Overheating and headgaskets

make , model, year, engine size ?

Are you loosing coolant ?

Is radiator fan working ?

Belt tight on waterpump and waterpump not leaking ?

By being 'in drive', do you mean the car is at speed, I hope?

Overheating at speed, or under load, can mean : (1) water pump not performing efficiently. Belts, or occasionally internal water pump problems (2) water pump suction hose (radiator hose) collapsing (3) cooling air not traversing radiator as it should do (but not usually a fan problem). Can be something like leaves and grass clippings between the air conditioner condensor and the radiator core, etc. (4) radiator partially blocked internally (5) thermostat not opening fully (6) air trapped in cooling system.. (7) leaking head gasket or cracked head or block

There may be others as well.-

make , model, year, engine size ? Subaru Outback wagon, 1996, 2.5 ltr, 178k miles

Are you loosing coolant ? Yes, but cannot see

Is radiator fan working ? Yes

Belt tight on waterpump and waterpump not leaking Yes/

What's the oil look like? Is it milky looking?

-LMB

What's the oil look like? Is it milky looking? No. It's okay.

By being 'in drive', do you mean the car is at speed, I hope? Correct.

So to summarize.

You are loosing a little coolent but can not see a leak. The car idles without overheating. (I assume you leave it idling long enough for the thermostat to open and circulate coolent.)

When driven the car overheats. How does this show itself ? temp gauge, temp light, steam from under hood ?

I would be tempted to open the radiator cap (when cold) and run the car in park until the coolent is circulating. You should be able to tell if there is a reasonable flow.

In my limited experiance if the head gasket is blown you will get either bad compression on one cylinder or a coolent leak into one cylinder.

You may check for antifreeze at the tailpipe but it may be caught at the catalytic converter or muffler.

Saw that your oil was okay. Given that:

Most likely: Do you have a heated intake manifold or throttle body? There may be an internal leak or a leaking gasket.

Maybe: HG leak coolant into cylinder, but you should see white smoke or smell sweet antifreeze at the exhaust. Are any of your spark plugs REALLY clean?

Long shots: HG leak exhaust into coolant - check radiator for exhaust bubbles or have antifreeze tested for HC presence

Check transmission fluid. Maybe the radiator is internally leaking into the transmission cooler.

Thanks to all for the advice!!

Good points to observe....

If you a losing a little coolant, without seeing where it is going, you may also want to take a look at all your Welch (freeze) plugs. A pinhole leak can gradually cause you to lose enough coolant to give overheating...As it sucks air, you can also get a lot of rust in the cooling system.

Another place coolant can go is into the passenger compartment via a leaky heater core. Usually you will get a syrupy sweet smell of glycol in the car if this happens.

A common symptom of a blown head gasket is steam out the exhaust.

After the car has warmed up, shut the engine off and feel of the radiator. It should be uniformly warm. If one side is hot and the other side is cold, you have a partially plugged radiator.

Another common problem is a bad thermostat. The engine will get hot but the radiator will stay relatively cool. The way to test it is to take it out, put it in a pan of water and heat it up on the stove. The thermostat should open before the water boils (unless you are at a VERY high altitude...).

Jerry