I'm having this problem with the airflow from all vents when I'm trying to get heat. I did a little research and figured out which actuator controls the blend door. So I removed the passenger side actuator and I manually turned the little knub in the opposite direction to cold which resulted in increased airflow. Turned it back to heat, airflow yet again gets restricted. Does this mean I need to replace the mixer box? If so, I have to remove the heater core too right? Are there any other possible options?

Air filter is brand new too, fan blows nice and strong at all speeds.

Thanks.

That suggests something is blocking airflow through the heater core but not through the AC evap. Some systems actually have 2 internal doors for that, a sliding door that sets the mix and a secondary flap-door that blocks off the heater core on max AC, so it doesn't "bleed" any heat into the flow. It's possible you have one of those flaps and it's broken internally. I'll try to find a flow diagram for your system.

https://drive.google.com/file/d/12uOvO0 ... sp=sharing

The link is to an airflow diagram for your system. As you can see, the heater core is located lower than the evap in the very bottom of the HVAC box. This means a flood condition from a blocked evap drain will cause loss of airflow through the core, but you'll still have full flow in the cold position. Check your evap drain underneath the car with the AC on, if you don't see drippage from the drain tube then you found your culprit, clean out the drain.

VStar650CL wrote: ↑

Thu Dec 17, 2020 5:37 pm

https://drive.google.com/file/d/12uOvO0 ... sp=sharing

The link is to an airflow diagram for your system. As you can see, the heater core is located lower than the evap in the very bottom of the HVAC box. This means a flood condition from a blocked evap drain will cause loss of airflow through the core, but you'll still have full flow in the cold position. Check your evap drain underneath the car with the AC on, if you don't see drippage from the drain tube then you found your culprit, clean out the drain.

You are awesome. I will do that. Thank you!!!

You're most welcome. Let us know if it's a right diagnosis.

VStar650CL wrote: ↑

Thu Dec 17, 2020 6:25 pm

You're most welcome. Let us know if it's a right diagnosis.

So, its definitely leaking under there with the AC on cold. A nice puddle at that.

Then you must have something else blocking the heater core inside the HVAC unit. There's no second door in the system, you can see on the diagram it's a simple slider with a sequential arrangement of evap and core. Because of the relative duct positions, you might get a bit more flow out the foot vents on heat and out the top vents on cold, but with the mode setting being the same, the total air should be the same. The only possibility I can see is that something went down a defroster duct and fell to the lowest point, which is the core, and is now blocking the airflow.

If anyone is trying to locate the evaporator drain in a Y34 (2003-2004 M45), don't waste your time looking in the engine compartment. It does not come through the firewall. It exits the evap/heater core case on the interior driver's side (LH drive) near the accelerator pedal. You can pull the carpet back to expose the drain line. It actually travels from the HVAC case to the floor pan and then exits the vehicle through the unibody frame near the front of the driver's seat. There is a short 3" drain line that comes out of the case then via a T-fitting connects to the remainder of the drain lines. Nissan utilized a main drain line and an overflow line in case the main line gets clogged. There is a plastic plug that you can remove to see the end of the drain line on the underside of the vehicle. There are three plugs in a row and it is the one closest to the front of the vehicle. If using an evaporator coil cleaner, I recommend disconnecting the the T-fitting near the case and introducing the foam there. Otherwise you will be sending foam up a 30" drain line that may or may not make it to the evaporator coils.