduces proper coolant flow, but also keeps in the heat the engine is trying to get rid of.

Several things come to mind.
Does the car also overheat if you keep driving on the freeway, or only after you get off?

First off are you losing (or have you lost) any coolant? Or have you done any repairs to the cooling system requiring replacement of coolant? If any the the answers are yes, you might have an airlock somewhere in the system.

Do you keep your car in a garage, and have furry animals that also occupy said garage? A nice coating of dog fur can act like a blanket on the face of the radiator.

Also do you drive on dirt roads, or in dusty areas? Small pebbles and dirt can get between the fins and block the airflow. To check for fur and dirt, shine a bright light on one side of the radiator and put your face on the other side. Can you see the light?

You could have a slightly blown head gasket, one that is not mixing oil and water. A good shop can check the coolant for the presence of CO.

You could also have a problem with the water pump not circulating the water fast enough at low RPM causing the coolant to overheat after a run on the freeway. If the pump is driven by a V belt, a belt issue could also cause iproblems. I have seen cases where the impeller of the pump has corroded to the point where it was smaller than it used to be, and it would not move enough coolant. I have also seen pumps with plastic impellers that melted off the shaft :eek:

NHTSA lists several service bulletins for your car. You can access them here:

None of the service bulletins I retrieved had to do with overheating. There are no outstanding safety recalls that have anything to do with this problem.

For engine and engine cooling there’s this:
Make : SUBARU; Model : FORESTER;
Service Bulletin Num : 029605
NHTSA Item Number : 10016869
Date of Bulletin : SEP 09, 2005

and for engine cooling, associated belts and pulleys there is:
Make : SUBARU; Model : FORESTER;
Service Bulletin Num : 02
NHTSA Item Number : 10019343
Date of Bulletin : FEB 01, 2006

To go further on the site you have to pay a fee, but I would guess that you might be able to find out what these are related to by going to the dealership, armed w/ the bulletin numbers. They may, or may not, have someting to do w/ the OP’s problem.

I had the exact same issue ('95 Honda Civic), not sure if the causal chain was:

Bad thermostat/fan didn’t run during idle/overheated during low speeds/idle-blew the head gasket, or

Fan didn’t run at all, overheated during low speeds/damaged the thermostat/blew the head gasket, or

Some other combination of the above, with a very hot Florida day as a possible aggravating factor. I thought I had it fixed on two separate occasions several years apart, but the condition came back three times in total. Finally sold it to a friend.

If you click get summary you will see:

Service Bulletin Number: 02
NHTSA Item Number: 10019343
Summary Description:

Service Bulletin Number: 029605
NHTSA Item Number: 10016869
Summary Description:

Like I said neither of them apply to this problem.

Since you say that the fans were still operating, I can’t say that your car’s computer might be at fault.

When my '92 Cutlass started having overheating problems, it was because the computer was hatched and would turn the electric fan off when the engine got hot.

And I had to mention this, quoting John DiFool:

“Finally sold it to a friend.”

That’s a :smiley: for me, but a :mad: for your (former) friend.

Enola, thank you , I will try that this morning

Rick, you are a great asset to the board. I will try to answer your questions .

  1. It only seems to happen after I slow down after getting off the highway.

  2. I don’t seem to be losing coolant, overflow tank is full. No recent repairs to any part of the car at all.

  3. Not in a garage, but sometimes in the barn. I live in the country and there are all sorts of furry creatures and dusty roads, I’ll clean off the radiator.

I have an appointment with a local garage this afternoon, thanks for the tip about checking the coolant.

Thanks to everyone else willing to help with this problem, too.

I’m having an episode of severe prognostication;
I’m going to say that you have a thermostat problem and that the cost to repair will be in the range of about $113.40. They’ll sell you on a coolant flush for another $39.99.
Have the check made out for $162.60.
You’re welcome.

Okay, before I left the house this morning I did **Enola Straight **'s trick. Then I stopped at a car wash and gave the radiator a good spray down and…
No overheating on the way in! The temperature needle stayed rock solid steady right at it’s normal spot. You guys ROCK!

Hey US, you have confirmed my skepticism in psychics! And when are we going to meet up for a brewski?

So, was there air in the radiator? If so, there’s a leak, and the air - and the problem - will return. Have the system tested for any leakage.

The coolant level in the overflow jar is NOT a reliable indicator of the level in the cooling system. Some leaks will suck air in through the leak when the engine cools, rather than suck coolant in from the overflow. To check the level in the system, remove the radiator cap - COLD ENGINE ONLY - and see if there’s liquid up to the bottom of the filler neck.

Overheating aluminum engines can also cause other serious problems, such as heads and block not aligning properly, so be careful as you sort it out.