The heat in my 2004 civic ex vtec only works when Im driving.. when I stop or go slow.. Im freezing.. thats been going on for a while.. more recently, now it overheats ..sometimes it cools down as I begin to drive again, or if Im on the highway and the heat actually works, that will cool it down too..

I have changed the thermostat, coolant is good and I switched all the relays and each one is good.. Yesterday I left it idle in the driveway and when it was completely overheating, the fan never kicked on under the hood. Although, when I turn the AC on, Both fans work properly..

What else do I do!!! Any suggestions?? Thanks

 

Does the radiator actually stay full of coolant? Don't just check the plastic tank.

Heater only works while driving is a classic symptom of low coolant level, which would then lead to overheating as it gets worse.

You may have head gasket damage from overheating, that will blow the coolant out of the radiator. To check that, take the radiator cap off (of course, never uncap unless engine is COLD), start the engine and rev it, see if any bubbles occur in radiator.

 

your blower motor is messed dude.
hence the heat only moving when your car is moving.
unless i guess you can feel it blowing through the vents

 

^Doubtful that the blower motor is bad. Much more likely a coolant issue.

 

Top off the coolant in the radiator and reservoir and then bleed the cooling system.

Bleeding:

Set the heater to max heat and then do this,

 

welllllll
why is his heat only working when he"s driving fast?

 

Working theory is that he's low on coolant. When the car isn't moving and the engine is idle, the air blows cold. When the car is moving and the RPMs are higher, there is enough pressure built up to move the hot coolant through the heater core, where before there was only air. When at idle, there isn't enough pressure to move the coolant through the block. Hence, overheating.

If this is the case, the next question is what caused him to be low on coolant. Is there a leaky hose? a damaged head gasket? a cracked radiator? Don't know. Hence, troubleshooting.

 

thank you for the clarifacation.
that would make beyond comon sence.

 

When you think of the cooling system as a whole and know what happens when the engine speed changes than it is common sense. Air in the heater core can also occur if you have someone flush the cooling system and not bleed it properly.