2006 wrx overheats going slow but not at speed

Question

my 2006 wrx heats up into the red going slow on back roads but is fine and shows no signs of any issue going fast. what is the deal? blown head gasket?

Answer 1

I don't think there's any reason to suspect a head gasket right off the bat like that. The first thing that comes to mind is the electric cooling fans. They are on the car specifically to keep it cool at slow speeds since you don't have natural air flow.

Get the car up to temperature, open the hood and wait for the fans to come on. If they come on you can start looking for other issues, but I suspect this is the problem.

If they don't come on, you'll have to check fuses first, then check the temperature sensor that makes them come on. If all of that is fine, you can check the fan connector for voltage. If everything is OK, but the fan still doesn't come on, you could have a bad fan motor.

Start with the simple stuff first. I would probably even check fuses before even looking at the fans because sometimes it is that easy.

Answer 2

Cools at speed, but not at slow vehicle speed. Definitely an engine cooling fan issue.

• It's either the fan itself or the sending units that tell the fan when to come on.
• Best bet is to pick up a Multimeter and a manual that tells you the right signals from all the sending units involved (normally an engine temp sender.) I'd recommend either a Haynes or a Chilton (New $30, from your local autoparts store.) When you look at that manual, it will tell you where all the correct sending units are located, and what values they should read (engine off). Normally these units offer a variable resistance based on temperature. You can read that resistance with the Multimeter.
• Obviously it could also be a wiring issue (power to the fan, or a signal input wire from a sending unit.) Wiring could be cut or worn (on sharp sheet metal) or burnt (by engine exhaust).
• There may be a separate fan controller module somewhere, something that takes a low current signal from the Engine control computer and converts that to a high amperage Pulse Width Modulated (PWM) signal to drive the engine cooling fan. A PWM system allows for your electric fan to spin at different speeds. This acts like a relay.. cooling fans take a whole lot of electrical current.
• Additionally you should ensure your cooling system is sound. Right amount of coolant, correctly operating fill cap (with an internal spring that controls the pressure of the cooling system.)
• You should ensure that there is no air blockage in front of the radiator / air conditioning condenser. You need all the air you can get.
• Please verify that the cooling fan shrouds are sealed very carefully around the radiator such that air is force thru the radiator when pulled by the cooling fan.

Let us know what you find.