Hey there I have have my 2003 sedan for over 6 months and have only been able to drive it 3 times. It was overheating then the heater is not working. So I have replaced the thermostat, radiator and hoses. Now my heater only blows cold air 🥶 and is overheating but only when the RPMs are higher. So this also means driving over 45mph. Someone please help me!! How can I fix this, I am dying to drive my baby and finish making her look sexy.

 

No offense but it sounds like you need to take it to someone who knows what their doing.

 

You didn't fill/bleed the coolant system properly, drain it all out into a clean container and start over.

-Open the coolant purge valve on the heater core hose, passenger side of the engine bay, looks like a plastic TEE with a plug in it that you can remove with a screwdriver. This part commonly breaks so be ready to go buy another one if that happens.

-Add coolant VERY SLOWLY, this isn't an old chevy truck that you just dump all the coolant in as fast as it will take it. A rate of 1 gallon per minute (pour very slowly, it should take 1 minute to add the entire gallon). This will allow the AIR in the system to bubble out past the coolant so you don't end up with a giant air pocket.

-Once coolant starts to spill out of the purge valve screw the cap back in and continue to slowly add coolant.

-Turn the ignition ON, turn your heat up TO MAX TEMP AND MAX FAN SPEED.

-Start the engine, let it run for one minute while you add more coolant because a little will suck down immediately.

-After one minute rev the engine a few times to 4k, check the coolant again but don't fill it all the way to the top of the neck, the coolant will expand during the next step while it heats up and it will just spill out if you add too much. I usually just keep it above the fins inside the radiator but below the fill neck.

Now you play the waiting game while you watch the thermostat, as soon as it gets up to the middle of the gauge you want to start revving the engine again to force water up against the back of the thermostat so it will open, it will usually start blasting hot air out of the AC when it finally burps.

Top off the radiator, top off the reservoir, reinstall the cap, go for a SHORT DRIVE.

During this drive you want to be pretty hard on the throttle to keep rpm's up, watch the temperature closely for any signs of overheating. Head back to the shop, cool the vehicle (fan in front of the radiator works best) remove the radiator cap, verify it's still topped off, reinstall cap. Verify the coolant reservoir is still topped off.

Now go for a real drive, just be aware that you might have to pull over and cool off so don't go hop on the I-405 in rush hour or anything stilly like that.

Once you have verified the coolant system is functioning properly ONLY NOW can you turn the heater off, watch the thermostat again for any overheat, finish your drive.

Please dispose of old coolant properly.

The End™

 

nice write up, to be clear start the whole process with the car COLD, and between the revs and checking/filling the levels let the car cool ALL THE WAY down?

 

Probably best to watch a few videos on YouTube so you can follow along visually. My first time took me a while to finally get it working, but after learning the tricks to it, you'll be able to do it in an hour.

I found that burping the valve with the car on an incline (front up higher than the back) really helped (Nissan does this aswell in my area, at least on my friends Maxima)

 

You can start the whole process hot or cold but it's easier to work with cold coolant.

Yes let the car cool down considerably, it's basically a verification step to make sure the radiator is properly pulling coolant out of the reservoir during cooling. It cools down really fast if you set a big fan in front of the radiator, takes maybe 15 minutes tops (EDIT: with the hood open).